Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (510) 668-7000 • FAX (510) 668-7017 • www.exar.com
xr XR16L2750
2.25V TO 5.5V DUART WITH 64-BYTE FIFO
APRIL 2005 REV. 1.2.1
GENERAL DESCRIPTION
The XR16L27501 (2750) is a low voltage dual
universal asynchronous receiver and transmitter
(UART) with 5 Volt tolerant inputs. The device
operates from 2.25 to 5.5 Volt supply range and is
pin-to-pin compatible to Exar’s ST16C2550 and
XR16C2850 except the 48-TQFP package. The 2750
register set is compatible to the ST16C2550 and the
XR16C2850 enhanced features. It supports the
Exar’s enhanced features of 64 bytes of TX and RX
FIFOs, programmable FIFO trigger level and FIFO
level counters, automatic hardware (RTS/CTS) and
software flow control, automatic RS-485 half duplex
direction control output and a complete modem
interface. Onboard registers provide the user with
operational status and data error flags. An internal
loopback capability allows system diagnostics.
Independent programmable baud rate generators are
provided in each channel to select data rates up to
6.25 Mbps at 5 Volt and 8X sampling clock. The 2750
is available in 48-pin TQFP and 44-pin PLCC
packages.
NOTE: 1 Covered by U.S. Patent #5,649,122 and #5,949,787
APPLICATIONS
•Portable Appliances
•Telecommunication Network Routers
•Ethernet Network Routers
•Cellular Data Devices
•Factory Automation and Process Controls
FEATURES
•2.25 to 5.5 Volt Operation
•5 Volt Tolerant Inputs
•Pin-to-pin compatible to Exar’s ST16C2550 and
TI’s TL16C752B on the 48-TQFP package
•Pin alike XR16C2850 48-TQFP package but
without CLK8/16, CLKSEL and HDCNTL inputs
•Two independent UART channels
■Reg set compatible to 16C2550 and 16C2850
■Up to 6.25 Mbps at 5 Volt, 4 Mbps at 3.3 Volt,
and 3 Mbps at 2.5 Volt with 8X sampling rate
■Transmit and Receive FIFOs of 64 bytes
■Programmable TX and RX FIFO Trigger Levels
■Transmit and Receive FIFO Level Counters
■Automatic Hardware (RTS/CTS) Flow Control
■Selectable Auto RTS Flow Control Hysteresis
■Automatic Software (Xon/Xoff) Flow Control
■Automatic RS-485 Half-duplex Direction
Control Output via RTS#
■Wireless Infrared (IrDA 1.0) Encoder/Decoder
■Automatic sleep mode
■Full modem interface
•Device Identification and Revision
•Crystal oscillator or external clock input
•Industrial and commercial temperature ranges
•48-TQFP and 44-PLCC packages
FIGURE 1. XR16L2750 BLOCK DIAGRAM
XTAL1
XTAL2
Crystal Osc/Buffer
TXA, RXA, DTRA#,
DSRA#, RTSA#,
DTSA#, CDA#, RIA#,
OP2A#
8-bit Data
Bus
Interface
UART Channel A
64 Byte TX FIFO
BRG
IR
ENDEC
TX & RX
UART
Regs
2.25 to 5.5 Volt VCC
GND
* 5 Volt Tolerant Inputs
(Except XTAL1)
2750BLK
TXB, RXB, DTRB#,
DSRB#, RTSB#,
CTSB#, CDB#, RIB#,
OP2B#
UART Channel B
(same as Channel A)
A2:A0
D7:D0
CSA#
CSB#
INTA
INTB
IOW#
IOR#
Reset
TXRDYA#
TXRDYB#
RXRDYA#
RXRDYB#
64 Byte RX FIFO
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
2
FIGURE 2. PIN OUT ASSIGNMENT
ORDERING INFORMATION
PART NUMBER PACKAGE OPERATING TEMPERATURE RANGE DEVICE STATUS
XR16L2750CJ 44-Lead PLCC 0°C to +70°C Active
XR16L2750IJ 44-Lead PLCC -40°C to +85°C Active
XR16L2750CM 48-Lead TQFP 0°C to +70°C Active
XR16L2750IM 48-Lead TQFP -40°C to +85°C Active
48
47
46
45
44
43
42
41
40
39
38
37
1
2
3
4
5
6
7
8
9
10
11
12
36
35
34
33
32
31
30
29
28
27
26
25
13
14
15
16
17
18
19
20
21
22
23
24
D5
D6
D7
RXB
RXA
TXRDYB#
TXA
TXB
OP2B#
CSA#
CSB#
NC
XTAL1
XTAL2
IOW#
CDB#
GND
RXRDYB#
IOR#
DSRB#
RIB#
RTSB#
CTSB#
NC
RESET
DTRB#
DTRA#
RTSA#
OP2A#
RXRDYA#
INTA
INTB
A0
A1
A2
NC
D4
D3
D2
D1
D0
TXRDYA#
VCC
RIA#
CDA#
DSRA#
CTSA#
NC
XR16L2750
48-pin TQFP
6
5
4
3
2
1
44
43
42
41
40
7
8
9
10
11
12
13
14
15
16
17
39
38
37
36
35
34
33
32
31
30
29
18
19
20
21
22
23
24
25
26
27
28
D5
D6
D7
RXB
RXA
TXRDYB#
TXA
TXB
OP2B#
CSA#
CSB#
RESET
DTRB#
DTRA#
RTSA#
OP2A#
RXRDYA#
INTA
INTB
A0
A1
A2
XTAL1
XTAL2
IOW#
CDB#
GND
RXRDYB#
IOR#
DSRB#
RIB#
RTSB#
CTSB#
D4
D3
D2
D1
D0
TXRDYA#
VCC
RIA#
CDA#
DSRA#
CTSA#
XR16L2750
44-pin PLCC
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
3
PIN DESCRIPTIONS
Pin Description
NAME 44-PLCC
PIN #
48-TQFP
PIN # TYPE DESCRIPTION
DATA BUS INTERFACE
A2
A1
A0
29
30
31
26
27
28
IAddress data lines [2:0]. These 3 address lines select one of the inter-
nal registers in UART channel A/B during a data bus transaction.
D7
D6
D5
D4
D3
D2
D1
D0
9
8
7
6
5
4
3
2
3
2
1
48
47
46
45
44
I/O Data bus lines [7:0] (bidirectional).
IOR# 24 19 IInput/Output Read Strobe (active low). The falling edge instigates an
internal read cycle and retrieves the data byte from an internal register
pointed to by the address lines [A2:A0]. The data byte is placed on the
data bus to allow the host processor to read it on the rising edge.
IOW# 20 15 IInput/Output Write Strobe (active low). The falling edge instigates an
internal write cycle and the rising edge transfers the data byte on the
data bus to an internal register pointed by the address lines.
CSA# 16 10 IUART channel A select (active low) to enable UART channel A in the
device for data bus operation.
CSB# 17 11 IUART channel B select (active low) to enable UART channel B in the
device for data bus operation.
INTA 33 30 OUART channel A Interrupt output. The output state is defined by the
user through the software setting of MCR[3]. INTA is set to the active
mode and OP2A# output LOW when MCR[3] is set to a logic 1. INTA is
set to the three state mode and OP2A# output HIGH when MCR[3] is
set to a logic 0 (default). See MCR[3].
INTB 32 29 OUART channel B Interrupt output. The output state is defined by the
user through the software setting of MCR[3]. INTB is set to the active
mode and OP2B# output LOW when MCR[3] is set to a logic 1. INTB is
set to the three state mode and OP2B# output HIGH when MCR[3] is
set to a logic 0 (default). See MCR[3].
TXRDYA# 143 OUART channel A Transmitter Ready (active low). The output provides
the TX FIFO/THR status for transmit channel A. See Table 2. If it is not
used, leave it unconnected.
RXRDYA# 34 31 OUART channel A Receiver Ready (active low). This output provides the
RX FIFO/RHR status for receive channel A. See Table 2. If it is not
used, leave it unconnected.
TXRDYB# 12 6 O UART channel B Transmitter Ready (active low). The output provides
the TX FIFO/THR status for transmit channel B. See Table 3. If it is not
used, leave it unconnected.
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
4
RXRDYB# 23 18 OUART channel B Receiver Ready (active low). This output provides the
RX FIFO/RHR status for receive channel B. See Table 2. If it is not
used, leave it unconnected.
MODEM OR SERIAL I/O INTERFACE
TXA 13 7 O UART channel A Transmit Data or infrared encoder data. Standard
transmit and receive interface is enabled when MCR[6] = 0. In this
mode, the TX signal will be HIGH during reset or idle (no data). Infrared
IrDA transmit and receive interface is enabled when MCR[6] = 1. In the
Infrared mode, the inactive state (no data) for the Infrared encoder/
decoder interface is LOW. If it is not used, leave it unconnected.
RXA 11 5 I UART channel A Receive Data or infrared receive data. Normal receive
data input must idle HIGH. The infrared receiver pulses typically idles at
LOW but can be inverted by software control prior going in to the
decoder, see MCR[6] and FCTR[2]. If this pin is not used, tie it to VCC
or pull it high via a 100k ohm resistor.
RTSA# 36 33 OUART channel A Request-to-Send (active low) or general purpose out-
put. This output must be asserted prior to using auto RTS flow control,
see EFR[6], MCR[1], FCTR[1:0], EMSR[5:4] and IER[6]. For auto
RS485 half-duplex direction control, see FCTR[3] and EMSR[3].
CTSA# 40 38 IUART channel A Clear-to-Send (active low) or general purpose input.
It can be used for auto CTS flow control, see EFR[7], and IER[7]. This
input should be connected to VCC when not used.
DTRA# 37 34 OUART channel A Data-Terminal-Ready (active low) or general purpose
output. If it is not used, leave it unconnected.
DSRA# 41 39 IUART channel A Data-Set-Ready (active low) or general purpose input.
This input should be connected to VCC when not used. This input has
no effect on the UART.
CDA# 42 40 IUART channel A Carrier-Detect (active low) or general purpose input.
This input should be connected to VCC when not used. This input has
no effect on the UART.
RIA# 43 41 IUART channel A Ring-Indicator (active low) or general purpose input.
This input should be connected to VCC when not used. This input has
no effect on the UART.
OP2A# 35 32 OOutput Port 2 Channel A - The output state is defined by the user and
through the software setting of MCR[3]. INTA is set to the active mode
and OP2A# output LOW when MCR[3] is set to a logic 1. INTA is set to
the three state mode and OP2A# output HIGH when MCR[3] is set to a
logic 0. See MCR[3]. This output should not be used as a general out-
put else it will disturb the INTA output functionality.
TXB 14 8 O UART channel B Transmit Data or infrared encoder data. Standard
transmit and receive interface is enabled when MCR[6] = 0. In this
mode, the TX signal will be HIGH during reset or idle (no data). Infrared
IrDA transmit and receive interface is enabled when MCR[6] = 1. In the
Infrared mode, the inactive state (no data) for the Infrared encoder/
decoder interface is LOW. If it is not used, leave it unconnected.
Pin Description
NAME 44-PLCC
PIN #
48-TQFP
PIN # TYPE DESCRIPTION
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
5
Pin type: I=Input, O=Output, I/O= Input/output, OD=Output Open Drain.
RXB 10 4 I UART channel B Receive Data or infrared receive data. Normal receive
data input must idle HIGH. The infrared receiver pulses typically idles at
logic 0 but can be inverted by software control prior going in to the
decoder, see MCR[6] and FCTR[2]. If this pin is not used, tie it to VCC
or pull it high via a 100k ohm resistor.
RTSB# 27 22 OUART channel B Request-to-Send (active low) or general purpose out-
put. This port must be asserted prior to using auto RTS flow control,
see EFR[6], MCR[1], FCTR[1:0], EMSR[5:4] and IER[6]. For auto
RS485 half-duplex direction control, see FCTR[3] and EMSR[3].
CTSB# 28 23 IUART channel B Clear-to-Send (active low) or general purpose input.
It can be used for auto CTS flow control, see EFR[7], and IER[7]. This
input should be connected to VCC when not used.
DTRB# 38 35 OUART channel B Data-Terminal-Ready (active low) or general purpose
output. If it is not used, leave it unconnected.
DSRB# 25 20 IUART channel B Data-Set-Ready (active low) or general purpose input.
This input should be connected to VCC when not used. This input has
no effect on the UART.
CDB# 21 16 IUART channel B Carrier-Detect (active low) or general purpose input.
This input should be connected to VCC when not used. This input has
no effect on the UART.
RIB# 26 21 IUART channel B Ring-Indicator (active low) or general purpose input.
This input should be connected to VCC when not used. This input has
no effect on the UART.
OP2B# 15 9 O Output Port 2 Channel B - The output state is defined by the user and
through the software setting of MCR[3]. INTB is set to the active mode
and OP2B# output LOW when MCR[3] is set to a logic 1. INTB is set to
the three state mode and OP2B# output HIGH when MCR[3] is set to a
logic 0. See MCR[3]. This output should not be used as a general out-
put else it will disturb the INTB output functionality.
ANCILLARY SIGNALS
XTAL1 18 13 ICrystal or external clock input. Caution: this input is not 5V tolerant.
XTAL2 19 14 OCrystal or buffered clock output.
RESET 39 36 IReset (active high) - A longer than 40 ns HIGH pulse on this pin will
reset the internal registers and all outputs. The UART transmitter output
will be held HIGH, the receiver input will be ignored and outputs are
reset during reset period (see External Reset Conditions).
VCC 44 42 Pwr 2.25V to 5.5V power supply. All input pins, except XTAL1, are 5V toler-
ant.
GND 22 17 Pwr Power supply common, ground.
N.C. none 12, 24, 25,
37
No Connection.
Pin Description
NAME 44-PLCC
PIN #
48-TQFP
PIN # TYPE DESCRIPTION
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
6
1.0 PRODUCT DESCRIPTION
The XR16L2750 (2750) integrates the functions of 2 enhanced 16C550 Universal Asynchronous Receiver and
Transmitter (UART). Each UART is independently controlled having its own set of device configuration
registers. The configuration registers set is 16550 UART compatible for control, status and data transfer.
Additionally, each UART channel has 64-bytes of transmit and receive FIFOs, automatic RTS/CTS hardware
flow control with hysteresis control, automatic Xon/Xoff and special character software flow control,
programmable transmit and receive FIFO trigger levels, FIFO level counters, infrared encoder and decoder
(IrDA ver 1.0), programmable baud rate generator with a prescaler of divide by 1 or 4, and data rate up to 6.25
Mbps with 8X sampling clock rate or 3.125 Mbps in the 16X rate. The XR16L2750 is a 2.25 to 5.5V device with
5 volt tolerant inputs. The 2750 is fabricated with an advanced CMOS process.
