Lucent Technologies Inc. 15
Prelim inary Data Sheet
February 2001 T8535/T8536 Quad Programmable Codec
Functional Description
Clocking Considerations
The PCM bus uses BCLK as the bit clock and the one-
going edge of FS to determine the location of the
beginning of a frame. These two clocks must be
derived from the same source. Internally, the device
develops all the internal clocks with a phase-locked
loop that uses BCLK as the timing source. BCLK and
FS must be continuously present and without gaps in
order for the device to operate correctly.
DCLK is used to clock the internal serial interface and
may be asynchronous to the other clocks. There is no
need to derive this clock from the same source as the
other clocks. The serial bus may be operated at any
speed up to 4.096 Mbits/s. DCLK can be gapped, how-
ever , additional clock cycles are required in and around
the command frame to process data, and during and
after a hardware or a software reset to ensure com-
plete clearing of internal logic. There is no limit on the
number of devices on the same serial bus.
The Control Interface
The device is controlled via a series of memory loca-
tions accessed by a serial data connection to the exter-
nal master controller. This interface operates using the
chip select lead to enable transmission of information.
All chip functions are enabled or disabled by setting or
clearing bits in the control memory. Filter coefficients
and gain adjustments are also stored in this memory.
The codec has both a serial input lead and a serial out-
put lead. These may be used individually for a 4-wire
serial interface, or tied together for a 2-wire interface.
The line driver circuitry is capable of driving relatively
high currents so that in the event that the line is long
enough to show significant transmission line effects, it
can be terminated in the characteristic impedance at
each end with resistors to VCC and ground.
All data transfers on the serial bus are byte oriented
with the least significant bit (shown in this data sheet as
bit 0) transmitted first, followed by the more significant
bits. For data fields, the least significant byte of the first
data byte is transmitted first, followed by the more sig-
nificant bytes, each byte transmitted LSB first. This for-
mat is compatible with the serial port on most
microcontrollers.
Modes
There are two different modes of operation for the
serial interface, the normal mode and the byte-by-byte
mode. These two modes differ in the manner in which
CS is used to control the transfer. Note that the CS
lead is used to control the transfer of serial data from
master controller to slave codec and in the reverse
direction.
In normal mode, (INTS pin open) the CS lead must go
low for the duration of the transfer . The only error check
performed by the codec is to verify that CS is low for an
integral number of bytes. Detection of an active (active-
low) chip select for other than an integral multiple of
8 bits results in the operation being terminated. The
next active excursion of chip select will be interpreted
as a new command; hence, the serial I/O interface can
always be initialized by asserting CS for a number of
clock periods th at is not an integral multiple of 8. CS is
captured using DCLK, so DCLK must be transitioned to
perform this initialization. Undefined command codes
are reserved for future use and may cause unwanted
oper ation of the device.
The byte-by-byte mode (INTS pin tied to ground) uses
CS to control each byte of the transfer. In this mode,
CS goes low for exactly 8 bits at a time, corresponding
to a 1-byte transfer either to or from the codec chip.
Repeated transitions of CS are used to control subse-
quent bytes of data to/from the codec. For a write com-
mand in this mode, CS must go low for each byte of the
transfer until the transfer is complete. For a read com-
mand, CS will go low for each of the 3 bytes of the read
command transferred to the device, then low again for
each byte to be read. Notice that the total number of
bytes transferred (and excursions on CS) is N + 3,
where N is the number of bytes to be read in the com-
mand. This mode of operation is useful in cases where
the master is a microprocessor with a built-in UART
that transfers 1 byte at a time. Error detection is limited
to detection of an active CS for other than an integral
multiple of 8 bits. Recovery is the same as normal
mode. Note that the clock phase is shifted in this mode.
Flow control can be accomplished by suspending the
transitions on DCLK by holding either state. During the
data transfer, CS must remain low while clock transi-
tions are suspended with DCLK in either state.