General Description
The DS17285, DS17485, DS17885, DS17287, DS17487,
and DS17887 real-time clocks (RTCs) are designed to be
successors to the industry-standard DS12885 and
DS12887. The DS17285, DS17485, and DS17885 (here-
after referred to as the DS17x85) provide a real-time
clock/calendar, one time-of-day alarm, three maskable
interrupts with a common interrupt output, a programma-
ble square wave, and 114 bytes of battery-backed NV
SRAM. The DS17x85 also incorporates a number of
enhanced functions including a silicon serial number,
power-on/off control circuitry, and 2k, 4k, or 8kbytes of
battery-backed NV SRAM. The DS17287, DS17487, and
DS17887 (hereafter referred to as the DS17x87) integrate
a quartz crystal and lithium energy source into a 24-pin
encapsulated DIP package. The DS17x85 and DS17x87
power-control circuitry allows the system to be powered
on by an external stimulus such as a keyboard or by a
time-and-date (wake-up) alarm. The PWR output pin is
triggered by one or either of these events, and is used to
turn on an external power supply. The PWR pin is under
software control, so that when a task is complete, the sys-
tem power can then be shut down.
For all devices, the date at the end of the month is auto-
matically adjusted for months with fewer than 31 days,
including correction for leap years. It also operates in
either 24-hour or 12-hour format with an AM/PM indicator.
A precision temperature-compensated circuit monitors
the status of VCC. If a primary power failure is detected,
the device automatically switches to a backup supply. A
lithium coin cell battery can be connected to the VBAT
input pin on the DS17x85 to maintain time and date oper-
ation when primary power is absent. The DS17x85 and
DS17x87 include a VBAUX input used to power auxiliary
functions such as PWR control. The device is accessed
through a multiplexed byte-wide interface.
Applications
Embedded Systems
Utility Meters
Security Systems
Network Hubs, Bridges, and Routers
Features
Incorporates Industry-Standard DS12887 PC
Clock Plus Enhanced Functions
RTC Counts Seconds, Minutes, Hours, Day, Date,
Month, and Year with Leap Year Compensation
Through 2099
Optional +3.0V or +5.0V Operation
SMI Recovery Stack
64-Bit Silicon Serial Number
Power-Control Circuitry Supports System Power-
On from Date/Time Alarm or Key Closure
Crystal Select Bit Allows Operation with 6pF or
12.5pF Crystal
12-Hour or 24-Hour Clock with AM and PM in
12-Hour Mode
114 Bytes of General-Purpose, Battery-Backed NV
SRAM
Extended Battery-Backed NV SRAM
2048 Bytes (DS17285/DS17287)
4096 Bytes (DS17485/DS17487)
8192 Bytes (DS17885/DS17887)
RAM Clear Function
Interrupt Output with Six Independently Maskable
Interrupt Flags
Time-of-Day Alarm Once per Second to Once per
Day
End of Clock Update Cycle Flag
Programmable Square-Wave Output
Automatic Power-Fail Detect and Switch Circuitry
Available in PDIP, SO, or TSOP Package
(DS17285, DS17485, DS17885)
Optional Encapsulated DIP (EDIP) Package with
Integrated Crystal and Battery (DS17287,
DS17487, DS17887)
Optional Industrial Temperature Range Available
Underwriters Laboratory (UL) Recognized
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
______________________________________________
Maxim Integrated Products
1
19-5222; Rev 1; 4/10
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
Ordering Information, Pin Configurations, and Typical
Operating Circuit appear at end of data sheet.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
2 _____________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
DC ELECTRICAL CHARACTERISTICS
(VCC = +4.5V to +5.5V, or VCC = +2.7V to +3.7V, TA= Over the operating temperature range, unless otherwise noted. Typical
values are with TA= +25°C, VCC = 5.0V or 3.0V and VBAT = 3.0V, unless otherwise noted.) (Note 2)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Voltage Range on VCC Pin Relative to Ground ....-0.3V to +6.0V
Operating Temperature Range (Noncondensing)
Commercial.........................................................0°C to +70°C
Industrial ..........................................................-40°C to +85°C
Storage Temperature Range
EDIP.................................................................-40°C to +85°C
PDIP, SO, TSOP.............................................-55°C to +125°C
Lead Temperature (soldering, 10s) .................................+260°C
(Note: EDIP is hand or wave-soldered only.)
Soldering Temperature (reflow) .......................................+260°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
(-5) 4.5 5.0 5.5
Supply Voltage (Note 3) VCC (-3) 2.7 3.0 3.7 V
VBAT Input Voltage VBAT (Note 3) 2.5 3.0 3.7 V
(-5) 2.5 3.0 5.2
VBAUX Input Voltage (Note 3) VBAUX (-3) 3.7 V
(-5) 2.2 VCC +
0.3
Input Logic 1 (Note 3) VIH
(-3) 2.0 VCC +
0.3
V
(-5) -0.3 +0.8
Input Logic 0 (Note 3) VIL (-3) -0.3 +0.6 V
(-5) 25 50
VCC Power-Supply Current
(Note 4) ICC1 (-3) 15 30 mA
(-5) 1.0 3.0
VCC Standby Current (Notes 4, 5) ICCS (-3) 0.5 2.0 mA
Input Leakage IIL -1.0 +1.0 µA
I/O Leakage IOL (Note 6) -1.0 +1.0 µA
(-5), -1.0mA 2.4
Output Logic 1 Voltage (Note 3) VOH (-3), -0.4mA 2.4 V
(-5), +2.1mA 0.4
Output Logic 0 Voltage
AD0–AD7, IRQ, SQW (Note 3) VOL (-3), +0.8mA 0.4 V
(-5), +10mA 0.4
Output Logic 0 Voltage
PWR (Note 3) VOL (-3), +4mA 0.4 V
(-5) 4.25 4.37 4.5
Power-Fail Voltage (Note 3) VPF (-3) 2.5 2.6 2.7 V
VRT Trip Point VRTTRIP (Note 3) 1.3 V
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