Enhanced Features
The 2750 DUART provides a solution that supports 64 bytes of transmit and receive FIFO memory, instead of
128 bytes provided in the XR16C2850 and 16 bytes in the ST16C2550, or one byte in the ST16C2450. The
2750 is designed to work with low supply voltage and high performance data communication systems, that
require fast data processing time. Increased performance is realized in the 2750 by the larger transmit and
receive FIFOs, FIFO trigger level control, FIFO level counters and automatic flow control mechanism. This
allows the external processor to handle more networking tasks within a given time. For example, the
ST16C2550 with a 16 byte FIFO, unloads 16 bytes of receive data in 1.53 ms (This example uses a character
length of 11 bits, including start/stop bits at 115.2 Kbps). This means the external CPU will have to service the
receive FIFO at 1.53 ms intervals. However with the 64 byte FIFO in the 2750, the data buffer will not require
unloading/loading for 6.1 ms. This increases the service interval giving the external CPU additional time for
other applications and reducing the overall UART interrupt servicing time. In addition, the programmable FIFO
level trigger interrupt and automatic hardware/software flow control is uniquely provided for maximum data
throughput performance especially when operating in a multi-channel system. The combination of the above
greatly reduces the CPU’s bandwidth requirement, increases performance, and reduces power consumption.
The 2750 supports a half-duplex output direction control signaling pin, RTS# A/B, to enable and disable the
external RS-485 transceiver operation. It automatically switches the logic state of the output pin to the receive
state after the last stop-bit of the last character has been shifted out of the transmitter. After receiving, the logic
state of the output pin switches back to the transmit state when a data byte is loaded in the transmitter. The
auto RS-485 direction control pin is not activated after reset. To activate the direction control function, user has
to set FCTR Bit-3 to “1”. This pin is normally high for receive state, low for transmit state.
Data Rate
The 2750 is capable of operation up to 3.125 Mbps at 5V with 16X internal sampling clock rate, and 6.25 Mbps
at 5V with 8X sampling clock rate. The device can operate with an external 24 MHz crystal on pins XTAL1 and
XTAL2, or external clock source of up to 50 MHz on XTAL1 pin. With a typical crystal of 14.7456 MHz and
through a software option, the user can set the prescaler bit for data rates of up to 1.84 Mbps.
The rich feature set of the 2750 is available through the internal registers. Automatic hardware/software flow
control, selectable transmit and receive FIFO trigger levels, selectable TX and RX baud rates, infrared
encoder/decoder interface, modem interface controls, and a sleep mode are all standard features.
Following a power on reset or an external reset, the 2750 is software compatible with previous generation of
UARTs, 16C450, 16C550 and 16C650A as well as the 16C850.
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
7
2.0 FUNCTIONAL DESCRIPTIONS
2.1 CPU Interface
The CPU interface is 8 data bits wide with 3 address lines and control signals to execute data bus read and
write transactions. The 2750 data interface supports the Intel compatible types of CPUs and it is compatible to
the industry standard 16C550 UART. No clock (oscillator nor external clock) is required to operate a data bus
transaction. Each bus cycle is asynchronous using CS#, IOR# and IOW# signals. Both UART channels share
the same data bus for host operations. The data bus interconnections are shown in Figure 3.
2.2 5-Volt Tolerant Inputs
The 2750 can accept up to 5V inputs even when operating at 3.3V or 2.5V. But note that if the 2750 is
operating at 2.5V, its VOH may not be high enough to meet the requirements of the VIH of a CPU or a serial
transceiver that is operating at 5V. Caution: XTAL1 is not 5 volt tolerant.
2.3 Device Reset
The RESET input resets the internal registers and the serial interface outputs in both channels to their default
state (see Table 16). An active high pulse of longer than 40 ns duration will be required to activate the reset
function in the device.
2.4 Device Identification and Revision
The XR16L2750 provides a Device Identification code and a Device Revision code to distinguish the part from
other devices and revisions. To read the identification code from the part, it is required to set the baud rate
generator registers DLL and DLM both to 0x00. Now reading the content of the DLM will provide 0x0A for the
XR16L2750 and reading the content of DLL will provide the revision of the part; for example, a reading of 0x01
means revision A.
2.5 Channel A and B Selection
The UART provides the user with the capability to bi-directionally transfer information between an external
CPU and an external serial communication device. A logic 0 on chip select pins, CSA# or CSB#, allows the
user to select UART channel A or B to configure, send transmit data and/or unload receive data to/from the
UART. Selecting both UARTs can be useful during power up initialization to write to the same internal registers,
but do not attempt to read from both uarts simultaneously. Individual channel select functions are shown in
Table 1.
FIGURE 3. XR16L2750 DATA BUS INTERCONNECTIONS
VCC
VCC
OP2A#
DSRA#
CTSA#
RTSA#
DTRA#
RXA
TXA
RIA#
CDA#
OP2B#
DSRB#
CTSB#
RTSB#
DTRB#
RXB
TXB
RIB#
CDB#
GND
A0
A1
A2
UART_CSA#
UART_CSB#
IOR#
IOW#
D0
D1
D2
D3
D4
D5
D6
D7
A0
A1
A2
CSA#
CSB#
D0
D1
D2
D3
D4
D5
D6
D7
IOR#
IOW#
UART
Channel A
UART
Channel B
UART_INTB
UART_INTA
INTB
INTA
RXRDYA#
TXRDYA#
RXRDYA#
TXRDYA#
RXRDYB#
TXRDYB#
RXRDYB#
TXRDYB#
UART_RESET RESET
Serial Interface of
RS-232, RS-485
Serial Interface of
RS-232, RS-485
2750int
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
8
2.6 Channel A and B Internal Registers
Each UART channel in the 2750 has a set of enhanced registers for control, monitoring and data loading and
unloading. The configuration register set is compatible to those already available in the standard single
16C550 and dual ST16C2550. These registers function as data holding registers (THR/RHR), interrupt status
and control registers (ISR/IER), a FIFO control register (FCR), receive line status and control registers (LSR/
LCR), modem status and control registers (MSR/MCR), programmable data rate (clock) divisor registers (DLL/
DLM), and a user accessible Scratchpad Register (SPR).
Beyond the general 16C2550 features and capabilities, the 2750 offers enhanced feature registers (EMSR,
FLVL, EFR, Xon/Xoff 1, Xon/Xoff 2, FCTR, TRG, FC) that provide automatic RTS and CTS hardware flow
control, Xon/Xoff software flow control, automatic RS-485 half-duplex direction output enable/disable, FIFO
trigger level control, and FIFO level counters. All the register functions are discussed in full detail later in
“Section 3.0, UART INTERNAL REGISTERS” on page 20.
2.7 DMA Mode
The device does not support direct memory access. The DMA Mode (a legacy term) in this document doesn’t
mean “direct memory access” but refers to data block transfer operation. The DMA mode affects the state of
the RXRDY# A/B and TXRDY# A/B output pins. The transmit and receive FIFO trigger levels provide additional
flexibility to the user for block mode operation. The LSR bits 5-6 provide an indication when the transmitter is
empty or has an empty location(s) for more data. The user can optionally operate the transmit and receive
FIFO in the DMA mode (FCR bit-3=1). When the transmit and receive FIFO are enabled and the DMA mode is
disabled (FCR bit-3 = 0), the 2750 is placed in single-character mode for data transmit or receive operation.
When DMA mode is enabled (FCR bit-3 = 1), the user takes advantage of block mode operation by loading or
unloading the FIFO in a block sequence determined by the programmed trigger level. In this mode, the 2750
sets the TXRDY# pin when the transmit FIFO becomes full, and sets the RXRDY# pin when the receive FIFO
becomes empty. The following table shows their behavior. Also see Figures 18 through 23.
TABLE 1: CHANNEL A AND B SELECT
CSA# CSB# FUNCTION
1 1 UART de-selected
0 1 Channel A selected
1 0 Channel B selected
0 0 Channel A and B selected
TABLE 2: TXRDY# AND RXRDY# OUTPUTS IN FIFO AND DMA MODE
PINS FCR BIT-0=0
(FIFO DISABLED)FCR BIT-0=1 (FIFO ENABLED)
FCR Bit-3 = 0
(DMA Mode Disabled)
FCR Bit-3 = 1
(DMA Mode Enabled)
RXRDY# A/B LOW = 1 byte.
HIGH = no data.
LOW = at least 1 byte in FIFO.
HIGH = FIFO empty.
HIGH to LOW transition when FIFO reaches the
trigger level, or time-out occurs.
LOW to HIGH transition when FIFO empties.
TXRDY# A/B LOW = THR empty.
HIGH = byte in THR.
LOW = FIFO empty.
HIGH = at least 1 byte in FIFO.
LOW = FIFO has at least 1 empty location.
HIGH = FIFO is full.
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
9
2.8 INTA and INTB Outputs
The INTA and INTB interrupt output changes according to the operating mode and enhanced features setup.
Table 3 and 4 summarize the operating behavior for the transmitter and receiver. Also see Figures 18
through 23.
2.9 Crystal Oscillator or External Clock Input
The 2750 includes an on-chip oscillator (XTAL1 and XTAL2) to produce a clock for both UART sections in the
device. The CPU data bus does not require this clock for bus operation. The crystal oscillator provides a
system clock to the Baud Rate Generators (BRG) section found in each of the UART. XTAL1 is the input to the
oscillator or external clock buffer input with XTAL2 pin being the output. Please note that the input XTAL1 is not
5V tolerant and so the maximum at the pin should be VCC. For programming details, see “Programmable
Baud Rate Generator.”
TABLE 3: INTA AND INTB PINS OPERATION FOR TRANSMITTER
AUTO RS485
MODE
FCR BIT-0 = 0
(FIFO DISABLED)FCR BIT-0 = 1 (FIFO ENABLED)
INTA/B Pin NO LOW = a byte in THR
HIGH = THR empty
LOW = FIFO above trigger level
HIGH = FIFO below trigger level or FIFO empty
INTA/B Pin YES LOW = a byte in THR
HIGH = transmitter empty
LOW = FIFO above trigger level
HIGH = FIFO below trigger level or transmitter empty
TABLE 4: INTA AND INTB PIN OPERATION FOR RECEIVER
FCR BIT-0 = 0
(FIFO DISABLED)
FCR BIT-0 = 1
(FIFO ENABLED)
INTA/B Pin LOW = no data
HIGH = 1 byte
LOW = FIFO below trigger level
HIGH = FIFO above trigger level
FIGURE 4. TYPICAL OSCILLATOR CONNECTIONS
C1
22-47 pF
C2
22-47 pF
Y1 1.8432 MHz
to
24 MHz
R1
0-120 Ω
(Optional)
R2
500 ΚΩ − 1 ΜΩ
XTAL1 XTAL2
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
10
The on-chip oscillator is designed to use an industry standard microprocessor crystal (parallel resonant,
fundamental frequency with 10-22 pF capacitance load, ESR of 20-120 ohms and 100 ppm frequency
tolerance) connected externally between the XTAL1 and XTAL2 pins (see Figure 4). The programmable Baud
Rate Generator is capable of operating with a crystal oscillator frequency of up to 24 MHz. However, with an
external clock input on XTAL1 pin and a 2K ohms pull-up resistor on XTAL2 pin (as shown in Figure 5) it can
extend its operation up to 50 MHz (6.25 Mbps serial data rate) at 5V with an 8X sampling rate.
For further reading on the oscillator circuit please see the Application Note DAN108 on the EXAR web site at
http://www.exar.com.
2.10 Programmable Baud Rate Generator
Each UART has its own Baud Rate Generator (BRG) with a prescaler. The prescaler is controlled by a
software bit in the MCR register. The MCR register bit-7 sets the prescaler to divide the input crystal or external
clock by 1 or 4. The clock output of the prescaler goes to the BRG. The BRG further divides this clock by a
programmable divisor between 1 and (216 -1) to obtain a 16X sampling rate clock of the serial data rate. The
sampling rate clock is used by the transmitter for data bit shifting and receiver for data sampling. The BRG
divisor defaults to the maximum baud rate (DLL = 0x01 and DLM = 0x00) upon power up.
Programming the Baud Rate Generator Registers DLM and DLL provides the capability of selecting the
operating data rate. Table 5 shows the standard data rates available with a 14.7456 MHz crystal or external
clock at 16X sampling rate clock rate. A 16X sampling clock is typically used. However, user can select the 8X
FIGURE 5. EXTERNAL CLOCK CONNECTION FOR EXTENDED DATA RATE
FIGURE 6. BAUD RATE GENERATOR AND PRESCALER
2K
XTAL1
XTAL2
R1
VCC
External Clock
vcc
gnd
XTAL1
XTAL2
Crystal
Osc/
Buffer
MCR Bit-7=0
(default)
MCR Bit-7=1
DLL and DLM
Registers
Prescaler
Divide by 1
Prescaler
Divide by 4
16X
Sampling
Rate Clock to
Transmitter
Baud Rate
Generator
Logic
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REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
11
sampling clock rate mode (EMSR bit-7=0) to double the operating data rate. When using a non-standard data
rate crystal or external clock, the divisor value can be calculated for DLL/DLM with the following equation.
2.11 Transmitter
The transmitter section comprises of an 8-bit Transmit Shift Register (TSR) and 64 bytes of FIFO which
includes a byte-wide Transmit Holding Register (THR). TSR shifts out every data bit with the 16X/8X internal
clock. A bit time is 16 (8) clock periods (see EMSR bit-7). The transmitter sends the start-bit followed by the
number of data bits, inserts the proper parity-bit if enabled, and adds the stop-bit(s). The status of the FIFO and
TSR are reported in the Line Status Register (LSR bit-5 and bit-6).
2.11.1 Transmit Holding Register (THR) - Write Only
The transmit holding register is an 8-bit register providing a data interface to the host processor. The host
writes transmit data byte to the THR to be converted into a serial data stream including start-bit, data bits,
parity-bit and stop-bit(s). The least-significant-bit (Bit-0) becomes first data bit to go out. The THR is the input
register to the transmit FIFO of 64 bytes when FIFO operation is enabled by FCR bit-0. Every time a write
operation is made to the THR, the FIFO data pointer is automatically bumped to the next sequential data
location.