_____________________________________________________________________ 3
DC ELECTRICAL CHARACTERISTICS
(VCC = 0V, VBAT = 3.0V, TA= Over the operating range, unless otherwise noted.) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
VBAT or VBAUX Current (Oscillator
On); TA = +25°C, VBAT = 3.0V IBAT (Note 7) 500 700 nA
VBAT or VBAUX Current
(Oscillator Off) IBATDR (Note 7) 50 400 nA
AC ELECTRICAL CHARACTERISTICS
(VCC = +4.5V to +5.5V, TA= Over the operating range, unless otherwise noted.) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Cycle Time tCYC 240 DC ns
Pulse Width, RD or WR Low PWRWL 120 ns
Pulse Width, RD or WR High PWRWH 80 ns
Input Rise and Fall tR, tF30 ns
Chip-Select Setup Time Before
RD or WR tCS 20 ns
Chip-Select Hold Time tCH 0ns
Read-Data Hold Time tDHR 10 50 ns
Write-Data Hold Time tDHW 0ns
Address Setup Time to ALE Fall tASL 20 ns
Address Hold Time to ALE Fall tAHL 10 ns
RD or WR High Setup to ALE
Rise tASD 25 ns
Pulse Width ALE High PWASH 40 ns
Delay Time ALE Low to RD Low tASED 30 ns
Output Data Delay Time from RD tDDR (Note 8) 20 120 ns
Data Setup Time tDSW 30 ns
IRQ Release from RD tIRD s
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
4 _____________________________________________________________________
AC ELECTRICAL CHARACTERISTICS
(VCC = +2.7V to +3.7V, TA= Over the operating range, unless otherwise noted.) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Cycle Time tCYC 360 DC ns
Pulse Width, RD or WR Low PWRWL 200 ns
Pulse Width, RD or WR High PWRWH 150 ns
Input Rise and Fall tR, tF30 ns
Chip-Select Setup Time Before
RD or WR tCS 20 ns
Chip-Select Hold Time tCH 0ns
Read-Data Hold Time tDHR 10 90 ns
Write-Data Hold Time tDHW 0ns
Address Setup Time to ALE Fall tASL 40 ns
Address Hold Time to ALE Fall tAHL 10 ns
RD or WR High Setup to ALE
Rise tASD 30 ns
Pulse Width ALE High PWASH 40 ns
Delay Time ALE Low to RD Low tASED 30 ns
Output Data Delay Time from RD tDDR (Note 8) 20 200 ns
Data Setup Time tDSW 70 ns
IRQ Release from RD tIRD s
Write Timing
PWASH
tASED
PWRWH PWRWL
tCS
tAHL
tASL tDSW tDHW
tCH
tASD
tASD
tCYC
CS
WR
AS
RD
AD0–AD7
WRITE
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
_____________________________________________________________________ 5
Read Timing
tASL tDDR
PWASH
CS
WR
ALE
RD
AD0–AD7
tASD
PWRWL
tCS
tDHR
tAHL
tCH
tCYC
PWRWH
tASED
IRQ
tIRD
tASD
AD0–AD7
CS, WR, RD
HIGH IMPEDANCE
DON'T CARE
VALID
RECOGNIZED RECOGNIZED
VALID
VCC
tF
VPF(MAX)
VPF(MIN)
tREC
tR
Power-Up/Power-Down Timing
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
6 _____________________________________________________________________
POWER-UP/POWER-DOWN CHARACTERISTICS
(TA= -40°C to +85°C) (Note 2)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Recovery at Power-Up tREC (Note 9) 20 150 ms
VCC Fall Time, VPF(MAX) to
VPF(MIN) tF300 µs
VCC Fall Time, VPF(MAX) to
VPF(MIN) tRs
DATA RETENTION (DS17x87 ONLY)
(TA= +25°C)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Expected Data Retention tDR (Note 9) 10 Years
CAPACITANCE
(TA= +25°C) (Note 10)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Capacitance on All Input Pins
Except X1 CIN (Note 10) 12 pF
Capacitance on IRQ, SQW, and
DQ0–DQ7 Pins CIO (Note 10) 12 pF
AC TEST CONDITIONS
PARAMETER CONDITIONS
Input Pulse Levels: 0 to 3.0V
Output Load Including Scope and Jig: 50pF + 1TTL Gate
Input and Output Timing Measurement Reference Levels: Input/Output: VIL max and VIH min
Input Pulse Rise and Fall Times: 5ns
Note 1: RTC modules can be successfully processed through conventional wave-soldering techniques as long as temperature
exposure to the lithium energy source contained within does not exceed +85°C. However, post-solder cleaning with water-
washing techniques is acceptable, provided that ultrasonic vibrations not used to prevent damage to the crystal.
Note 2: Limits at -40°C are guaranteed by design and not production tested.
Note 3: All voltages are referenced to ground.
Note 4: All outputs are open.
Note 5: Specified with CS = RD = WR = VCC, ALE, AD0–AD7 = 0.
Note 6: Applies to the AD0–AD7 pins, IRQ, and SQW when each is in a high-impedance state.
Note 7: Measured with a 32.768kHz crystal attached to X1 and X2.
Note 8: Measured with a 50pF capacitance load plus 1TTL gate.
Note 9: If the oscillator is disabled in software, or if the countdown chain is in reset, tREC is bypassed, and the part becomes
immediately accessible.
Note 10: Guaranteed by design. Not production tested.
WARNING: Negative undershoots below -0.3V while the part is in battery-backed mode can cause loss of
data.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
_____________________________________________________________________
7
SUPPLY CURRENT
vs. INPUT VOLTAGE
DS17285/87 toc01
VBAT (V)
SUPPLY CURRENT (nA)
3.53.33.02.8
250
300
350
400
200
2.5 3.8
VCC = 0V
SUPPLY CURRENT
vs. TEMPERATURE
DS17285/87 toc02
TEMPERATURE (°C)
SUPPLY CURRENT (nA)
65503520
5
-10-25
300
350
400
250
-40 80
VBAT = 3.0V
OSCILLATOR FREQUENCY
vs. SUPPLY VOLTAGE
DS17285/87 toc03
SUPPLY VOLTAGE (V)
OSCILLATOR FREQUENCY (Hz)
5.04.54.03.53.0
32768.1
32768.2
32768.3
32768.4
32768.5
32768.6
32768.7
32768.0
2.5 5.5
Typical Operating Characteristics
(VCC = +3.3V, TA = +25°C, unless otherwise noted.)
Pin Description
PIN
24 28
NAME FUNCTION
1 8 PWR
Active-Low Power-On Reset. This open-drain output pin is intended for use as an on/off
control for the system power. With VCC voltage removed from the device, PWR can be
automatically activated from a kickstart input by the KS pin or from a wake-up interrupt.