2.11.2 Transmitter Operation in non-FIFO Mode
The host loads transmit data to THR one character at a time. The THR empty flag (LSR bit-5) is set when the
data byte is transferred to TSR. THR flag can generate a transmit empty interrupt (ISR bit-1) when it is enabled
by IER bit-1. The TSR flag (LSR bit-6) is set when TSR becomes completely empty.
divisor (decimal) = (XTAL1 clock frequency / prescaler) / (serial data rate x 16), with 16XMode [EMSR bit-7] = 1
divisor (decimal) = (XTAL1 clock frequency / prescaler) / (serial data rate x 8), with 16XMode [EMSR bit-7] = 0
TABLE 5: TYPICAL DATA RATES WITH A 14.7456 MHZ CRYSTAL OR EXTERNAL CLOCK
OUTPUT Data Rate
MCR Bit-7=1
OUTPUT Data Rate
MCR Bit-7=0
(DEFAULT)
DIVISOR FOR 16x
Clock (Decimal)
DIVISOR FOR 16x
Clock (HEX)
DLM
PROGRAM
VALUE (HEX)
DLL
PROGRAM
VALUE (HEX)
DATA RATE
ERROR (%)
100 400 2304 900 09 00 0
600 2400 384 180 01 80 0
1200 4800 192 C0 00 C0 0
2400 9600 96 60 00 60 0
4800 19.2k 48 30 00 30 0
9600 38.4k 24 18 00 18 0
19.2k 76.8k 12 0C 00 0C 0
38.4k 153.6k 606 00 06 0
57.6k 230.4k 404 00 04 0
115.2k 460.8k 202 00 02 0
230.4k 921.6k 101 00 01 0
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
12
2.11.3 Transmitter Operation in FIFO Mode
The host may fill the transmit FIFO with up to 64 bytes of transmit data. The THR empty flag (LSR bit-5) is set
whenever the FIFO is empty. The THR empty flag can generate a transmit empty interrupt (ISR bit-1) when the
amount of data in the FIFO falls below its programmed trigger level. The transmit empty interrupt is enabled by
IER bit-1. The TSR flag (LSR bit-6) is set when TSR/FIFO becomes empty.
2.12 Receiver
The receiver section contains an 8-bit Receive Shift Register (RSR) and 64 bytes of FIFO which includes a
byte-wide Receive Holding Register (RHR). The RSR uses the 16X/8X clock (EMSR bit-7) for timing. It verifies
and validates every bit on the incoming character in the middle of each data bit. On the falling edge of a start or
false start bit, an internal receiver counter starts counting at the 16X/8X clock rate. After 8 clocks (or 4 if 8X) the
start bit period should be at the center of the start bit. At this time the start bit is sampled and if it is still a logic
0 it is validated. Evaluating the start bit in this manner prevents the receiver from assembling a false character.
The rest of the data bits and stop bits are sampled and validated in this same manner to prevent false framing.
If there were any error(s), they are reported in the LSR register bits 2-4. Upon unloading the receive data byte
from RHR, the receive FIFO pointer is bumped and the error tags are immediately updated to reflect the status
of the data byte in RHR register. RHR can generate a receive data ready interrupt upon receiving a character
or delay until it reaches the FIFO trigger level. Furthermore, data delivery to the host is guaranteed by a
receive data ready time-out interrupt when data is not received for 4 word lengths as defined by LCR[1,0] plus
12 bits time. This is equivalent to 3.7-4.6 character times. The RHR interrupt is enabled by IER bit-0.
FIGURE 7. TRANSMITTER OPERATION IN NON-FIFO MODE
FIGURE 8. TRANSMITTER OPERATION IN FIFO AND FLOW CONTROL MODE
Transmit
Holding
Register
(THR)
Transmit Shift Register (TSR)
Data
Byte
L
S
B
M
S
B
THR Interrupt (ISR bit-1)
Enabled by IER bit-1
TXNOFIFO1
16X or 8X
Clock
(EMSR Bit-7)
Transmit Data Shift Register
(TSR)
Transmit
Data Byte THR Interrupt (ISR bit-1) falls
below the programmed Trigger
Level and then when becomes
empty. FIFO is Enabled by FCR
bit-0=1
Transmit
FIFO
16X or 8X Clock
(EMSR bit-7)
Auto CTS Flow Control (CTS# pin)
Auto Software Flow Control
Flow Control Characters
(Xoff1/2 and Xon1/2 Reg.
TXFIFO1
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REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
13
2.12.1 Receive Holding Register (RHR) - Read-Only
The Receive Holding Register is an 8-bit register that holds a receive data byte from the Receive Shift
Register. It provides the receive data interface to the host processor. The RHR register is part of the receive
FIFO of 64 bytes by 11-bits wide, the 3 extra bits are for the 3 error tags to be reported in LSR register. When
the FIFO is enabled by FCR bit-0, the RHR contains the first data character received by the FIFO. After the
RHR is read, the next character byte is loaded into the RHR and the errors associated with the current data
byte are immediately updated in the LSR bits 2-4.
NOTE: Table-B selected as Trigger Table for Figure 10 (Table 10).
FIGURE 9. RECEIVER OPERATION IN NON-FIFO MODE
FIGURE 10. RECEIVER OPERATION IN FIFO AND AUTO RTS FLOW CONTROL MODE
Receive Data Shift
Register (RSR)
Receive
Data Byte
and Errors
RHR Interrupt (ISR bit-2)
Receive Data
Holding Register
(RHR)
RXFIFO1
16X or 8X Clock
(EMSR bit-7)
Receive Data Characters
Data Bit
Validation
Error
Tags in
LSR bits
4:2
Receive Data Shift
Register (RSR)
RXFIFO1
16X or 8X Clock
(EMSR bit-7)
Error Tags
(64-sets)
Error Tags in
LSR bits 4:2
64 bytes by 11-bit
wide
FIFO
Receive Data Characters
FIFO
Trigger=16
Example
:
- RX FIFO trigger level selected at 16
bytes
(See Note Below)
Data fills to
24
Data falls to
8
Data Bit
Validation
Receive
Data FIFO
Receive
Data
Receive Data
Byte and Errors
RHR Interrupt (ISR bit-2) programmed for
desired FIFO trigger level.
FIFO is Enabled by FCR bit-0=1
RTS# de-asserts when data fills above the flow
control trigger level to suspend remote transmitter.
Enable by EFR bit-6=1, MCR bit-1.
RTS# re-asserts when data falls below the flow
control trigger level to restart remote transmitter.
Enable by EFR bit-6=1, MCR bit-1.
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
14
2.13 Auto RTS (Hardware) Flow Control
Automatic RTS hardware flow control is used to prevent data overrun to the local receiver FIFO. The RTS#
output is used to request remote unit to suspend/resume data transmission. The auto RTS flow control
features is enabled to fit specific application requirement (see Figure 11):
•Enable auto RTS flow control using EFR bit-6.
•The auto RTS function must be started by asserting RTS# output pin (MCR bit-1 to logic 1 after it is enabled).
If using the Auto RTS interrupt:
•Enable RTS interrupt through IER bit-6 (after setting EFR bit-4). The UART issues an interrupt when the
RTS# pin makes a transition from low to high: ISR bit-5 will be set to logic 1.
2.14 Auto RTS Hysteresis
The 2750 has a new feature that provides flow control trigger hysteresis while maintaining compatibility with
the XR16C850, ST16C650A and ST16C550 family of UARTs. With the Auto RTS function enabled, an interrupt
is generated when the receive FIFO reaches the programmed RX trigger level. The RTS# pin will not be forced
HIGH (RTS off) until the receive FIFO reaches the upper limit of the hysteresis level. The RTS# pin will return
LOW after the RX FIFO is unloaded to the lower limit of the hysteresis level. Under the above described
conditions, the 2750 will continue to accept data until the receive FIFO gets full. The Auto RTS function is
initiated when the RTS# output pin is asserted LOW (RTS On). Tab l e 13 shows the complete details for the
Auto RTS# Hysteresis levels. Please note that this table is for programmable trigger levels only (Table D). The
hysteresis values for Tables A-C are the next higher and next lower trigger levels in the corresponding table.
2.15 Auto CTS Flow Control
Automatic CTS flow control is used to prevent data overrun to the remote receiver FIFO. The CTS# input is
monitored to suspend/restart the local transmitter. The auto CTS flow control feature is selected to fit specific
application requirement (see Figure 11):
•Enable auto CTS flow control using EFR bit-7.
If using the Auto CTS interrupt:
•Enable CTS interrupt through IER bit-7 (after setting EFR bit-4). The UART issues an interrupt when the
CTS# pin is de-asserted (HIGH): ISR bit-5 will be set to 1, and UART will suspend transmission as soon as
the stop bit of the character in process is shifted out. Transmission is resumed after the CTS# input is re-
asserted (LOW), indicating more data may be sent.
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REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
15
FIGURE 11. AUTO RTS AND CTS FLOW CONTROL OPERATION
The local UART (UARTA) starts data transfer by asserting RTSA# (1). RTSA# is normally connected to CTSB# (2) of
remote UART (UARTB). CTSB# allows its transmitter to send data (3). TXB data arrives and fills UARTA receive FIFO
(4). When RXA data fills up to its receive FIFO trigger level, UARTA activates its RXA data ready interrupt (5) and con-
tinues to receive and put data into its FIFO. If interrupt service latency is long and data is not being unloaded, UARTA
monitors its receive data fill level to match the upper threshold of RTS delay and de-assert RTSA# (6). CTSB# follows
(7) and request UARTB transmitter to suspend data transfer. UARTB stops or finishes sending the data bits in its trans-
mit shift register (8). When receive FIFO data in UARTA is unloaded to match the lower threshold of RTS delay (9),
UARTA re-asserts RTSA# (10), CTSB# recognizes the change (11) and restarts its transmitter and data flow again until
next receive FIFO trigger (12). This same event applies to the reverse direction when UARTA sends data to UARTB
with RTSB# and CTSA# controlling the data flow.
RTSA# CTSB#
RXA TXB Transmitter
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
Auto CTS
Monitor
RTSA#
TXB
RXA FIFO
CTSB#
Remote UART
UARTB
Local UART
UARTA
ON OFF ON
Suspend Restart
RTS High
Threshold
Data Starts
ON OFF ON
Assert RTS# to Begin
Transmission
1
2
3
4
5
6
7
Receive
Data RTS Low
Threshold
9
10
11
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
Transmitter
Auto CTS
Monitor
RTSB#CTSA#
RXBTXA
INTA
(RXA FIFO
Interrupt)
RX FIFO
Trigger Level
RX FIFO
Trigger Level
8
12
RTSCTS1
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
16
2.16 Auto Xon/Xoff (Software) Flow Control
When software flow control is enabled (See Table 15), the 2750 compares one or two sequential receive data
characters with the programmed Xon or Xoff-1,2 character value(s). If receive character(s) (RX) match the
programmed values, the 2750 will halt transmission (TX) as soon as the current character has completed
transmission. When a match occurs, the Xoff (if enabled via IER bit-5) flag will be set and the interrupt output
pin will be activated. Following a suspension due to a match of the Xoff character, the 2750 will monitor the
receive data stream for a match to the Xon-1,2 character. If a match is found, the 2750 will resume operation
and clear the flags (ISR bit-4).
Reset initially sets the contents of the Xon/Xoff 8-bit flow control registers to a logic 0. Following reset the user
can write any Xon/Xoff value desired for software flow control. Different conditions can be set to detect Xon/
Xoff characters (See Table 15) and suspend/resume transmissions. When double 8-bit Xon/Xoff characters are
selected, the 2750 compares two consecutive receive characters with two software flow control 8-bit values
(Xon1, Xon2, Xoff1, Xoff2) and controls TX transmissions accordingly. Under the above described flow control
mechanisms, flow control characters are not placed (stacked) in the user accessible RX data buffer or FIFO.
In the event that the receive buffer is overfilling and flow control needs to be executed, the 2750 automatically
sends an Xoff message (when enabled) via the serial TX output to the remote modem. The 2750 sends the
Xoff-1,2 characters two-character-times (= time taken to send two characters at the programmed baud rate)
after the receive FIFO crosses the programmed trigger level (for all trigger tables A-D). To clear this condition,
the 2750 will transmit the programmed Xon-1,2 characters as soon as receive FIFO is less than one trigger
level below the programmed trigger level (for Trigger Tables A, B, and C) or when receive FIFO is less than the
trigger level minus the hysteresis value (for Trigger Table D). This hysteresis value is the same as the Auto
RTS Hysteresis value in Tab l e 14. Tab l e 6 below explains this when Trigger Table-B (See Tab l e 10) is selected.
* After the trigger level is reached, an xoff character is sent after a short span of time (= time required to send 2 characters);
for example, after 2.083ms has elapsed for 9600 baud and 10-bit word length setting.
2.17 Special Character Detect
A special character detect feature is provided to detect an 8-bit character when bit-5 is set in the Enhanced
Feature Register (EFR). When this character (Xoff2) is detected, it will be placed in the FIFO along with normal
incoming RX data.
The 2750 compares each incoming receive character with Xoff-2 data. If a match exists, the received data will
be transferred to FIFO and ISR bit-4 will be set to indicate detection of special character. Although the Internal
Register Table shows Xon, Xoff Registers with eight bits of character information, the actual number of bits is
dependent on the programmed word length. Line Control Register (LCR) bits 0-1 defines the number of
character bits, i.e., either 5 bits, 6 bits, 7 bits, or 8 bits. The word length selected by LCR bits 0-1 also
determines the number of bits that will be used for the special character comparison. Bit-0 in the Xon, Xoff
Registers corresponds with the LSB bit for the receive character.
2.18 Auto RS485 Half-duplex Control
The auto RS485 half-duplex direction control changes the behavior of the transmitter when enabled by FCTR
bit-3. By default, it de-asserts RTS# (HIGH) output following the last stop bit of the last character that has been
transmitted. This helps in turning around the transceiver to receive the remote station’s response. When the
host is ready to transmit next polling data packet again, it only has to load data bytes to the transmit FIFO. The
TABLE 6: AUTO XON/XOFF (SOFTWARE) FLOW CONTROL
RX TRIGGER LEVEL INT PIN ACTIVATION XOFF CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
XON CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
8 8 8* 0
16 16 16* 8
24 24 24* 16
28 28 28* 24
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REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
17
transmitter automatically re-asserts RTS# (LOW) output prior to sending the data. The RS485 half-duplex
direction control output can be inverted by enabling EMSR bit-3.