Once the system is powered on, the state of PWR can be controlled by bits in the control
registers. The PWR pin can be connected through a pullup resistor to a positive supply. For
5V operation, the voltage of the pullup supply should be no greater than 5.7V. For 3V
operation, the voltage on the pullup supply should be no greater than 3.9V.
2, 3 9, 10 X1, X2
Connections for Standard 32.768kHz Quartz Crystal. The internal oscillator circuitry is
designed for operation with a crystal having a specified load capacitance (CL) of 6pF or
12.5pF. Pin X1 is the input to the oscillator and can optionally be connected to an external
32.768kHz oscillator. The output of the internal oscillator, pin X2, is left unconnected if an
external oscillator is connected to pin X1. These pins are missing (N.C.) on the EDIP
package.
4–11 1217,
19, 20 AD0–AD7
Multiplexed Bidirectional Address/Data Bus. The addresses are presented during the first
portion of the bus cycle and latched into the device by the falling edge of ALE. Write data is
latched by the rising edge of WR. In a read cycle, the device outputs data during the latter
portion of the RD low. The read cycle is terminated and the bus returns to a high-impedance
state as RD transitions high.
12, 16 21, 22, 26 GND Ground
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
8 _____________________________________________________________________
Pin Description (continued)
PIN
24 28
NAME FUNCTION
13 23 CS
Active-Low Chip-Select Input. This pin must be asserted low during a bus cycle for the
device to be accessed. CS must be kept in the active state during RD and WR. Bus cycles
that take place without asserting CS latch addresses, but no access occurs.
14 24 ALE
Address Latch Enable Input, Active High. This input pin is used to demultiplex the
address/data bus. The falling edge of ALE causes the address to be latched within the
device.
15 25 WR Active-Low Write Input. This pin defines the period during which data is written to the
addressed register.
17 27 RD Active-Low Read Input. This pin identifies the period when the device drives the bus with
read data. It is an enable signal for the output buffers of the device.
18 28 KS
Active-Low Kickstart Input. When VCC is removed from the device, the system can be
powered on in response to an active-low transition on the KS pin, as might be generated
from a key closure. VBAUX must be present and auxiliary-battery-enable bit (ABE) must be
set to 1 if the kickstart function is used, and the KS pin must be pulled up to the VBAUX
supply. While VCC is applied, the KS pin can be used as an interrupt input. If not used, KS
must be grounded and ABE set to 0.
19 1 IRQ
Active-Low Interrupt Request. This pin is an active-low output that can be used as an
interrupt input to a processor. The IRQ output remains low as long as the status bit causing
the interrupt is present and the corresponding interrupt-enable bit is set. To clear the IRQ
pin, the application software must clear all enabled flag bits contributing to the pin’s active
state. When no interrupt conditions are present, the IRQ level is in the high-impedance
state. Multiple interrupting devices can be connected to an IRQ bus, provided that they are
all open drain. The IRQ pin requires an external pullup resistor to VCC.
20 2 VBAT
Connection for Primary Battery. This supply input is used to power the normal clock
functions when VCC is absent. Diodes placed in series between VBAT and the battery can
prevent proper operation. If VBAT is not required, the pin must be grounded. UL recognized
to ensure against reverse charging current when used with a lithium battery (www.maxim-
ic.com/qa/info/ul). This pin is missing (N.C.) on the EDIP package.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
_____________________________________________________________________ 9
Pin Description (continued)
PIN
24 28
NAME FUNCTION
21 3 RCLR
Active-Low RAM Clear Input. This pin is used to clear (set to logic 1) all the 114 bytes of
general-purpose RAM but does not affect the RAM associated with the real time clock or
extended RAM. RCLR may be invoked while the part is powered from any supply. The
RCLR function is designed to be used via a human interface (shorting to ground manually
or by a switch) and not to be driven with external buffers. This pin is internally pulled up. Do
not use an external pullup resistor on this pin.
22 4 VBAUX
Auxiliary Battery Input. Required for kickstart and wake-up functions. This input also
supports clock/calendar and user RAM if VBAT is at lower voltage or is not used. A standard
+3V lithium cell or other energy source can be used. Diodes placed in series between
VBAUX and the battery may prevent proper operation. UL recognized to ensure against
reverse charging current when used with a lithium battery (www.maxim-ic.com/qa/info/ul/).
For 3V VCC operation, VBAUX must be held between +2.5V and +3.7V. For 5V VCC operation,
VBAUX must be held between +2.5V and +5.2V. If VBAUX is not used it should be grounded
and the auxiliary-battery-enable bit bank 1, register 4BH, should = 0.
23 5 SQW
Square-Wave Output. When VCC rises above VPF, bits DV1 and E32k are set to 1. This
condition enables a 32kHz square-wave output. A square wave is output if either SQWE = 1
or E32k = 1. If E32k = 1, then 32kHz is output regardless of the other control bits. If E32k =
0, then the output frequency is dependent on the control bits in Register A. The SQW pin
can output a signal from one of 13 taps provided by the 15 internal divider stages of the
RTC. The frequency of the SQW pin can be changed by programming Register A, as shown
in Table 3. The SQW signal can be turned on and off using the SQWE bit in Register B or the
E32k bit in extended register 4Bh. A 32kHz square wave is also available when VCC is less
than VPF if E32k = 1, ABE = 1, and voltage is applied to the VBAUX pin. When disabled,
SQW is high impedance when VCC is below VPF.
24 6, 7 VCC
DC Power Pin for Primary Power Supply. When VCC is applied within normal limits, the
device is fully accessible and data can be written and read. When VCC is below VPF reads
and writes are inhibited.
2, 3, 16,
20
(DS17x87
only)
11, 18 N.C. No Connection
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
10 ____________________________________________________________________
X1
OSCILLATOR
POWER
CONTROL
X2
DS17x87
ONLY
VBAT
GND
DIVIDE
BY 8
REGISTERS A, B, C, D
CLOCK/CALENDAR
UPDATE LOGIC
EXTENDED
USER RAM
2k/4k/8k
BYTES
SELECT
EXTENDED RAM ADDR/
DATA REGISTERS
EXTENDED CONTROL/
STATUS REGISTERS
64-BIT SERIAL NUMBER
CENTURY COUNTER
DATE ALARM
RTC ADDRESS-2
RTC ADDRESS-3
DIVIDE BY
64
DIVIDE BY
64
16:1 MUX
SQUARE-
WAVE
GENERATOR
SQW
IRQ
PWR
KS
RLCR
IRQ
GENERATOR
VCC
VBAUX
BUS
INTERFACE
CS
WR
RD
ALE
AD0–AD7
CLOCK/CALENDAR AND
ALARM REGISTERS
BUFFERED CLOCK/
CALENDAR AND ALARM
REGISTERS
USER RAM
114 BYTES
RAM
CLEAR
LOGIC
DS17x85/87
Figure 1. Functional Diagram
Detailed Description
The DS17x85 is a successor to the DS1285 real-time
clock (RTC). The device provides 18 bytes of real-time
clock/calendar, alarm, and control/status registers and
114 bytes of nonvolatile battery-backed RAM. The
device also provides additional extended RAM in either
2k/4k/8kbytes (DS17285/DS17485/DS17885). A time-
of-day alarm, six maskable interrupts with a common
interrupt output, and a programmable square-wave
output are available. It also operates in either 24-hour
or 12-hour format with an AM/PM indicator. A precision
temperature-compensated circuit monitors the status of
VCC. If a primary power-supply failure is detected, the
device automatically switches to a backup supply. The
backup supply input supports a primary battery, such
as a lithium coin cell. The device is accessed by a mul-
tiplexed address/data bus.
Oscillator Circuit
The DS17x85 uses an external 32.768kHz crystal. The
oscillator circuit does not require any external resistors
or capacitors to operate. Table 1 specifies several
crystal parameters for the external crystal, and Figure 2
shows a functional schematic of the oscillator circuit.
The oscillator is controlled by an enable bit in the con-
trol register. Oscillator startup times are highly depen-
dent upon crystal characteristics, PC board leakage,
and layout. High ESR and excessive capacitive loads
are the major contributors to long startup times. A cir-
cuit using a crystal with the recommended characteris-
tics and proper layout usually starts within one second.
An external 32.768kHz oscillator can also drive the
DS17x85. In this configuration, the X1 pin is connected
to the external oscillator signal and the X2 pin is left
unconnected.
Clock Accuracy
The accuracy of the clock is dependent upon the accu-
racy of the crystal and the accuracy of the match
between the capacitive load of the oscillator circuit and
the capacitive load for which the crystal was trimmed.
Additional error will be added by crystal frequency drift
caused by temperature shifts. External circuit noise
coupled into the oscillator circuit may result in the clock
running fast. Figure 3 shows a typical PC board layout
for isolation of the crystal and oscillator from noise.
Refer to
Application Note 58: Crystal Considerations
with Dallas Real-Time Clocks
for detailed information.
Clock Accuracy (DS17287,
DS17487, and DS17887)
The encapsulated DIP (EDIP) modules are trimmed at
the factory to ±1 minute per month accuracy at 25°C.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
____________________________________________________________________ 11
PARAMETER SYMBOL MIN TYP MAX UNITS
Nominal fO32.768 kHz
Series
Resistance ESR 50 kΩ
Load
Capacitance CL6 or
12.5 pF
Table 1. Crystal Specifications* (DS17x85
Only)
*
The crystal, traces, and crystal input pins should be isolated
from RF generating signals. Refer to Application Note 58:
Crystal Considerations for Dallas Real-Time Clocks for addi-
tional specifications.
COUNTDOWN
CHAIN
X1 X2
CRYSTAL
CL1C
L2RTC REGISTERS
DS17285/87
DS17485/87
DS17885/87
Figure 2. Oscillator Circuit Showing Internal Bias Network
LOCAL GROUND PLANE (TOP LAYER)
CRYSTAL
GND
X2
X1
NOTE: AVOID ROUTING SIGNAL LINES
IN THE CROSSHATCHED AREA
(UPPER LEFT QUADRANT) OF
THE PACKAGE UNLESS THERE IS
A GROUND PLANE BETWEEN THE
SIGNAL LINE AND THE DEVICE PACKAGE.
Figure 3. Layout Example
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Power-Down/Power-Up
Considerations
The RTC function continues to operate, and all the
RAM, time, calendar, and alarm memory locations
remain nonvolatile regardless of the level of the VCC
input. VBAT or VBAUX must remain within the minimum
and maximum limits when VCC is not applied. When
VCC falls below VPF, the device inhibits all access,
putting the part into a low-power mode. When VCC is
applied and exceeds VPF (power-fail trip point), the
device becomes accessible after tREC, if the oscillator
is running and the oscillator countdown chain is not in
reset (Register A). This time period allows the system to
stabilize after power is applied. If the oscillator is not
enabled, the oscillator enable bit is enabled on power-
up, and the device becomes immediately accessible.
Power Control
The power control function is provided by a precise,
temperature-compensated voltage reference and a
comparator circuit that monitors the VCC level. The
device is fully accessible and data can be written and
read when VCC is greater than VPF. However, when
VCC falls below VPF, the device inhibits read and write
access. If VPF is less than VBAT, the device power is
switched from VCC to the higher of VBAT or VBAUX
when VCC drops below VPF. If VPF is greater than the
higher of VBAT or VBAUX, the device power is switched
from VCC to the higher of VBAT or VBAUX when VCC
drops below the higher backup source. The registers
are maintained from the VBAT or VBAUX source until
VCC is returned to nominal levels. After VCC returns
above VPF, read and write access is allowed after tREC.
Time, Calendar, and Alarm
Locations
The time and calendar information is obtained by read-
ing the appropriate register bytes. The time, calendar,
and alarm are set or initialized by writing the appropri-
ate register bytes. The contents of the 12 time, calen-
dar, and alarm bytes can be either binary or
binary-coded decimal (BCD) format. Tables 3A and 3B
show the BCD and binary formats of the 12 time, date,
and alarm registers, control registers A to D, plus the
two extended registers that reside in bank 1 only (bank
0 and bank 1 switching is explained later in this text).
The day-of-week register increments at midnight, incre-
menting from 1 through 7. The day-of-week register is
used by the daylight saving function, and so the value
1 is defined as Sunday. The date at the end of the
month is automatically adjusted for months with fewer
than 31 days, including correction for leap years.
Before writing the internal time, calendar, and alarm
registers, the SET bit in Register B should be written to
logic 1 to prevent updates from occurring while access
is being attempted. In addition to writing the 12 time,
calendar, and alarm registers in a selected format
(binary or BCD), the data mode bit (DM) of Register B
must be set to the appropriate logic level. All 12 time,
calendar, and alarm bytes must use the same data
mode. The set bit in Register B should be cleared after
the data mode bit has been written to allow the real
time clock to update the time and calendar bytes. Once
initialized, the real time clock makes all updates in the
selected mode. The data mode cannot be changed
without reinitializing the 12 data bytes. Tables 3A and
3B show the BCD and binary formats of the 12 time,
calendar, and alarm locations.