2.19 Infrared Mode
The 2750 UART includes the infrared encoder and decoder compatible to the IrDA (Infrared Data Association)
version 1.0. The IrDA 1.0 standard that stipulates the infrared encoder sends out a 3/16 of a bit wide HIGH-
pulse for each “0” bit in the transmit data stream. This signal encoding reduces the on-time of the infrared LED,
hence reduces the power consumption. See Figure 12 below.
The infrared encoder and decoder are enabled by setting MCR register bit-6 to a ‘1’. When the infrared feature
is enabled, the transmit data output, TX, idles at logic zero level. Likewise, the RX input assumes an idle level
of logic zero from a reset and power up, see Figure 12.
Typically, the wireless infrared decoder receives the input pulse from the infrared sensing diode on the RX pin.
Each time it senses a light pulse, it returns a logic 1 to the data bit stream. However, this is not true with some
infrared modules on the market which indicate a logic 0 by a light pulse. So the 2750 has a provision to invert
the input polarity to accommodate this. In this case user can enable FCTR bit-2 to invert the input signal.
FIGURE 12. INFRARED TRANSMIT DATA ENCODING AND RECEIVE DATA DECODING
Character
Data Bits
Start
Stop
0000 0
11 111
Bit Time
1/16 Clock Delay
IRdecoder
-
RX Data
Receive
IR Pulse
(RX pin)
Character
Data Bits
Start
Stop
0000 0
11 111
TX Data
Transmit
IR Pulse
(TX Pin)
Bit Time 1/2 Bit Time
3/16 Bit Time
IrEncoder-1
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
18
2.20 Sleep Mode with Auto Wake-Up
The 2750 supports low voltage system designs, hence, a sleep mode is included to reduce its power
consumption when the chip is not actively used.
All of these conditions must be satisfied for the 2750 to enter sleep mode:
■no interrupts pending for both channels of the 2750 (ISR bit-0 = 1)
■sleep mode of both channels are enabled (IER bit-4 = 1)
■modem inputs are not toggling (MSR bits 0-3 = 0)
■RX input pins are idling HIGH
The 2750 stops its crystal oscillator to conserve power in the sleep mode. User can check the XTAL2 pin for
no clock output as an indication that the device has entered the sleep mode.
The 2750 resumes normal operation by any of the following:
■a receive data start bit transition (HIGH to LOW)
■a data byte is loaded to the transmitter, THR or FIFO
■a change of logic state on any of the modem or general purpose serial inputs: CTS#, DSR#, CD#, RI#
If the 2750 is awakened by any one of the above conditions, it will return to the sleep mode automatically after
all interrupting conditions have been serviced and cleared. If the 2750 is awakened by the modem inputs, a
read to the MSR is required to reset the modem inputs. In any case, the sleep mode will not be entered while
an interrupt is pending from channel A or B. The 2750 will stay in the sleep mode of operation until it is disabled
by setting IER bit-4 to a logic 0.
If the address lines, data bus lines, IOW#, IOR#, CSA#, CSB#, and modem input lines remain steady when the
2750 is in sleep mode, the maximum current will be in the microamp range as specified in the DC Electrical
Characteristics on page 37. If the input lines are floating or are toggling while the 2750 is in sleep mode, the
current can be up to 100 times more. If any of those signals are toggling or floating, then an external buffer
would be required to keep the address, data and control lines steady to achieve the low current. As an
alternative, please refer to the XR16L2751 which is pin-to-pin and software compatible with the 2750 but with
(some additional pins and) the PowerSave feature that eliminates any unnecessary external buffer.
A word of caution: owing to the starting up delay of the crystal oscillator after waking up from sleep mode, the
first few receive characters may be lost. The number of characters lost during the restart also depends on your
operating data rate. More characters are lost when operating at higher data rate. Also, it is important to keep
RX A/B inputs idling HIGH or “marking” condition during sleep mode to avoid receiving a “break” condition
upon the restart. This may occur when the external interface transceivers (RS-232, RS-485 or another type)
are also put to sleep mode and cannot maintain the “marking” condition. To avoid this, the designer can use a
47k-100k ohm pull-up resistor on the RXA and RXB pins.
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REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
19
2.21 Internal Loopback
The 2750 UART provides an internal loopback capability for system diagnostic purposes. The internal
loopback mode is enabled by setting MCR register bit-4 to logic 1. All regular UART functions operate normally.
Figure 13 shows how the modem port signals are re-configured. Transmit data from the transmit shift register
output is internally routed to the receive shift register input allowing the system to receive the same data that it
was sending. The TX, RTS# and DTR# pins are held while the CTS#, DSR# CD# and RI# inputs are ignored.
Caution: the RX input pin must be held HIGH during loopback test else upon exiting the loopback test the
UART may detect and report a false “break” signal. Also, Auto RTS/CTS flow control is not supported during
internal loopback.
FIGURE 13. INTERNAL LOOP BACK IN CHANNEL A AND B
TXA/TXB
RXA/RXB
Modem / General Purpose Control Logic
Internal Data Bus Lines and Control Signals
RTSA#/RTSB#
MCR bit-4=1
VCC
VCC
Transmit Shift Register
(THR/FIFO)
Receive Shift Register
(RHR/FIFO)
CTSA#/CTSB#
DTRA#/DTRB#
DSRA#/DSRB#
RIA#/RIB#
CDA#/CDB#
OP1#
OP2#
RTS#
CTS#
DTR#
DSR#
RI#
CD#
VCC
VCC
OP2A#/OP2B#
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
20
3.0 UART INTERNAL REGISTERS
Each of the UART channel in the 2750 has its own set of configuration registers selected by address lines A0,
A1 and A2 with CSA# or CSB# selecting the channel. The complete register set is shown on Table 7 and
Table 8.
TABLE 7: UART CHANNEL A AND B UART INTERNAL REGISTERS
ADDRESSES
A2 A1 A0 REGISTER READ/WRITE COMMENTS
16C550 COMPATIBLE REGISTERS
0 0 0 RHR - Receive Holding Register
THR - Transmit Holding Register
Read-only
Write-only LCR[7] = 0
0 0 0 DLL - Div Latch Low Byte Read/Write
LCR[7] = 1, LCR ≠ 0xBF
0 0 1 DLM - Div Latch High Byte Read/Write
0 0 0 DREV - Device Revision Code Read-only DLL, DLM = 0x00,
LCR[7] = 1, LCR ≠ 0xBF
0 0 1 DVID - Device Identification Code Read-only
0 0 1 IER - Interrupt Enable Register Read/Write LCR[7] = 0
0 1 0 ISR - Interrupt Status Register
FCR - FIFO Control Register
Read-only
Write-only LCR ≠ 0xBF
0 1 1 LCR - Line Control Register Read/Write
1 0 0 MCR - Modem Control Register Read/Write
LCR ≠ 0xBF
1 0 1 LSR - Line Status Register
Reserved
Read-only
Write-only
1 1 0 MSR - Modem Status Register
Reserved
Read-only
Write-only
1 1 1 SPR - Scratch Pad Register Read/Write LCR ≠ 0xBF, FCTR[6] = 0
1 1 1 FLVL - RX/TX FIFO Level Counter Register Read-only
LCR ≠ 0xBF, FCTR[6] = 1
1 1 1 EMSR - Enhanced Mode Select Register Write-only
ENHANCED REGISTERS
0 0 0 TRG - RX/TX FIFO Trigger Level Register
FC - RX/TX FIFO Level Counter Register
Write-only
Read-only
LCR = 0xBF
0 0 1 FCTR - Feature Control Register Read/Write
0 1 0 EFR - Enhanced Function Register Read/Write
1 0 0 Xon-1 - Xon Character 1 Read/Write
1 0 1 Xon-2 - Xon Character 2 Read/Write
1 1 0 Xoff-1 - Xoff Character 1 Read/Write
1 1 1 Xoff-2 - Xoff Character 2 Read/Write
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
21
.
TABLE 8: INTERNAL REGISTERS DESCRIPTION. SHADED BITS ARE ENABLED WHEN EFR BIT-4=1
ADDRESS
A2-A0
REG
NAME
READ/
WRITE BIT-7 BIT-6 BIT-5 BIT-4 BIT-3 BIT-2 BIT-1 BIT-0 COMMENT
16C550 Compatible Registers
0 0 0 RHR RD Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
LCR[7]=0
0 0 0 THR WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
0 0 1 IER RD/WR 0/ 0/ 0/ 0/ Modem
Stat. Int.
Enable
RX Line
Stat.
Int.
Enable
TX
Empty
Int
Enable
RX
Data
Int.
Enable
CTS Int.
Enable
RTS Int.
Enable
Xoff Int.
Enable
Sleep
Mode
Enable
0 1 0 ISR RD FIFOs
Enabled
FIFOs
Enabled
0/ 0/ INT
Source
Bit-3
INT
Source
Bit-2
INT
Source
Bit-1
INT
Source
Bit-0
LCR ≠ 0xBF
INT
Source
Bit-5
INT
Source
Bit-4
0 1 0 FCR WR RX FIFO
Trigger
RX FIFO
Trigger
0/ 0/ DMA
Mode
Enable
TX
FIFO
Reset
RX
FIFO
Reset
FIFOs
Enable
TX FIFO
Trigger
TX FIFO
Trigger
0 1 1 LCR RD/WR Divisor
Enable
Set TX
Break
Set Par-
ity
Even
Parity
Parity
Enable
Stop
Bits
Word
Length
Bit-1
Word
Length
Bit-0
1 0 0 MCR RD/WR 0/ 0/ 0/ Internal
Lopback
Enable
OP2#/INT
Output
Enable
Rsrvd
(OP1#)
RTS#
Output
Control
DTR#
Output
Control
LCR ≠ 0xBF
BRG
Pres-
caler
IR Mode
ENable
XonAny
1 0 1 LSR RD RX FIFO
Global
Error
THR &
TSR
Empty
THR
Empty
RX
Break
RX Fram-
ing Error
RX
Parity
Error
RX
Over-
run
Error
RX
Data
Ready
1 1 0 MSR RD CD#
Input
RI#
Input
DSR#
Input
CTS#
Input
Delta
CD#
Delta
RI#
Delta
DSR#
Delta
CTS#
1 1 1 SPR RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0 LCR ≠ 0xBF
FCTR[6]=0
1 1 1 EMSR WR 16X
Sam-
pling
Rate
Mode
LSR
Error
Inter-
rupt.
Imd/Dly#
Auto
RTS
Hyst.
bit-3
Auto
RTS
Hyst.
bit-2
Auto
RS485
Output
Inversion
Rsrvd Rx/Tx
FIFO
Count
Rx/Tx
FIFO
Count
LCR ≠ 0xBF
FCTR[6]=1
1 1 1 FLVL RD Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
22
4.0 INTERNAL REGISTER DESCRIPTIONS
4.1 Receive Holding Register (RHR) - Read- Only
See “Receiver” on page 12.
4.2 Transmit Holding Register (THR) - Write-Only
See “Transmitter” on page 11.
4.3 Interrupt Enable Register (IER) - Read/Write
The Interrupt Enable Register (IER) masks the interrupts from receive data ready, transmit empty, line status
and modem status registers. These interrupts are reported in the Interrupt Status Register (ISR).
Baud Rate Generator Divisor
0 0 0 DLL RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0 LCR[7]=1
LCR ≠ 0xBF
0 0 1 DLM RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
0 0 0 DREV RD Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0 LCR[7]=1
LCR ≠ 0xBF
DLL=0x00
DLM=0x00
0 0 1 DVID RD 0 0 0 0 1 0 1 0
Enhanced Registers
0 0 0 TRG WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
LCR=0XBF
0 0 0 FC RD Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
0 0 1 FCTR RD/WR RX/TX
Mode
SCPAD
Swap
Trig
Tab le
Bit-1
Trig
Tab le
Bit-0
Auto
RS485
Direction
Control
RX IR
Input
Inv.
Auto
RTS
Hyst
Bit-1
Auto
RTS
Hyst
Bit-0
0 1 0 EFR RD/WR Auto
CTS
Enable
Auto
RTS
Enable
Special
Char
Select
Enable
IER [7:4],
ISR [5:4],
FCR[5:4],
MCR[7:5]
Soft-
ware
Flow
Cntl
Bit-3
Soft-
ware
Flow
Cntl
Bit-2
Soft-
ware
Flow
Cntl
Bit-1
Soft-
ware
Flow
Cntl
Bit-0
1 0 0 XON1 RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
1 0 1 XON2 RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
1 1 0 XOFF1 RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
1 1 1 XOFF2 RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0
TABLE 8: INTERNAL REGISTERS DESCRIPTION. SHADED BITS ARE ENABLED WHEN EFR BIT-4=1
ADDRESS
A2-A0
REG
NAME
READ/
WRITE BIT-7 BIT-6 BIT-5 BIT-4 BIT-3 BIT-2 BIT-1 BIT-0 COMMENT
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
23
4.3.1 IER versus Receive FIFO Interrupt Mode Operation
When the receive FIFO (FCR BIT-0 = 1) and receive interrupts (IER BIT-0 = 1) are enabled, the RHR interrupts
(see ISR bits 2 and 3) status will reflect the following:
A. The receive data available interrupts are issued to the host when the FIFO has reached the programmed
trigger level. It will be cleared when the FIFO drops below the programmed trigger level.
B. FIFO level will be reflected in the ISR register when the FIFO trigger level is reached. Both the ISR register
status bit and the interrupt will be cleared when the FIFO drops below the trigger level.
C. The receive data ready bit (LSR BIT-0) is set as soon as a character is transferred from the shift register to
the receive FIFO. It is reset when the FIFO is empty.
4.3.2 IER versus Receive/Transmit FIFO Polled Mode Operation
When FCR BIT-0 equals a logic 1 for FIFO enable; resetting IER bits 0-3 enables the XR16L2750 in the FIFO
polled mode of operation. Since the receiver and transmitter have separate bits in the LSR either or both can
be used in the polled mode by selecting respective transmit or receive control bit(s).
A. LSR BIT-0 indicates there is data in RHR or RX FIFO.
B. LSR BIT-1 indicates an overrun error has occurred and that data in the FIFO may not be valid.
C. LSR BIT 2-4 provides the type of receive data errors encountered for the data byte in RHR, if any.
D. LSR BIT-5 indicates THR is empty.
E. LSR BIT-6 indicates when both the transmit FIFO and TSR are empty.
F. LSR BIT-7 indicates a data error in at least one character in the RX FIFO.
IER[0]: RHR Interrupt Enable
The receive data ready interrupt will be issued when RHR has a data character in the non-FIFO mode or when
the receive FIFO has reached the programmed trigger level in the FIFO mode.