The 24-12 bit cannot be changed without reinitializing
the hour locations. When the 12-hour format is selected,
the high order bit of the hours byte represents PM when
it is logic 1. The time, calendar, and alarm bytes are
always accessible because they are double-buffered.
Once per second, the eight bytes are advanced by one
second and checked for an alarm condition.
If a read of the time and calendar data occurs during
an update, a problem exists where seconds, minutes,
hours, etc., may not correlate. The probability of read-
ing incorrect time and calendar data is low. Several
methods of avoiding any possible incorrect time and
calendar reads are covered later in this text.
Real-Time Clocks
12 ____________________________________________________________________
Table 2. Power Control
SUPPLY CONDITION READ/WRITE
ACCESS POWERED BY
VCC < VPF, VCC <
(VBAT | VBAUX)No VBAT or VBAUX
VCC < VPF, VCC >
(VBAT | VBAUX)No VCC
VCC > VPF, VCC <
(VBAT | VBAUX)Yes VCC
VCC > VPF, VCC >
(VBAT | VBAUX)Yes VCC
The alarm bytes can be used in two ways. First, when
the alarm time is written in the appropriate hours, min-
utes, and seconds alarm locations, the alarm interrupt
is initiated at the specified time each day, if the alarm
enable bit is high. In this mode, the “0” bits in the alarm
registers and the corresponding time registers must
always be written to 0 (see Table 3A and 3B). Writing
the 0 bits in the alarm and/or time registers to 1 can
result in undefined operation.
The second use condition is to insert a “don’t care”
state in one or more of the alarm bytes. The don’t care
code is any hexadecimal value from C0 to FF. The two
most significant bits of each byte set the don’t care
condition when at logic 1. An alarm will be generated
each hour when the “don’t care” bits are set in the
hours byte. Similarly, an alarm is generated every
minute with don’t care codes in the hours and minute
alarm bytes. An alarm is generated every second with
don’t care codes in the hours, minutes, and seconds
alarm bytes.
All 128 bytes can be directly written or read except for
the following:
1) Registers C and D are read-only.
2) Bit 7 of register A is read-only.
3) The MSB of the seconds byte is read-only.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
____________________________________________________________________ 13
Table 3A. Time, Calendar, and Alarm Data Modes—BCD Mode (DM = 0)
Note: Unless otherwise specified, the state of the registers is not defined when power is first applied. Except for the seconds regis-
ter, 0 bits in the time and date registers can be written to 1, but can be modified when the clock updates. 0 bits should always be
written to 0 except for alarm mask bits.
ADDRESS BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 FUNCTION RANGE
00h 0 10 Seconds Seconds Seconds 00–59
01h 0 10 Seconds Seconds Seconds Alarm 00–59
02h 0 10 Minutes Minutes Minutes 00–59
03h 0 10 Minutes Minutes Minutes Alarm 00–59
AM/PM 0 10 Hour
04h 0010 Hour Hours Hours 1–12 +AM/PM
00–23
AM/PM 0 10 Hour
05h 0010 Hour Hours Hours Alarm 1–12 +AM/PM
00–23
06h 0 0 0 0 0 Day Day 01–07
07h 0 0 10 Date Date Date 01–31
08h 0 0 0 10 Month Month Month 01–12
09h 10 Year Year Year 00–99
0Ah UIP DV2 DV1 DV0 RS3 RS2 RS1 RS0 Control
0Bh SET PIE AIE UIE SQWE DM 24/12 DSE Control
0Ch IRQF PF AF UF 0 0 0 0 Control
0Dh VRT 0 0 0 0 0 0 0 Control
Bank 1, 48h 10 Century Century Century 00–99
Bank 1, 49h 10 Date Date Date Alarm 01–31
Bit 7: Update In Progress (UIP). This bit is a status
flag that can be monitored. When the UIP bit is 1, the
update transfer will soon occur. When UIP is 0, the
update transfer does not occur for at least 244µs. The
time, calendar, and alarm information in RAM is fully
available for access when the UIP bit is 0. The UIP bit is
read-only. Writing the SET bit in Register B to 1 inhibits
any update transfer and clears the UIP status bit.
Bits 6, 5, and 4: DV2, DV1, and DV0. These bits are
used to turn the oscillator on or off and to reset the
countdown chain. A pattern of 01X is the only combina-
tion of bits that turns the oscillator on and allows the RTC
to keep time. A pattern of 11X enables the oscillator but
holds the countdown chain in reset. The next update
occurs at 500ms after a pattern of 01X is written to DV0,
DV1, and DV2. DV0 is used to select bank 0 or bank 1 as
defined in Table 5. When DV0 is set to 0, bank 0 is
selected. When DV0 is set to 1, bank 1 is selected.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
14 ____________________________________________________________________
Table 3B. Time, Calendar, and Alarm Data Modes—Binary Mode (DM = 1)
Note: Unless otherwise specified, the state of the registers is not defined when power is first applied. Except for the seconds regis-
ter, 0 bits in the time and date registers can be written to 1, but can be modified when the clock updates. 0 bits should always be
written to 0 except for alarm mask bits.
ADDRESS BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 FUNCTION RANGE
00h 0 0 Seconds Seconds 00–3B
01h 0 0 Seconds Seconds Alarm 00–3B
02h 0 0 Minutes Minutes 00–3B
03h 0 0 Minutes Minutes Alarm 00–3B
AM/PM 0 Hours
04h 000 Hours Hours 1–0C +AM/PM
00–17
AM/PM 0 Hours
05h 000 Hours Hours Alarm 1–0C +AM/PM
00–17
06h 0 0 0 0 0 Day Day 01–07
07h 0 0 0 Date Date 01–1F
08h 0 0 0 0 Month Month 01–0C
09h 0 Year Year 00–63
0Ah UIP DV2 DV1 DV0 RS3 RS2 RS1 RS0 Control
0Bh SET PIE AIE UIE SQWE DM 24/12 DSE Control
0Ch IRQF PF AF UF 0 0 0 0 Control
0Dh VRT 0 0 0 0 0 0 0 Control
Bank 1, 48h 10 Century Century Century 00–63
Bank 1, 49h 10 Date Date Date Alarm 01–1F
Control Registers
The four control registers (A, B, C, and D) reside in
both bank 0 and bank 1. These registers are accessi-
ble at all times, even during the update cycle.