•Logic 0 = Disable the receive data ready interrupt (default).
•Logic 1 = Enable the receiver data ready interrupt.
IER[1]: THR Interrupt Enable
This bit enables the Transmit Ready interrupt which is issued whenever the THR becomes empty in the non-
FIFO mode or when data in the FIFO falls below the programmed trigger level in the FIFO mode. If the THR is
empty when this bit is enabled, an interrupt will be generated.
•Logic 0 = Disable Transmit Ready interrupt (default).
•Logic 1 = Enable Transmit Ready interrupt.
IER[2]: Receive Line Status Interrupt Enable
If any of the LSR register bits 1, 2, 3 or 4 is a logic 1, it will generate an interrupt to inform the host controller
about the error status of the current data byte in FIFO. LSR bit-1 generates an interrupt immediately when the
character has been received. LSR bits 2-4 generate an interrupt when the character with errors is read out of
the FIFO (default). Instead, LSR bits 2-4 can be programmed to generate an interrupt immediately, by setting
EMSR bit-6 to a logic 1.
•Logic 0 = Disable the receiver line status interrupt (default).
•Logic 1 = Enable the receiver line status interrupt.
IER[3]: Modem Status Interrupt Enable
•Logic 0 = Disable the modem status register interrupt (default).
•Logic 1 = Enable the modem status register interrupt.
IER[4]: Sleep Mode Enable (requires EFR bit-4 = 1)
•Logic 0 = Disable Sleep Mode (default).
•Logic 1 = Enable Sleep Mode. See Sleep Mode section for further details.
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
24
IER[5]: Xoff Interrupt Enable (requires EFR bit-4=1)
•Logic 0 = Disable the software flow control, receive Xoff interrupt. (default)
•Logic 1 = Enable the software flow control, receive Xoff interrupt. See Software Flow Control section for
details.
IER[6]: RTS# Output Interrupt Enable (requires EFR bit-4=1)
•Logic 0 = Disable the RTS# interrupt (default).
•Logic 1 = Enable the RTS# interrupt. The UART issues an interrupt when the RTS# pin makes a transition
from low to high.
IER[7]: CTS# Input Interrupt Enable (requires EFR bit-4=1)
•Logic 0 = Disable the CTS# interrupt (default).
•Logic 1 = Enable the CTS# interrupt. The UART issues an interrupt when CTS# pin makes a transition from
low to high.
4.4 Interrupt Status Register (ISR) - Read-Only
The UART provides multiple levels of prioritized interrupts to minimize external software interaction. The
Interrupt Status Register (ISR) provides the user with six interrupt status bits. Performing a read cycle on the
ISR will give the user the current highest pending interrupt level to be serviced, others are queued up to be
serviced next. No other interrupts are acknowledged until the pending interrupt is serviced. The Interrupt
Source Table, Tabl e 9, shows the data values (bit 0-5) for the interrupt priority levels and the interrupt sources
associated with each of these interrupt levels.
4.4.1 Interrupt Generation:
•LSR is by any of the LSR bits 1, 2, 3 and 4.
•RXRDY is by RX trigger level.
•RXRDY Time-out is by a 4-char plus 12 bits delay timer.
•TXRDY is by TX trigger level or TX FIFO empty (or transmitter empty in auto RS-485 control).
•MSR is by any of the MSR bits 0, 1, 2 and 3.
•Receive Xoff/Special character is by detection of a Xoff or Special character.
•CTS# is when its transmitter toggles the input pin (from LOW to HIGH) during auto CTS flow control.
•RTS# is when its receiver toggles the output pin (from LOW to HIGH) during auto RTS flow control.
4.4.2 Interrupt Clearing:
•LSR interrupt is cleared by a read to the LSR register.
•RXRDY interrupt is cleared by reading data until FIFO falls below the trigger level.
•RXRDY Time-out interrupt is cleared by reading RHR.
•TXRDY interrupt is cleared by a read to the ISR register or writing to THR.
•MSR interrupt is cleared by a read to the MSR register.
•Xoff interrupt is cleared by a read to ISR or when Xon character(s) is received.
•Special character interrupt is cleared by a read to ISR or after the next character is received.
•RTS# and CTS# flow control interrupts are cleared by a read to the MSR register.
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
25
]
ISR[0]: Interrupt Status
•Logic 0 = An interrupt is pending and the ISR contents may be used as a pointer to the appropriate interrupt
service routine.
•Logic 1 = No interrupt pending (default condition).
ISR[3:1]: Interrupt Status
These bits indicate the source for a pending interrupt at interrupt priority levels (See Interrupt Source Table 9).
ISR[5:4]: Interrupt Status
These bits are enabled when EFR bit-4 is set to a logic 1. ISR bit-4 indicates that the receiver detected a data
match of the Xoff character(s). Note that once set to a logic 1, the ISR bit-4 will stay a logic 1 until a Xon
character is received. ISR bit-5 indicates that CTS# or RTS# has changed state.
ISR[7:6]: FIFO Enable Status
These bits are set to a logic 0 when the FIFOs are disabled. They are set to a logic 1 when the FIFOs are
enabled.
4.5 FIFO Control Register (FCR) - Write-Only
This register is used to enable the FIFOs, clear the FIFOs, set the transmit/receive FIFO trigger levels, and
select the DMA mode. The DMA, and FIFO modes are defined as follows:
FCR[0]: TX and RX FIFO Enable
•Logic 0 = Disable the transmit and receive FIFO (default).
•Logic 1 = Enable the transmit and receive FIFOs. This bit must be set to logic 1 when other FCR bits are
written or they will not be programmed.
FCR[1]: RX FIFO Reset
This bit is only active when FCR bit-0 is a ‘1’.
•Logic 0 = No receive FIFO reset (default)
•Logic 1 = Reset the receive FIFO pointers and FIFO level counter logic (the receive shift register is not
cleared or altered). This bit will return to a logic 0 after resetting the FIFO.
TABLE 9: INTERRUPT SOURCE AND PRIORITY LEVEL
PRIORITY ISR REGISTER STATUS BITS SOURCE OF INTERRUPT
LEVEL BIT-5 BIT-4 BIT-3 BIT-2 BIT-1 BIT-0
1 0 0 0 1 1 0 LSR (Receiver Line Status Register)
2 0 0 1 1 0 0 RXRDY (Receive Data Time-out)
3 0 0 0 1 0 0 RXRDY (Received Data Ready)
4 0 0 0 0 1 0 TXRDY (Transmit Ready)
5 0 0 0 0 0 0 MSR (Modem Status Register)
6 0 1 0 0 0 0 RXRDY (Received Xoff or Special character)
7 1 0 0 0 0 0 CTS#, RTS# change of state
- 0 0 0 0 0 1 None (default)
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
26
FCR[2]: TX FIFO Reset
This bit is only active when FCR bit-0 is a ‘1’.
•Logic 0 = No transmit FIFO reset (default).
•Logic 1 = Reset the transmit FIFO pointers and FIFO level counter logic (the transmit shift register is not
cleared or altered). This bit will return to a logic 0 after resetting the FIFO.
FCR[3]: DMA Mode Select
Controls the behavior of the TXRDY# and RXRDY# pins. See DMA operation section for details.
•Logic 0 = Normal Operation (default).
•Logic 1 = DMA Mode.
FCR[5:4]: Transmit FIFO Trigger Select
(logic 0 = default, TX trigger level = 1)
These 2 bits set the trigger level for the transmit FIFO. The UART will issue a transmit interrupt when the
number of characters in the FIFO falls below the selected trigger level, or when it gets empty in case that the
FIFO did not get filled over the trigger level on last re-load. Tabl e 10 below shows the selections. EFR bit-4
must be set to ‘1’ before these bits can be accessed. Note that the receiver and the transmitter cannot use
different trigger tables. Whichever selection is made last applies to both the RX and TX side.
FCR[7:6]: Receive FIFO Trigger Select
(logic 0 = default, RX trigger level =1)
The FCTR Bits 5-4 are associated with these 2 bits. These 2 bits are used to set the trigger level for the receive
FIFO. The UART will issue a receive interrupt when the number of the characters in the FIFO crosses the
trigger level. Table 10 shows the complete selections. Note that the receiver and the transmitter cannot use
different trigger tables. Whichever selection is made last applies to both the RX and TX side.
TABLE 10: TRANSMIT AND RECEIVE FIFO TRIGGER TABLE AND LEVEL SELECTION
TRIGGER
TABLE
FCTR
BIT-5
FCTR
BIT-4
FCR
BIT-7
FCR
BIT-6
FCR
BIT-5
FCR
BIT-4
RECEIVE
TRIGGER LEVEL
TRANSMIT
TRIGGER
LEVEL
COMPATIBILITY
Table-A 0 0
0
0
1
1
0
1
0
1
0 0
1 (default)
4
8
14
1 (default) 16C550, 16C2550,
16C2552, 16C554,
16C580
Table-B 0 1
0
0
1
1
0
1
0
1
0
0
1
1
0
1
0
1
8
16
24
28
16
8
24
30
16C650A
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
27
4.6 Line Control Register (LCR) - Read/Write
The Line Control Register is used to specify the asynchronous data communication format. The word or
character length, the number of stop bits, and the parity are selected by writing the appropriate bits in this
register.
LCR[1:0]: TX and RX Word Length Select
These two bits specify the word length to be transmitted or received.
LCR[2]: TX and RX Stop-bit Length Select
The length of stop bit is specified by this bit in conjunction with the programmed word length.
LCR[3]: TX and RX Parity Select
Parity or no parity can be selected via this bit. The parity bit is a simple way used in communications for data
integrity check. See Tabl e 11 for parity selection summary below.
•Logic 0 = No parity.
•Logic 1 = A parity bit is generated during the transmission while the receiver checks for parity error of the
data character received.
Table-C 1 0
0
0
1
1
0
1
0
1
0
0
1
1
0
1
0
1
8
16
56
60
8
16
32
56
16C654
Table-D 1 1 X X X X Programmable
via TRG
register.
FCTR[7] = 0.
Programmable
via TRG
register.
FCTR[7] = 1.
16L2752, 16C2850,
16C2852, 16C850,
16C854, 16C864
BIT-1 BIT-0 WORD LENGTH
0 0 5 (default)
0 1 6
1 0 7
1 1 8
BIT-2 WORD LENGTH STOP BIT LENGTH
(BIT TIME(S))
05,6,7,8 1 (default)
1 5 1-1/2
16,7,8 2
TABLE 10: TRANSMIT AND RECEIVE FIFO TRIGGER TABLE AND LEVEL SELECTION
TRIGGER
TABLE
FCTR
BIT-5
FCTR
BIT-4
FCR
BIT-7
FCR
BIT-6
FCR
BIT-5
FCR
BIT-4
RECEIVE
TRIGGER LEVEL
TRANSMIT
TRIGGER
LEVEL
COMPATIBILITY
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
28
LCR[4]: TX and RX Parity Select
If the parity bit is enabled with LCR bit-3 set to a logic 1, LCR BIT-4 selects the even or odd parity format.
•Logic 0 = ODD Parity is generated by forcing an odd number of logic 1’s in the transmitted character. The
receiver must be programmed to check the same format (default).
•Logic 1 = EVEN Parity is generated by forcing an even number of logic 1’s in the transmitted character. The
receiver must be programmed to check the same format.
LCR[5]: TX and RX Parity Select
If the parity bit is enabled, LCR BIT-5 selects the forced parity format.
•LCR BIT-5 = logic 0, parity is not forced (default).
•LCR BIT-5 = logic 1 and LCR BIT-4 = logic 0, parity bit is forced to a logical 1 for the transmit and receive
data.
•LCR BIT-5 = logic 1 and LCR BIT-4 = logic 1, parity bit is forced to a logical 0 for the transmit and receive
data.
LCR[6]: Transmit Break Enable
When enabled, the Break control bit causes a break condition to be transmitted (the TX output is forced to a
“space", LOW state). This condition remains, until disabled by setting LCR bit-6 to a logic 0.
•Logic 0 = No TX break condition. (default)
•Logic 1 = Forces the transmitter output (TX) to a “space”, LOW, for alerting the remote receiver of a line
break condition.
LCR[7]: Baud Rate Divisors Enable
Baud rate generator divisor (DLL/DLM) enable.
•Logic 0 = Data registers are selected. (default)
•Logic 1 = Divisor latch registers are selected.
4.7 Modem Control Register (MCR) or General Purpose Outputs Control - Read/Write
The MCR register is used for controlling the serial/modem interface signals or general purpose inputs/outputs.
MCR[0]: DTR# Output
The DTR# pin is a modem control output. If the modem interface is not used, this output may be used as a
general purpose output.
•Logic 0 = Force DTR# output HIGH (default).
•Logic 1 = Force DTR# output LOW.
TABLE 11: PARITY SELECTION
LCR BIT-5 LCR BIT-4 LCR BIT-3 PARITY SELECTION
X X 0 No parity
0 0 1 Odd parity
0 1 1 Even parity
1 0 1 Force parity to mark, “1”
1 1 1 Forced parity to space, “0”
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
29
MCR[1]: RTS# Output
The RTS# pin is a modem control output and may be used for automatic hardware flow control by enabled by
EFR bit-6. If the modem interface is not used, this output may be used as a general purpose output.
•Logic 0 = Force RTS# HIGH (default).
•Logic 1 = Force RTS# LOW.
MCR[2]: Reserved
OP1# is not available as an output pin on the 2750. But it is available for use during Internal Loopback Mode.
In the Loopback Mode, this bit is used to write the state of the modem RI# interface signal.
MCR[3]: OP2# Output / INT Output Enable
This bit enables or disables the operation of INT, interrupt output. If INT output is not used, OP2# can be used
as a general purpose output.
•Logic 0 = INT (A-B) outputs disabled (three state mode) and OP2# output set HIGH(default).
•Logic 1 = INT (A-B) outputs enabled (active mode) and OP2# output set LOW.
MCR[4]: Internal Loopback Enable
•Logic 0 = Disable loopback mode (default).
•Logic 1 = Enable local loopback mode, see loopback section and Figure 13.
MCR[5]: Xon-Any Enable
•Logic 0 = Disable Xon-Any function (for 16C550 compatibility, default).
•Logic 1 = Enable Xon-Any function. In this mode, any RX character received will resume transmit operation.
The RX character will be loaded into the RX FIFO, unless the RX character is an Xon or Xoff character and
the 2750 is programmed to use the Xon/Xoff flow control.