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
UIP DV2 DV1 DV0 RS3 RS2 RS1 RS0
Register A (0Ah)
MSB LSB
Bits 3 to 0: Rate Selector Bits (RS3 to RS0). These
four rate-selection bits select one of the 13 taps on the
15-stage divider or disable the divider output. The tap
selected can be used to generate an output square
wave (SQW pin) and/or a periodic interrupt. The user
can do one of the following:
1) Enable the interrupt with the PIE bit;
2) Enable the SQW output pin with the SQWE or E32k
bits;
3) Enable both at the same time and the same rate; or
4) Enable neither.
Table 4 lists the periodic interrupt rates and the square-
wave frequencies that can be chosen with the RS bits.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
____________________________________________________________________ 15
Table 4. Periodic Interrupt Rate and Square-Wave Output Frequency
EXT REG B SELECT BITS REGISTER A
E32K RS3 RS2 RS1 RS0
tPI PERIODIC INTERRUPT
RATE SQW OUTPUT FREQUENCY
0 0 0 0 0 None None
0 0 0 0 1 3.90625ms 256Hz
0 0 0 1 0 7.8125ms 128Hz
0 0 0 1 1 122.070µs 8.192kHz
0 0 1 0 0 244.141µs 4.096kHz
0 0 1 0 1 488.281µs 2.048kHz
0 0 1 1 0 976.5625µs 1.024kHz
0 0 1 1 1 1.953125ms 512Hz
0 1 0 0 0 3.90625ms 256Hz
0 1 0 0 1 7.8125ms 128Hz
0 1 0 1 0 15.625ms 64Hz
0 1 0 1 1 31.25ms 32Hz
0 1 1 0 0 62.5ms 16Hz
0 1 1 0 1 125ms 8Hz
0 1 1 1 0 250ms 4Hz
0 1 1 1 1 500ms 2Hz
1 X X X X * 32.768kHz
*
RS3 to RS0 determine periodic interrupt rates as listed for E32K = 0.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Bit 7: SET. When the SET bit is 0, the update transfer
functions normally by advancing the counts once per
second. When the SET bit is written to 1, any update
transfer is inhibited, and the program can initialize the
time and calendar bytes without an update occurring in
the midst of initializing. Read cycles can be executed in
a similar manner. SET is a read/write bit and is not
affected by any internal functions of the DS17x85.
Bit 6: Periodic Interrupt Enable (PIE). This bit is a
read/write bit that allows the periodic interrupt flag (PF)
bit in Register C to drive the IRQ pin low. When PIE is
set to 1, periodic interrupts are generated by driving
the IRQ pin low at a rate specified by the RS3–RS0 bits
of Register A. A 0 in the PIE bit blocks the IRQ output
from being driven by a periodic interrupt, but the PF bit
is still set at the periodic rate. PIE is not modified by
any internal DS17x85 functions.
Bit 5: Alarm Interrupt Enable (AIE). This bit is a
read/write bit that, when set to 1, permits the alarm flag
(AF) bit in Register C to assert IRQ. An alarm interrupt
occurs for each second that the three time bytes equal
the three alarm bytes, including a don’t care alarm
code of binary 11XXXXXX. When the AIE bit is set to 0,
the AF bit does not initiate the IRQ signal. The internal
functions of the DS17x285/87 do not affect the AIE bit.
Bit 4: Update-Ended Interrupt Enable (UIE). This bit is
a read/write bit that enables the update-end flag (UF)
bit in Register C to assert IRQ. The SET bit going high
clears the UIE bit.
Bit 3: Square-Wave Enable (SQWE). When this bit is
set to 1 and E32k = 0, a square-wave signal at the fre-
quency set by RS3–RS0 is driven out on the SQW pin.
When the SQWE bit is set to 0 and E32k = 0, the SQW
pin is held low. SQWE is a read/write bit. SQWE is set
to 1 when VCC is powered up.
Bit 2: Data Mode (DM). This bit indicates whether time
and calendar information is in binary or BCD format.
The program sets the DM bit to the appropriate format
and can be read as required. This bit is not modified by
internal functions. A 1 in DM signifies binary data, while
a 0 in DM specifies binary-coded decimal (BCD) data.
Bit 1: 24/12 Control (24/12). This bit establishes the
format of the hours byte. A 1 indicates the 24-hour
mode and a 0 indicates the 12-hour mode. This bit is
read/write and is not affected by internal functions.
Bit 0: Daylight Saving Enable (DSE). This bit is a
read/write bit that enables two daylight saving adjust-
ments when DSE is set to 1. On the first Sunday in
April, the time increments from 1:59:59AM to
3:00:00AM. On the last Sunday in October when the
time first reaches 1:59:59AM, it changes to 1:00:00AM.
When DSE is enabled, the internal logic tests for the
first/last Sunday condition at midnight. If the DSE bit is
not set when the test occurs, the daylight saving func-
tion does not operate correctly. These adjustments do
not occur when the DSE bit is zero. This bit is not
affected by internal functions.
Real-Time Clocks
16 ____________________________________________________________________
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
SET PIE AIE UIE SQWE DM 24/12 DSE
Register B (0Bh)
MSB LSB
Bit 7: Interrupt Request Flag (IRQF). This bit is set to
1 when any of the following are true:
PF = PIE = 1 WF = WIE = 1
AF = AIE = 1 KF = KSE = 1
UF = UIE = 1 RF = RIE = 1
Any time the IRQF bit is 1, the IRQ pin is driven low.
Flag bits PF, AF, and UF are cleared after reading
Register C.
Bit 6: Periodic Interrupt Flag (PF). This is a read-only
bit that is set to 1 when an edge is detected on the
selected tap of the divider chain. The RS3–RS0 bits
establish the periodic rate. PF is set to 1 independent
of the state of the PIE bit. When both PF and PIE are 1s,
the IRQ signal is active and sets the IRQF bit. Reading
Register C clears this bit.
Bit 5: Alarm Interrupt Flag (AF). A 1 in this bit indicates
that the current time has matched the alarm time. If the
AIE bit is also 1, the IRQ pin goes low and a 1 appears in
the IRQF bit. Reading Register C clears this bit.
Bit 4: Update-Ended Interrupt Flag (UF). This bit is
set after each update cycle. When the UIE bit is set to
1, the 1 in UF causes the IRQF bit to be 1, which
asserts IRQ. Reading Register C clears this bit.