MCR[6]: Infrared Encoder/Decoder Enable
•Logic 0 = Enable the standard modem receive and transmit input/output interface. (Default)
•Logic 1 = Enable infrared IrDA receive and transmit inputs/outputs. The TX/RX output/input are routed to the
infrared encoder/decoder. The data input and output levels conform to the IrDA infrared interface
requirement. While in this mode, the infrared TX output will be a logic 0 during idle data conditions.
MCR[7]: Clock Prescaler Select
•Logic 0 = Divide by one. The input clock from the crystal or external clock is fed directly to the Programmable
Baud Rate Generator without further modification, i.e., divide by one (default).
•Logic 1 = Divide by four. The prescaler divides the input clock from the crystal or external clock by four and
feeds it to the Programmable Baud Rate Generator, hence, data rates become one forth.
4.8 Line Status Register (LSR) - Read Only
This register provides the status of data transfers between the UART and the host.
LSR[0]: Receive Data Ready Indicator
•Logic 0 = No data in receive holding register or FIFO (default).
•Logic 1 = Data has been received and is saved in the receive holding register or FIFO.
LSR[1]: Receiver Overrun Error Flag
•Logic 0 = No overrun error (default).
•Logic 1 = Overrun error. A data overrun error condition occurred in the receive shift register. This happens
when additional data arrives while the FIFO is full. In this case the previous data in the receive shift register
is overwritten. Note that under this condition the data byte in the receive shift register is not transferred into
the FIFO, therefore the data in the FIFO is not corrupted by the error.
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
30
LSR[2]: Receive Data Parity Error Tag
•Logic 0 = No parity error (default).
•Logic 1 = Parity error. The receive character in RHR does not have correct parity information and is suspect.
This error is associated with the character available for reading in RHR.
LSR[3]: Receive Data Framing Error Tag
•Logic 0 = No framing error (default).
•Logic 1 = Framing error. The receive character did not have a valid stop bit(s). This error is associated with
the character available for reading in RHR.
LSR[4]: Receive Break Error Tag
•Logic 0 = No break condition (default).
•Logic 1 = The receiver received a break signal (RX was a logic 0 for at least one character frame time). In the
FIFO mode, only one break character is loaded into the FIFO.
LSR[5]: Transmit Holding Register Empty Flag
This bit is the Transmit Holding Register Empty indicator. The THR bit is set to a logic 1 when the last data byte
is transferred from the transmit holding register to the transmit shift register. The bit is reset to logic 0
concurrently with the data loading to the transmit holding register by the host. In the FIFO mode this bit is set
when the transmit FIFO is empty, it is cleared when the transmit FIFO contains at least 1 byte.
LSR[6]: THR and TSR Empty Flag
This bit is set to a logic 1 whenever the transmitter goes idle. It is set to logic 0 whenever either the THR or
TSR contains a data character. In the FIFO mode this bit is set to a logic 1 whenever the transmit FIFO and
transmit shift register are both empty.
LSR[7]: Receive FIFO Data Error Flag
•Logic 0 = No FIFO error (default).
•Logic 1 = A global indicator for the sum of all error bits in the RX FIFO. At least one parity error, framing error
or break indication is in the FIFO data. This bit clears when there is no more error(s) in any of the bytes in the
RX FIFO.
4.9 Modem Status Register (MSR) - Read Only
This register provides the current state of the modem interface input signals. Lower four bits of this register are
used to indicate the changed information. These bits are set to a logic 1 whenever a signal from the modem
changes state. These bits may be used for general purpose inputs when they are not used with modem
signals.
MSR[0]: Delta CTS# Input Flag
•Logic 0 = No change on CTS# input (default).
•Logic 1 = The CTS# input has changed state since the last time it was monitored. A modem status interrupt
will be generated if MSR interrupt is enabled (IER bit-3).
MSR[1]: Delta DSR# Input Flag
•Logic 0 = No change on DSR# input (default).
•Logic 1 = The DSR# input has changed state since the last time it was monitored. A modem status interrupt
will be generated if MSR interrupt is enabled (IER bit-3).
MSR[2]: Delta RI# Input Flag
•Logic 0 = No change on RI# input (default).
•Logic 1 = The RI# input has changed from a LOW to HIGH, ending of the ringing signal. A modem status
interrupt will be generated if MSR interrupt is enabled (IER bit-3).
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
31
MSR[3]: Delta CD# Input Flag
•Logic 0 = No change on CD# input (default).
•Logic 1 = Indicates that the CD# input has changed state since the last time it was monitored. A modem
status interrupt will be generated if MSR interrupt is enabled (IER bit-3).
MSR[4]: CTS Input Status
CTS# pin may function as automatic hardware flow control signal input if it is enabled and selected by Auto
CTS (EFR bit-7). Auto CTS flow control allows starting and stopping of local data transmissions based on the
modem CTS# signal. A HIGH on the CTS# pin will stop UART transmitter as soon as the current character has
finished transmission, and a LOW will resume data transmission. Normally MSR bit-4 bit is the compliment of
the CTS# input. However in the loopback mode, this bit is equivalent to the RTS# bit in the MCR register. The
CTS# input may be used as a general purpose input when the modem interface is not used.
MSR[5]: DSR Input Status
Normally this bit is the complement of the DSR# input. In the loopback mode, this bit is equivalent to the DTR#
bit in the MCR register. The DSR# input may be used as a general purpose input when the modem interface is
not used.
MSR[6]: RI Input Status
Normally this bit is the complement of the RI# input. In the loopback mode this bit is equivalent to bit-2 in the
MCR register. The RI# input may be used as a general purpose input when the modem interface is not used.
MSR[7]: CD Input Status
Normally this bit is the complement of the CD# input. In the loopback mode this bit is equivalent to bit-3 in the
MCR register. The CD# input may be used as a general purpose input when the modem interface is not used.
4.10 Scratch Pad Register (SPR) - Read/Write
This is a 8-bit general purpose register for the user to store temporary data. The content of this register is
preserved during sleep mode but becomes 0xFF (default) after a reset or a power off-on cycle.
4.11 Enhanced Mode Select Register (EMSR)
This register replaces SPR (during a Write) and is accessible only when FCTR[6] = 1.
EMSR[1:0]: Receive/Transmit FIFO Level Count (Write-Only)
When Scratchpad Swap (FCTR[6]) is asserted, EMSR bits 1-0 controls what mode the FIFO Level Counter is
operating in.
During Alternate RX/TX FIFO Level Counter Mode, the first value read after EMSR bits 1-0 have been
asserted will always be the RX FIFO Level Counter. The second value read will correspond with the TX FIFO
Level Counter. The next value will be the RX FIFO Level Counter again, then the TX FIFO Level Counter and
so on and so forth.
EMSR[2]: Reserved
EMSR[3]: Automatic RS485 Half-Duplex Control Output Inversion
•Logic 0 = RTS# output is LOW during TX and HIGH during RX (default, compatible with 16C2850).
•Logic 1 = RTS# output is HIGH during TX and LOW during RX.
TABLE 12: SCRATCHPAD SWAP SELECTION
FCTR[6] EMSR[1] EMSR[0] SCRATCHPAD IS
0 X X Scratchpad
1 X 0 RX FIFO Level Counter Mode
1 0 1 TX FIFO Level Counter Mode
1 1 1 Alternate RX/TX FIFO Counter Mode
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
32
EMSR[5:4]: Extended RTS Hysteresis
EMSR[6]: LSR Interrupt Mode
•Logic 0 = LSR Interrupt Delayed (for 16C2550 compatibility, default). LSR bits 2, 3, and 4 will generate an
interrupt when the character with the error is in the RHR.
•Logic 1 = LSR Interrupt Immediate. LSR bits 2, 3, and 4 will generate an interrupt as soon as the character is
received into the FIFO.
EMSR[7]: 16X Sampling Rate Mode
Logic 0 = 8X Sampling Rate.
Logic 1 = 16X Sampling Rate (for 16C550 compatibility, default).
4.12 FIFO Level Register (FLVL) - Read-Only
The FIFO Level Register replaces the Scratchpad Register (during a Read) when FCTR[6] = 1. Note that this is
not identical to the FIFO Data Count Register which can be accessed when LCR = 0xBF.
FLVL[7:0]: FIFO Level Register
This register provides the FIFO counter level for the RX FIFO or the TX FIFO or both depending on EMSR[1:0].
See Table 12 for details.
4.13 Baud Rate Generator Registers (DLL and DLM) - Read/Write
The concatenation of the contents of DLM and DLL gives the 16-bit divisor value which is used to calculate the
baud rate:
•Baud Rate = (Clock Frequency / 16) / Divisor
See MCR bit-7 and the baud rate table also.
4.14 Device Identification Register (DVID) - Read Only
This register contains the device ID (0x0A for XR16L2750). Prior to reading this register, DLL and DLM should
be set to 0x00.
TABLE 13: AUTO RTS HYSTERESIS
EMSR
BIT-5
EMSR
BIT-4
FCTR
BIT-1
FCTR
BIT-0
RTS#
HYSTERESIS
(CHARACTERS)
0
0
0
0
0
0
0
0
0
0
1
1
0
1
0
1
0
±4
±6
±8
0
0
0
0
1
1
1
1
0
0
1
1
0
1
0
1
±8
±16
±24
±32
1
1
1
1
0
0
0
0
0
0
1
1
0
1
0
1
±40
±44
±48
±52
1
1
1
1
1
1
1
1
0
0
1
1
0
1
0
1
±12
±20
±28
±36
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
33
4.15 Device Revision Register (DREV) - Read Only
This register contains the device revision information. For example, 0x01 means revision A. Prior to reading
this register, DLL and DLM should be set to 0x00.
4.16 Trigger Level Register (TRG) - Write-Only
User Programmable Transmit/Receive Trigger Level Register.
TRG[7:0]: Trigger Level Register
These bits are used to program desired trigger levels when trigger Table-D is selected. FCTR bit-7 selects
between programming the RX Trigger Level (a logic 0) and the TX Trigger Level (a logic 1).
4.17 RX/TX FIFO Level Count Register (FC) - Read-Only
This register is accessible when LCR = 0xBF. Note that this register is not identical to the FIFO Level Count
Register which is located in the general register set when FCTR bit-6 = 1 (Scratchpad Register Swap). It is
suggested to read the FIFO Level Count Register at the Scratchpad Register location when FCTR bit-6 = 1.
See Tabl e 12.
FC[7:0]: RX/TX FIFO Level Count
Receive/Transmit FIFO Level Count. Number of characters in Receiver FIFO (FCTR[7] = 0) or Transmitter
FIFO (FCTR[7] = 1) can be read via this register.
4.18 Feature Control Register (FCTR) - Read/Write
This register controls the XR16L2750 new functions that are not available in ST16C2450 or ST16C2550.
FCTR[1:0]: RTS Hysteresis
User selectable RTS# hysteresis levels for hardware flow control application. After reset, these bits are set to
“0” to select the next trigger level for hardware flow control. See Ta b l e 13 for more details.
FCTR[2]: IrDa RX Inversion
•Logic 0 = Select RX input as encoded IrDa data (Idle state will be LOW).
•Logic 1 = Select RX input as inverted encoded IrDa data (Idle state will be HIGH).
FCTR[3]: Auto RS-485 Direction Control
•Logic 0 = Standard ST16C550 mode. Transmitter generates an interrupt when transmit holding register
becomes empty and transmit shift register is shifting data out.
•Logic 1 = Enable Auto RS485 Direction Control function. The direction control signal, RTS# pin, changes its
output logic state from LOW to HIGH one bit time after the last stop bit of the last character is shifted out.
Also, the Transmit interrupt generation is delayed until the transmitter shift register becomes empty. The
RTS# output pin will automatically return to a LOW when a data byte is loaded into the TX FIFO. However,
RTS# behavior can be inverted by setting EMSR[3] = 1.
FCTR[5:4]: Transmit/Receive Trigger Table Select
See Tabl e 10 for more details.
TABLE 14: TRIGGER TABLE SELECT
FCTR
BIT-5
FCTR
BIT-4 TABLE
0 0 Table-A (TX/RX)
0 1 Table-B (TX/RX)
1 0 Table-C (TX/RX)
1 1 Table-D (TX/RX)
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
34
FCTR[6]: Scratchpad Swap
•Logic 0 = Scratch Pad register is selected as general read and write register. ST16C550 compatible mode.
•Logic 1 = FIFO Count register (Read-Only), Enhanced Mode Select Register (Write-Only). Number of
characters in transmit or receive FIFO can be read via scratch pad register when this bit is set. Enhanced
Mode Select Register is selected when it is written into.
FCTR[7]: Programmable Trigger Register Select
•Logic 0 = Registers TRG and FC selected for RX.
•Logic 1 = Registers TRG and FC selected for TX.
4.19 Enhanced Feature Register (EFR)
Enhanced features are enabled or disabled using this register. Bit 0-3 provide single or dual consecutive
character software flow control selection (see Table 15). When the Xon1 and Xon2 and Xoff1 and Xoff2 modes
are selected, the double 8-bit words are concatenated into two sequential characters. Caution: note that
whenever changing the TX or RX flow control bits, always reset all bits back to logic 0 (disable) before
programming a new setting.
EFR[3:0]: Software Flow Control Select
Single character and dual sequential characters software flow control is supported. Combinations of software
flow control can be selected by programming these bits.
TABLE 15: SOFTWARE FLOW CONTROL FUNCTIONS
EFR BIT-3
CONT-3
EFR BIT-2
CONT-2
EFR BIT-1
CONT-1
EFR BIT-0
CONT-0 TRANSMIT AND RECEIVE SOFTWARE FLOW CONTROL
0 0 0 0 No TX and RX flow control (default and reset)
0 0 X X No transmit flow control
1 0 X X Transmit Xon1, Xoff1
0 1 X X Transmit Xon2, Xoff2
1 1 X X Transmit Xon1 and Xon2, Xoff1 and Xoff2
X X 0 0 No receive flow control
X X 1 0 Receiver compares Xon1, Xoff1
X X 0 1 Receiver compares Xon2, Xoff2
1 0 1 1 Transmit Xon1, Xoff1
Receiver compares Xon1 or Xon2, Xoff1 or Xoff2
0 1 1 1 Transmit Xon2, Xoff2
Receiver compares Xon1 or Xon2, Xoff1 or Xoff2
1 1 1 1 Transmit Xon1 and Xon2, Xoff1 and Xoff2,
Receiver compares Xon1 and Xon2, Xoff1 and Xoff2
0 0 1 1 No transmit flow control,
Receiver compares Xon1 and Xon2, Xoff1 and Xoff2
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
35
EFR[4]: Enhanced Function Bits Enable
Enhanced function control bit. This bit enables IER bits 4-7, ISR bits 4-5, FCR bits 4-5, and MCR bits 5-7 to be
modified. After modifying any enhanced bits, EFR bit-4 can be set to a logic 0 to latch the new values. This
feature prevents legacy software from altering or overwriting the enhanced functions once set. Normally, it is
recommended to leave it enabled, logic 1.
•Logic 0 = modification disable/latch enhanced features. IER bits 4-7, ISR bits 4-5, FCR bits 4-5, and MCR
bits 5-7 are saved to retain the user settings. After a reset, the IER bits 4-7, ISR bits 4-5, FCR bits 4-5, and
MCR bits 5-7 are set to a logic 0 to be compatible with ST16C550 mode (default).
•Logic 1 = Enables the above-mentioned register bits to be modified by the user.
EFR[5]: Special Character Detect Enable
•Logic 0 = Special Character Detect Disabled (default).
•Logic 1 = Special Character Detect Enabled. The UART compares each incoming receive character with
data in Xoff-2 register. If a match exists, the receive data will be transferred to FIFO and ISR bit-4 will be set
to indicate detection of the special character. Bit-0 corresponds with the LSB bit of the receive character. If
flow control is set for comparing Xon1, Xoff1 (EFR [1:0]= ‘10’) then flow control and special character work
normally. However, if flow control is set for comparing Xon2, Xoff2 (EFR[1:0]= ‘01’) then flow control works
normally, but Xoff2 will not go to the FIFO, and will generate an Xoff interrupt and a special character
interrupt, if enabled via IER bit-5.
EFR[6]: Auto RTS Flow Control Enable
RTS# output may be used for hardware flow control by setting EFR bit-6 to logic 1. When Auto RTS is
selected, an interrupt will be generated when the receive FIFO is filled to the programmed trigger level and
RTS de-asserts HIGH at the next upper trigger level or hysteresis level. RTS# will return LOW when FIFO data
falls below the next lower trigger level. The RTS# output must be asserted (LOW) before the auto RTS can
take effect. RTS# pin will function as a general purpose output when hardware flow control is disabled.
•Logic 0 = Automatic RTS flow control is disabled (default).
•Logic 1 = Enable Automatic RTS flow control.
EFR[7]: Auto CTS Flow Control Enable
Automatic CTS Flow Control.
•Logic 0 = Automatic CTS flow control is disabled (default).
•Logic 1 = Enable Automatic CTS flow control. Data transmission stops when CTS# input de-asserts HIGH.
Data transmission resumes when CTS# returns LOW.
4.19.1 Software Flow Control Registers (XOFF1, XOFF2, XON1, XON2) - Read/Write
These registers are used as the programmable software flow control characters xoff1, xoff2, xon1, and xon2.
For more details, see Table 6.
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
36
TABLE 16: UART RESET CONDITIONS FOR CHANNEL A AND B
REGISTERS RESET STATE
DLM and DLL DLM = 0x00 and DLL = 0x01. Only resets to these values during a power up.
They do not reset when the Reset Pin is asserted.
RHR Bits 7-0 = 0xXX
THR Bits 7-0 = 0xXX
IER Bits 7-0 = 0x00
FCR Bits 7-0 = 0x00
ISR Bits 7-0 = 0x01
LCR Bits 7-0 = 0x00
MCR Bits 7-0 = 0x00
LSR Bits 7-0 = 0x60
MSR Bits 3-0 = Logic 0
Bits 7-4 = Logic levels of the inputs inverted
SPR Bits 7-0 = 0xFF
EMSR Bits 7-0 = 0x80
FLVL Bits 7-0 = 0x00
EFR Bits 7-0 = 0x00
XON1 Bits 7-0 = 0x00
XON2 Bits 7-0 = 0x00
XOFF1 Bits 7-0 = 0x00
XOFF2 Bits 7-0 = 0x00
FC Bits 7-0 = 0x00
I/O SIGNALS RESET STATE
TX HIGH
OP2# HIGH
RTS# HIGH
DTR# HIGH
RXRDY# HIGH
TXRDY# LOW
INT Three-State Condition
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
37
Test 1: The following inputs must remain steady at VCC or GND state to minimize Sleep current: A0-A2, D0-
D7, IOR#, IOW#, CSA#, CSB# and all modem inputs. Also, RXA and RXB inputs must idle at logic 1 state
while asleep. Floating inputs will result in sleep currents in the mA range. For PowerSave feature that isolates
address, data and control signals, please see the XR16L2751 datasheet.
ABSOLUTE MAXIMUM RATINGS
Power Supply Range 7 Volts
Voltage at Any Pin GND-0.3V to 7V
Operating Temperature -40o to +85oC
Storage Temperature -65o to +150oC
Package Dissipation 500 mW
TYPICAL PACKAGE THERMAL RESISTANCE DATA (MARGIN OF ERROR: ± 15%)
Thermal Resistance (48-TQFP) theta-ja =59oC/W, theta-jc = 16oC/W
Thermal Resistance (44-PLCC) theta-ja = 50oC/W, theta-jc = 21oC/W
ELECTRICAL CHARACTERISTICS
DC ELECTRICAL CHARACTERISTICS
TA=0O TO 70OC (-40O TO +85OC FOR INDUSTRIAL GRADE PACKAGE), VCC IS 2.25V TO 5.5V
SYMBOL PARAMETER
LIMITS
2.5V
MIN MAX
LIMITS
3.3V
MIN MAX
LIMITS
5.0V
MIN MAX
UNITS CONDITIONS
VILCK Clock Input Low Level -0.3 0.2 -0.3 0.6 -0.5 0.6 V
VIHCK Clock Input High Level 2.0 VCC 2.4 VCC 3.0 VCC V
VIL Input Low Voltage -0.3 0.6 -0.3 0.8 -0.5 0.8 V
VIH Input High Voltage 2.0 5.5 2.0 5.5 2.2 5.5 V
VOL Output Low Voltage
0.4
0.4
0.4 V
V
V
IOL = 6 mA
IOL = 4 mA
IOL = 2 mA
VOH Output High Voltage
1.8
2.0
2.4 V
V
V
IOH = -6 mA
IOH = -1 mA
IOH = -400 uA
IIL Input Low Leakage Current ±10 ±10 ±10 uA
IIH Input High Leakage Current ±10 ±10 ±10 uA
CIN Input Pin Capacitance 555pF
ICC Power Supply Current 1.2 2 5 mA
ISLEEP Sleep Current 615 30 uA See Test 1
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
38
AC ELECTRICAL CHARACTERISTICS
UNLESS OTHERWISE NOTED: TA=0O TO 70OC (-40O TO +85OC FOR INDUSTRIAL GRADE PACKAGE), VCC=2.25 - 5.5V,
70 PF LOAD WHERE APPLICABLE
SYMBOL PARAMETER
LIMITS
2.5
MIN MAX
LIMITS
3.3
MIN MAX
LIMITS
5.0
MIN MAX
UNIT
-Crystal Frequency 16 20 24 MHz
CLK External Clock Low/High Time 20 15 10 ns
OSC External Clock Frequency 24 33 50 MHz
TAS Address Setup Time 10 10 10 ns
TAH Address Hold Time 10 10 10 ns
TCS Chip Select Width 150 75 50 ns
TRD IOR# Strobe Width 150 75 50 ns
TDY Read Cycle Delay 150 75 50 ns
TRDV Data Access Time 135 70 45 ns
TDD Data Disable Time 045 030 030 ns
TWR IOW# Strobe Width 150 75 50 ns
TDY Write Cycle Delay 150 75 50 ns
TDS Data Setup Time 25 20 15 ns
TDH Data Hold Time 15 10 10 ns
TWDO Delay From IOW# To Output 150 75 50 ns
TMOD Delay To Set Interrupt From MODEM Input 150 75 50 ns
TRSI Delay To Reset Interrupt From IOR# 150 75 50 ns
TSSI Delay From Stop To Set Interrupt 111Bclk
TRRI Delay From IOR# To Reset Interrupt 150 75 50 ns
TSI Delay From Stop To Interrupt 150 75 50 ns
TINT Delay From Initial INT Reset To Transmit
Start
824 824 824 Bclk
TWRI Delay From IOW# To Reset Interrupt 150 75 50 ns
TSSR Delay From Stop To Set RXRDY# 111Bclk
TRR Delay From IOR# To Reset RXRDY# 150 75 50 ns
TWT Delay From IOW# To Set TXRDY# 150 75 50 ns
TSRT Delay From Center of Start To Reset TXRDY# 888Bclk
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
39
TRST Reset Pulse Width 40 40 40 ns
NBaud Rate Divisor 1216-1 1216-1 1216-1 -
Bclk Baud Clock 16X or 8X of data rate Hz
FIGURE 14. CLOCK TIMING
FIGURE 15. MODEM INPUT/OUTPUT TIMING FOR CHANNELS A & B
AC ELECTRICAL CHARACTERISTICS
UNLESS OTHERWISE NOTED: TA=0O TO 70OC (-40O TO +85OC FOR INDUSTRIAL GRADE PACKAGE), VCC=2.25 - 5.5V,
70 PF LOAD WHERE APPLICABLE
SYMBOL PARAMETER
LIMITS
2.5
MIN MAX
LIMITS
3.3
MIN MAX
LIMITS
5.0
MIN MAX
UNIT
OSC
CLK
CLK
EXTERNAL
CLOCK
IOW #
RTS#
DTR#
CD#
CTS#
DSR#
INT
IOR#
RI#
TWDO
TMOD TMOD
TRSI
TMOD
Active
Active
Change of state Change of state
Active Active Active
Change of state Change of state
Change of state
Active Active
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
40
FIGURE 16. DATA BUS READ TIMING
FIGURE 17. DATA BUS WRITE TIMING
TAS
TDD
TAH
TRD
TRDV
TDY
TDD
TRDV
TAH
TAS
TCS
Valid Address Valid Address
Valid Data Valid Data
A0-A2
CSA#/
CSB#
IOR#
D0-D7
RDTm
TCS
TRD
16Write
TAS
TDH
TAH
TWR
TDS
TDY
TDH
TDS
TAH
TAS
TCS
Valid Address Valid Address
Valid Data Valid Data
A0-A2
CSA#/
CSB#
IOW#
D0-D7
TCS
TWR
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
41
FIGURE 18. RECEIVE READY & INTERRUPT TIMING [NON-FIFO MODE] FOR CHANNELS A & B
FIGURE 19. TRANSMIT READY & INTERRUPT TIMING [NON-FIFO MODE] FOR CHANNELS A & B
RX
RXRDY#
IOR#
INT
D0:D7
Start
Bit D0:D7
Stop
Bit D0:D7
TSSR
1 Byte
in RHR
Active
Data
Ready
Active
Data
Ready
Active
Data
Ready
1 Byte
in RHR
1 Byte
in RHR
TSSR TSSR
RXNFM
TRR TRR TRR
TSSR TSSR TSSR
(Reading data
out of RHR)
TX
TXRDY#
IOW#
INT*
D0:D7
Start
Bit D0:D7
Stop
Bit D0:D7
TWT
TXNonFIFO
TWT TWT
TWRI TWRI TWRI
TSRT TSRT TSRT
*INT is cleared when the ISR is read or when data is loaded into the THR.
ISR is read ISR is readISR is read
(Loading data
into THR)
IER[1]
enabled
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
42
FIGURE 20. RECEIVE READY & INTERRUPT TIMING [FIFO MODE, DMA DISABLED] FOR CHANNELS A & B
FIGURE 21. RECEIVE READY & INTERRUPT TIMING [FIFO MODE, DMA ENABLED] FOR CHANNELS A & B
RX
RXRDY#
IOR#
INT
D0:D7
S
TSSR
RXINTDMA#
RX FIFO fills up to RX
Trigger Level or RX Data
Timeout
RX FIFO drops
below RX
Trigger Level
FIFO
Empties
First Byte is
Received in
RX FIFO
D0:D7
SD0:D7TD0:D7
SD0:D7
S
TD0:D7
S
TTD0:D7
ST
Start
Bit
Stop
Bit
TRR
TRRI
TSSI
(Reading data out
of RX FIFO)
RX
RXRDY#
IOR#
INT
D0:D7
S
TSSR
RXFIFODMA
RX FIFO fills up to RX
Trigger Level or RX Data
Timeout
RX FIFO drops
below RX
Trigger Level
FIFO
Empties
D0:D7
SD0:D7TD0:D7
SD0:D7
S
TD0:D7
S
TTD0:D7
ST
Start
Bit
Stop
Bit
TRR
TRRI
TSSI
(Reading data out
of RX FIFO)
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
43
FIGURE 22. TRANSMIT READY & INTERRUPT TIMING [FIFO MODE, DMA MODE DISABLED] FOR CHANNELS A & B
FIGURE 23. TRANSMIT READY & INTERRUPT TIMING [FIFO MODE, DMA MODE ENABLED] FOR CHANNELS A & B
TX
TXRDY#
IOW#
INT*
TXDMA#
D0:D7
SD0:D7TD0:D7
SD0:D7
S
TD0:D7
S
TTD0:D7
ST
Start
Bit
Stop
Bit
TWRI
(Unloading)
(Loading data
into FIFO)
Last Data Byte
Transmitted
TX FIFO fills up
to trigger level TX FIFO drops
below trigger level
Data in
TX FIFO
TX FIFO
Empty
TWT
TSRT
TX FIFO
Empty
T
TS
TSI
ISR is read
IER[1]
enabled
ISR is read
*INT is cleared when the ISR is read or when TX FIFO fills up to the trigger level.
TX
TXRDY#
IOW#
INT*
D0:D7
S
TXDMA
D0:D7
SD0:D7
TD0:D7
SD0:D7S
TD0:D7
S
TTD0:D7ST
Start
Bit
Stop
Bit
TWRI
T
(Unloading)
(Loading data
into FIFO)
Last Data Byte
Transmitted
TX FIFO fills up
to trigger level
TX FIFO drops
below trigger level
At least 1
empty location
in FIFO
TSRT
TX FIFO
Full
TWT
TSI
ISR Read ISR Read
*INT cleared when the ISR is read or when TX FIFO fills up to trigger level.