Bits 3 to 0: Unused. These unused bits always read 0
and cannot be written.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
____________________________________________________________________ 17
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
IRQF PF AF UF 0000
Register C (0Ch)
MSB LSB
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
VRT0000000
Register D (0Dh)
MSB LSB
Register D (0Dh)
Bit 7: Valid RAM and Time (VRT). This bit indicates
the condition of the battery connected to the VBAT and
VBAUX pin. If either supply is above the internal voltage
threshold, VRTTRIP, the bit will be high. This bit is not
writeable and should always be a 1 when read. If a 0 is
ever present, an exhausted internal lithium energy
source is indicated and both the contents of the RTC
data and RAM data are questionable.
Bits 6 to 0: Unused. These bits cannot be written and,
when read, always read 0.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Nonvolatile RAM
The user RAM bytes are not dedicated to any special
function within the DS17x85. They can be used by the
processor program as battery-backed memory and are
fully available during the update cycle.
The user RAM is divided into two separate memory
banks. When the bank 0 is selected, the 14 real-time
clock registers and 114 bytes of user RAM are accessi-
ble. When bank 1 is selected, an additional 2kbytes,
4kbytes, or 8kbytes of user RAM are accessible
through the extended RAM address and data registers.
Interrupts
The RTC includes six separate, fully automatic sources
of interrupt for a processor:
1) Alarm Interrupt
2) Periodic Interrupt
3) Update-Ended Interrupt
4) Wake-Up Interrupt
5) Kickstart Interrupt
6) RAM Clear Interrupt
The conditions that generate each of these indepen-
dent interrupt conditions are described in detail in other
sections of this data sheet. This section describes the
overall control of the interrupts.
The application software can select which interrupts, if
any, are to be used. There are 6 bits, including 3 bits in
Register B and 3 bits in Extended Register 4B, that
enable the interrupts. The extended register locations
are described later. Writing logic 1 to an interrupt-
enable bit permits that interrupt to be initiated when the
event occurs. A logic 0 in the interrupt-enable bit pro-
hibits the IRQ pin from being asserted from that interrupt
condition. If an interrupt flag is already set when an
interrupt is enabled, IRQ is immediately set at an active
level, although the event initiating the interrupt condition
might have occurred much earlier. Therefore, there are
cases where the software should clear these earlier
generated interrupts before first enabling new interrupts.
When an interrupt event occurs, the relating flag bit is
set to logic 1 in Register C or in Extended Register 4A.
These flag bits are set regardless of the setting of the
corresponding enable bit located either in Register B or
in Extended Register 4B. The flag bits can be used in a
polling mode without enabling the corresponding
enable bits.
However, care should be taken when using the flag bits
of Register C as they are automatically cleared to 0
immediately after they are read. Double latching is
implemented on these bits so that set bits remain sta-
ble throughout the read cycle. All bits that were set are
cleared when read and new interrupts that are pending
during the read cycle are held until after the cycle is
completed. One, two, or three bits can be set when
reading Register C. Each used flag bit should be exam-
ined when read to ensure that no interrupts are lost.
The flag bits in Extended Register 4A are not automati-
cally cleared following a read. Instead, each flag bit
can be cleared to 0 only by writing 0 to that bit.
When using the flag bits with fully enabled interrupts,
the IRQ line is driven low when an interrupt flag bit is
set and its corresponding enable bit is also set. IRQ is
held low as long as at least one of the six possible
interrupt sources has its flag and enable bits both set.
The IRQF bit in Register C is 1 whenever the IRQ pin is
being driven low as a result of one of the six possible
active sources. Therefore, determination that the
DS17x85/DS17x87 initiated an interrupt is accom-
plished by reading Register C and finding IRQF = 1.
IRQF remains set until all enabled interrupt flag bits are
cleared to 0.
Oscillator Control Bits
A pattern of 01X in bits 4 to 6 of Register A turns the
oscillator on and enables the countdown chain. A pat-
tern of 11X (DV2 = 1, DV1 = 1, DV0 = X) turns the oscil-
lator on, but holds the countdown chain of the oscillator
in reset. All other combinations of bits 4 to 6 keep the
oscillator off.
When the DS17x87 is shipped from the factory, the
internal oscillator is turned off. This feature prevents the
lithium energy cell from being used until it is installed in
a system.
Square-Wave Output Selection
Thirteen of the 15 divider taps are made available to a
1-of-16 multiplexer, as shown in Figure 1. The square
wave and periodic interrupt generators share the out-
put of the multiplexer. The RS0–RS3 bits in Register A
establish the output frequency of the multiplexer. These
frequencies are listed in Table 4. Once the frequency is
selected, the output of the SQW pin can be turned on
and off under program control with the square-wave
enable bit (SQWE).
If E32K = 0, the square-wave output is determined by
the RS3 to RS0 bits. If E32K = 1, a 32kHz square wave
is output on the SQW pin, regardless of the RS3 to RS0
bits’ state. If E32K = ABE = 1 and a valid voltage is
applied to VBAUX, a 32kHz square wave is output on
SQW when VCC is below VTP.
Real-Time Clocks
18 ____________________________________________________________________
Periodic Interrupt Selection
The periodic interrupt causes the IRQ pin to go to an
active state from once every 500ms to once every
122µs. This function is separate from the alarm inter-
rupt, which can be output from once per second to
once per day. The periodic interrupt rate is selected
using the same Register A bits that select the square-
wave frequency (see Table 4). Changing the Register A
bits affects both the square-wave frequency and the
periodic interrupt output. However, each function has a
separate enable bit in Register B. The SQWE and E32k
bits control the square-wave output. Similarly, the peri-
odic interrupt is enabled by the PIE bit in Register B.
The periodic interrupt can be used with software coun-
ters to measure inputs, create output intervals, or await
the next needed software function.
Update Cycle
The DS17x85 executes an update cycle once per sec-
ond regardless of the SET bit in Register B. When the
SET bit in Register B is set to 1, the user copy of the
double-buffered time, calendar, and alarm bytes is
frozen and does not update as the time increments.
However, the time countdown chain continues to
update the internal copy of the buffer. This feature
allows time to maintain accuracy independent of read-
ing or writing the time, calendar, and alarm buffers, and
also guarantees that time and calendar information is
consistent. The update cycle also compares each
alarm byte with the corresponding time byte and issues
an alarm if a match or if a don’t care code is present in
all alarm locations.
There are three methods that can handle access of the
RTC that avoid any possibility of accessing inconsistent
time and calendar data. The first method uses the
update-ended interrupt. If enabled, an interrupt occurs
after every update cycle that indicates that over 999ms
are available to read valid time and date information. If
this interrupt is used, the IRQF bit in Register C should
be cleared before leaving the interrupt routine.
A second method uses the update-in-progress (UIP) bit
in Register A to determine if the update cycle is in
progress. The UIP bit pulses once per second. After
the UIP bit goes high, the update transfer occurs 244µs
later. If a low is read on the UIP bit, the user has at least
244µs before the time/calendar data is changed.
Therefore, the user should avoid interrupt service rou-
tines that would cause the time needed to read valid
time/calendar data to exceed 244µs.
The third method uses a periodic interrupt to determine
if an update cycle is in progress. The UIP bit in Register
A is set high between the setting of the PF bit in
Register C (see Figure 4). Periodic interrupts that occur
at a rate of greater than tBUC allow valid time and date
information to be reached at each occurrence of the
periodic interrupt. The reads should be complete within
1 (tPI/2 + tBUC) to ensure that data is not read during
the update cycle.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
____________________________________________________________________ 19
UIP
UF
PF
tBUC = DELAY TIME BEFORE UPDATE CYCLE = 244μs.
1 SECOND
tPI
tPI/2 tPI/2
tBUC
Figure 4. UIP and Periodic Interrupt Timing
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Extended Functions
The extended functions provided by the DS17x85/
DS17x87 that are new to the RAMified RTC family are
accessed by a software-controlled bank-switching
scheme, as illustrated in Table 5. In bank 0, the
clock/calendar registers and 50 bytes of user RAM are
in the same locations as for the DS1287. As a result,
existing routines implemented within BIOS, DOS, or
application software packages can gain access to the
DS17x85/DS17x87 clock registers with no changes.
Also in bank 0, an extra 64 bytes of RAM are provided
at addresses just above the original locations for a total
of 114 directly addressable bytes of user RAM.
When bank 1 is selected, the clock/calendar registers
and the original 50 bytes of user RAM still appear as
bank 0. However, the extended registers that provide
control and status for the extended functions are
accessed in place of the additional 64 bytes of user
RAM. The major extended functions controlled by the
extended registers are listed below:
64-Bit Silicon Serial Number
Century Counter
RTC Write Counter
Date Alarm
Auxiliary Battery Control/Status
Wake-Up
Kickstart
RAM Clear Control/Status
Extended RAM Access
The bank selection is controlled by the state of the DV0
bit in register A. To access bank 0 the DV0 bit should
be written to a 0. To access bank 1, DV0 should be
written to 1. Register locations designated as reserved
in the bank 1 map are reserved for future use by Dallas
Semiconductor. Bits in these locations cannot be writ-
ten and return a 0 if read.
Silicon Serial Number
A unique 64-bit lasered serial number is located in
bank 1, registers 40h–47h. This serial number is divid-
ed into three parts. The first byte in register 40h con-
tains a model number to identify the device type of the
DS17x85/DS17x87. Registers 41h–46h contain a
unique binary number. Register 47h contains a CRC
byte used to validate the data in registers 40h–46h. The
CRC polynomial is X8+ X5+ X4+ 1. See Figure 5. All 8
bytes of the serial number are read-only registers. The
DS17x85/DS17x87 is manufactured such that no two
devices contain an identical number in locations
41h–47h.
Real-Time Clocks
20 ____________________________________________________________________
1ST
STAGE
2ND
STAGE
3RD
STAGE
4TH
STAGE
5TH
STAGE
6TH
STAGE
7TH
STAGE
8TH
STAGE
INPUT DATA
POLYNOMIAL = X8 + X5 + X4 + 1
X0 X1 X2 X3 X4 X5 X6 X7 X8
Figure 5. CRC Polynomial
DEVICE MODEL NUMBER
DS17285/87 72h
DS17485/87 74h
DS17885/87 78h
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
____________________________________________________________________ 21
Table 5. Extended Bank Register Bank Definition
Note: Reserved bits can be written to any value, but always read back as zeros.
Bank 0 Bank 1
DV0 = 0 DV0 = 1
00h
0Dh
Timekeeping and Control
00h
0Dh
Timekeeping and Control
0Eh
3Fh
50 Bytes – User RAM
0Eh
3Fh
50 Bytes – User RAM
40h 40h Model Number Byte
41h 1st Byte Serial Number
42h 2nd Byte Serial Number
43h 3rd Byte Serial Number
44h 4th Byte Serial Number
45h 5th Byte Serial Number
46h 6th Byte Serial Number
47h CRC Byte
48h Century Byte
49h Date Alarm
4Ah Extended Control Register 4A
4Bh Extended Control Register 4B
4Ch Reserved
4Dh Reserved
4Eh RTC Address – 2
4Fh RTC Address – 3
64 Bytes – User RAM 50h Extended RAM Address LSB
51h Extended RAM Address MSB
52h Reserved
53h Extended RAM Data Port
54h Reserved
55h Reserved
56h Reserved
57h Reserved
58h Reserved
59h Reserved
5Ah Reserved
5Bh Reserved
5Ch Reserved
5Dh Reserved
5Eh RTC Write Counter
7Fh
5Fh
7Fh
Reserved
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
Real-Time Clocks
22 ____________________________________________________________________
Century Counter
A register has been added in bank 1, location 48H, to
keep track of centuries. The value is read in either bina-
ry or BCD according to the setting of the DM bit.
RTC Write Counter
An 8-bit counter located in extended register bank 1,
5Eh, counts the number of times the RTC is written to.
This counter is incremented on the rising edge of the
WR signal every time that the CS signal qualifies it. This
counter is a read-only register and rolls over after 256
RTC write pulses. This counter can be used to deter-
mine if and how many RTC writes have occurred since
the last time this register was read.
Auxiliary Battery
The VBAUX input is provided to supply power from an
auxiliary battery for the DS17x85/DS17x87 kickstart,
wake-up, and SQW output in the absence of VCC func-
tions. This power source must be available to use these
auxiliary functions when no VCC is applied to the device.
The auxiliary battery enable (ABE; bank 1, register
04BH) bit in Extended Control Register 4B is used to
turn the auxiliary battery on and off for the above func-
tions in the absence of VCC. When set to 1, VBAUX bat-