IER[1]
enabled
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
44
PACKAGE DIMENSIONS (48 PIN TQFP - 7 X 7 X 1 mm)
Note: The control dimension is the millimeter column
INCHES MILLIMETERS
SYMBOL MIN MAX MIN MAX
A0.039 0.047 1.00 1.20
A10.002 0.006 0.05 0.15
A20.037 0.041 0.95 1.05
B0.007 0.011 0.17 0.27
C0.004 0.008 0.09 0.20
D0.346 0.362 8.80 9.20
D10.272 0.280 6.90 7.10
e0.020 BSC 0.50 BSC
L0.018 0.030 0.45 0.75
α0°7°0°7°
36 25
24
13
1
1
2
37
48
D
D
1
D
D
1
B
e
α
A
2
A
1
A
Seating
Plane
L
C
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
45
PACKAGE DIMENSIONS (44 PIN PLCC)
Note: The control dimension is the millimeter column
INCHES MILLIMETERS
SYMBOL MIN MAX MIN MAX
A0.165 0.180 4.19 4.57
A10.090 0.120 2.29 3.05
A20.020 --- 0.51 ---
B0.013 0.021 0.33 0.53
B10.026 0.032 0.66 0.81
C0.008 0.013 0.19 0.32
D0.685 0.695 17.40 17.65
D10.650 0.656 16.51 16.66
D20.590 0.630 14.99 16.00
D30.500 typ. 12.70 typ.
e0.050 BSC 1.27 BSC
H10.042 0.056 1.07 1.42
H20.042 0.048 1.07 1.22
R0.025 0.045 0.64 1.14
44 LEAD PLASTIC LEADED CHIP CARRIER
(PLCC)
Rev. 1.00
1
D
D
1
A
A
1
D D
1
D
3
B
A
2
B
1
e
Seating Plane
D
2
244
D
3
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XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
46
NOTICE
EXAR Corporation reserves the right to make changes to the products contained in this publication in order to
improve design, performance or reliability. EXAR Corporation assumes no responsibility for the use of any
circuits described herein, conveys no license under any patent or other right, and makes no representation that
the circuits are free of patent infringement. Charts and schedules contained here in are only for illustration
purposes and may vary depending upon a user’s specific application. While the information in this publication
has been carefully checked; no responsibility, however, is assumed for inaccuracies.
EXAR Corporation does not recommend the use of any of its products in life support applications where the
failure or malfunction of the product can reasonably be expected to cause failure of the life support system or
to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless
EXAR Corporation receives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage has
been minimized; (b) the user assumes all such risks; (c) potential liability of EXAR Corporation is adequately
protected under the circumstances.
Copyright 2005 EXAR Corporation
Datasheet April 2005.
Send your UART technical inquiry with technical details to hotline: uarttechsupport@exar.com.
Reproduction, in part or whole, without the prior written consent of EXAR Corporation is prohibited.
REVISION HISTORY
DATE REVISION DESCRIPTION
November 2001 Rev P1.0.0 Preliminary data sheet.
March 2002 Rev P1.1.0 Corrected INTA/B pin descriptions and reset state. Clarified MCR bit-3 pin descrip-
tion. Renamed Sclk to Bclk.
September 2002 Rev 1.0.0 Release into production. Clarified RTS# pin descriptions, XTAL1 pin description,
external clock description, auto RS485 half-duplex control description, EMSR bit-3
description and updated 2.5 V, ICC and ISLEEP DC Electrical Characteristics.
March 2003 Rev 1.1.0 Updated AC Electrical Characteristics.
August 2004 Rev 1.2.0 Added Device Status to Ordering Information. Clarified pin descriptions- changed
from using logic 1 and logic 0 to HIGH (VCC) and LOW (GND) for input and output
pin descriptions.
April 2005 Rev 1.2.1 Updated the Data Access Times (TRDV) in AC Electrical Characteristics.
xr XR16L2750
REV. 1.2.1 2.25V TO 5.5V DUART WITH 64-BYTE FIFO
I
TABLE OF CONTENTS
GENERAL DESCRIPTION................................................................................................. 1
APPLICATIONS ............................................................................................................................................... 1
FEATURES ..................................................................................................................................................... 1
FIGURE 1. XR16L2750 BLOCK DIAGRAM ......................................................................................................................................... 1
FIGURE 2. PIN OUT ASSIGNMENT ..................................................................................................................................................... 2
ORDERING INFORMATION ................................................................................................................................2
PIN DESCRIPTIONS ......................................................................................................... 3
1.0 PRODUCT DESCRIPTION .................................................................................................................... 6
2.0 FUNCTIONAL DESCRIPTIONS ............................................................................................................ 7
2.1 CPU INTERFACE ............................................................................................................................................. 7
FIGURE 3. XR16L2750 DATA BUS INTERCONNECTIONS.................................................................................................................... 7
2.2 5-VOLT TOLERANT INPUTS ........................................................................................................................... 7
2.3 DEVICE RESET ................................................................................................................................................ 7
2.4 DEVICE IDENTIFICATION AND REVISION .................................................................................................... 7
2.5 CHANNEL A AND B SELECTION ................................................................................................................... 7
TABLE 1: CHANNEL A AND B SELECT ............................................................................................................................................... 8
2.6 CHANNEL A AND B INTERNAL REGISTERS ................................................................................................ 8
2.7 DMA MODE ...................................................................................................................................................... 8
TABLE 2: TXRDY# AND RXRDY# OUTPUTS IN FIFO AND DMA MODE............................................................................................. 8
2.8 INTA AND INTB OUTPUTS .............................................................................................................................. 9
TABLE 3: INTA AND INTB PINS OPERATION FOR TRANSMITTER ........................................................................................................ 9
TABLE 4: INTA AND INTB PIN OPERATION FOR RECEIVER ............................................................................................................... 9
2.9 CRYSTAL OSCILLATOR OR EXTERNAL CLOCK INPUT ............................................................................. 9
FIGURE 4. TYPICAL OSCILLATOR CONNECTIONS................................................................................................................................. 9
FIGURE 5. EXTERNAL CLOCK CONNECTION FOR EXTENDED DATA RATE .......................................................................................... 10
2.10 PROGRAMMABLE BAUD RATE GENERATOR ......................................................................................... 10
FIGURE 6. BAUD RATE GENERATOR AND PRESCALER ..................................................................................................................... 10
TABLE 5: TYPICAL DATA RATES WITH A 14.7456 MHZ CRYSTAL OR EXTERNAL CLOCK ...................................................................... 11
2.11 TRANSMITTER ............................................................................................................................................. 11
2.11.1 TRANSMIT HOLDING REGISTER (THR) - WRITE ONLY....................................................................................... 11
2.11.2 TRANSMITTER OPERATION IN NON-FIFO MODE ................................................................................................ 11
FIGURE 7. TRANSMITTER OPERATION IN NON-FIFO MODE .............................................................................................................. 12
2.11.3 TRANSMITTER OPERATION IN FIFO MODE ......................................................................................................... 12
FIGURE 8. TRANSMITTER OPERATION IN FIFO AND FLOW CONTROL MODE ..................................................................................... 12
2.12 RECEIVER .................................................................................................................................................... 12
2.12.1 RECEIVE HOLDING REGISTER (RHR) - READ-ONLY .......................................................................................... 13
FIGURE 9. RECEIVER OPERATION IN NON-FIFO MODE .................................................................................................................... 13
FIGURE 10. RECEIVER OPERATION IN FIFO AND AUTO RTS FLOW CONTROL MODE ....................................................................... 13
2.13 AUTO RTS (HARDWARE) FLOW CONTROL ............................................................................................. 14
2.14 AUTO RTS HYSTERESIS ........................................................................................................................... 14
2.15 AUTO CTS FLOW CONTROL ..................................................................................................................... 14
FIGURE 11. AUTO RTS AND CTS FLOW CONTROL OPERATION....................................................................................................... 15
2.16 AUTO XON/XOFF (SOFTWARE) FLOW CONTROL ................................................................................... 16
TABLE 6: AUTO XON/XOFF (SOFTWARE) FLOW CONTROL ............................................................................................................... 16
2.17 SPECIAL CHARACTER DETECT ............................................................................................................... 16
2.18 AUTO RS485 HALF-DUPLEX CONTROL .................................................................................................. 16
2.19 INFRARED MODE ........................................................................................................................................ 17
FIGURE 12. INFRARED TRANSMIT DATA ENCODING AND RECEIVE DATA DECODING.......................................................................... 17
2.20 SLEEP MODE WITH AUTO WAKE-UP ...................................................................................................... 18
2.21 INTERNAL LOOPBACK .............................................................................................................................. 19
FIGURE 13. INTERNAL LOOP BACK IN CHANNEL A AND B ................................................................................................................ 19
3.0 UART INTERNAL REGISTERS ........................................................................................................... 20
TABLE 7: UART CHANNEL A AND B UART INTERNAL REGISTERS ...................................................................................... 20
TABLE 8: INTERNAL REGISTERS DESCRIPTION. SHADED BITS ARE ENABLED WHEN EFR BIT-4=1......................................... 21
4.0 INTERNAL REGISTER DESCRIPTIONS ............................................................................................ 22
4.1 RECEIVE HOLDING REGISTER (RHR) - READ- ONLY ............................................................................... 22
4.2 TRANSMIT HOLDING REGISTER (THR) - WRITE-ONLY ............................................................................ 22
4.3 INTERRUPT ENABLE REGISTER (IER) - READ/WRITE ............................................................................. 22
4.3.1 IER VERSUS RECEIVE FIFO INTERRUPT MODE OPERATION ............................................................................. 23
XR16L2750 xr
2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.2.1
II
4.3.2 IER VERSUS RECEIVE/TRANSMIT FIFO POLLED MODE OPERATION................................................................ 23
4.4 INTERRUPT STATUS REGISTER (ISR) - READ-ONLY ............................................................................... 24
4.4.1 INTERRUPT GENERATION: ...................................................................................................................................... 24
4.4.2 INTERRUPT CLEARING: ........................................................................................................................................... 24
TABLE 9: INTERRUPT SOURCE AND PRIORITY LEVEL ....................................................................................................................... 25
4.5 FIFO CONTROL REGISTER (FCR) - WRITE-ONLY ...................................................................................... 25
TABLE 10: TRANSMIT AND RECEIVE FIFO TRIGGER TABLE AND LEVEL SELECTION .......................................................................... 26
4.6 LINE CONTROL REGISTER (LCR) - READ/WRITE ...................................................................................... 27
TABLE 11: PARITY SELECTION ........................................................................................................................................................ 28
4.7 MODEM CONTROL REGISTER (MCR) OR GENERAL PURPOSE OUTPUTS CONTROL - READ/WRITE 28
4.8 LINE STATUS REGISTER (LSR) - READ ONLY ........................................................................................... 29
4.9 MODEM STATUS REGISTER (MSR) - READ ONLY .................................................................................... 30
4.10 SCRATCH PAD REGISTER (SPR) - READ/WRITE .................................................................................... 31
4.11 ENHANCED MODE SELECT REGISTER (EMSR) ...................................................................................... 31
TABLE 12: SCRATCHPAD SWAP SELECTION .................................................................................................................................... 31
TABLE 13: AUTO RTS HYSTERESIS ................................................................................................................................................ 32
4.12 FIFO LEVEL REGISTER (FLVL) - READ-ONLY .......................................................................................... 32
4.13 BAUD RATE GENERATOR REGISTERS (DLL AND DLM) - READ/WRITE .............................................. 32
4.14 DEVICE IDENTIFICATION REGISTER (DVID) - READ ONLY .................................................................... 32
4.15 DEVICE REVISION REGISTER (DREV) - READ ONLY .............................................................................. 33
4.16 TRIGGER LEVEL REGISTER (TRG) - WRITE-ONLY ................................................................................. 33
4.17 RX/TX FIFO LEVEL COUNT REGISTER (FC) - READ-ONLY .................................................................... 33
4.18 FEATURE CONTROL REGISTER (FCTR) - READ/WRITE ........................................................................ 33
TABLE 14: TRIGGER TABLE SELECT................................................................................................................................................ 33
4.19 ENHANCED FEATURE REGISTER (EFR) .................................................................................................. 34
TABLE 15: SOFTWARE FLOW CONTROL FUNCTIONS ........................................................................................................................ 34
4.19.1 SOFTWARE FLOW CONTROL REGISTERS (XOFF1, XOFF2, XON1, XON2) - READ/WRITE ............................ 35
TABLE 16: UART RESET CONDITIONS FOR CHANNEL A AND B............................................................................................ 36
ABSOLUTE MAXIMUM RATINGS...................................................................................37
TYPICAL PACKAGE THERMAL RESISTANCE DATA (MARGIN OF ERROR: ± 15%) 37
ELECTRICAL CHARACTERISTICS ................................................................................37
DC ELECTRICAL CHARACTERISTICS ..............................................................................................................37
TA=0o to 70oC (-40o to +85oC for industrial grade package), Vcc is 2.25V to 5.5V ..............................................37
AC ELECTRICAL CHARACTERISTICS ..............................................................................................................38
Unless otherwise noted: TA=0o to 70oC (-40o to +85oC for industrial grade package), Vcc=2.25 - 5.5V, 70 pF load
where applicable........................................................................................................................................................ 38
FIGURE 14. CLOCK TIMING............................................................................................................................................................. 39
FIGURE 15. MODEM INPUT/OUTPUT TIMING FOR CHANNELS A & B ................................................................................................. 39
FIGURE 17. DATA BUS WRITE TIMING............................................................................................................................................. 40
FIGURE 16. DATA BUS READ TIMING .............................................................................................................................................. 40
FIGURE 18. RECEIVE READY & INTERRUPT TIMING [NON-FIFO MODE] FOR CHANNELS A & B ......................................................... 41
FIGURE 19. TRANSMIT READY & INTERRUPT TIMING [NON-FIFO MODE] FOR CHANNELS A & B ....................................................... 41
FIGURE 20. RECEIVE READY & INTERRUPT TIMING [FIFO MODE, DMA DISABLED] FOR CHANNELS A & B........................................ 42
FIGURE 21. RECEIVE READY & INTERRUPT TIMING [FIFO MODE, DMA ENABLED] FOR CHANNELS A & B......................................... 42
FIGURE 22. TRANSMIT READY & INTERRUPT TIMING [FIFO MODE, DMA MODE DISABLED] FOR CHANNELS A & B............................ 43
FIGURE 23. TRANSMIT READY & INTERRUPT TIMING [FIFO MODE, DMA MODE ENABLED] FOR CHANNELS A & B ............................ 43
PACKAGE DIMENSIONS (48 PIN TQFP - 7 X 7 X 1 MM) ....................................................................................44
PACKAGE DIMENSIONS (44 PIN PLCC) .........................................................................................................45
REVISION HISTORY.......................................................................................................................................46
