Cirrus Logic CS5460A, Manual

Page: 23 / 64

CS5460A
DS284PP4 23
3.2 Serial Port Interface
The CS5460A’s slave-mode serial interface con-
sists of two control lines and two data lines, which
have the following pin-names: CS, SCLK, SDI,
SDO. Each control line is now described.
CS Chip Select (input pin), is the control line
which enables access to the serial port. When CS
is set to logic 1, the SDI, SDO, and SCLK pins will
be held at high impedance. When the CS pin is set
to logic 0, the SDI, SDO, and SCLK pins have the
following functionality:
SDI Serial Data In (input pin), is the user-generat-
ed signal used to transfer (send) data/command/ad-
dress/etc. bits into the device.
SDO Serial Data Out (output pin), is the data sig-
nal used to read output data bits from the device’s
registers.
SCLK Serial Clock (input pin), is the serial
bit-clock which controls the transfer rate of data
to/from the ADC’s serial port. To accommodate
opto-isolators,
SCLK is designed with a
Schmitt-trigger input to allow an opto-isolator with
slower rise and fall times to directly drive the pin.
Additionally, SDO is capable of sinking or sourc-
ing up to 5 mA to directly drive an opto-isolator
LED. SDO will have less than a 400 mV loss in the
drive voltage when sinking or sourcing 5 mA.
3.3 Serial Read and Write
The state machine decodes the command word as it
is received. Data is written to and read from the
CS5460A by using the Register Read/Write com-
mand. Figure 1 illustrates the serial sequence nec-
essary to write to or read from the serial port
buffers. As shown in Figure 1, a transfer of data is
always initiated by sending the appropriate 8-bit
command (MSB first) to the serial port (SDI pin).
It is important to note that some commands use in-
formation from the Cycle-Count Register and Con-
figuration Register to perform the function. For
those commands, it is important that the correct in-
formation is written to those registers first.
3.3.1 Register Write
When a command involves a write operation, the
serial port will continue to clock in the data bits
(MSB first) on the SDI pin for the next 24 SCLK
cycles.
Command words instructing a register
write must be followed by 24 bits of data. For in-
stance, to write the Configuration Register, the user
would transmit the command (0x40) to initiate a
write to the Configuration Register. The CS5460A
will then acquire the serial data input from the
(SDI) pin when the user pulses the serial clock
(SCLK) 24 times. Once the data is received, the
state machine writes the data to the Configuration
Register and then waits to receive another valid
command.
3.3.2 Register Read
When a read command is initiated, the serial port
will start transferring register content bits (MSB
first) on the SDO pin for the next 8, 16, or 24 SCLK
cycles. Command words instructing a register read
may be terminated at 8-bit boundaries (e.g., read
transfers may be 8, 16, or 24 bits in length). Also
data register reads allow “command chaining”.
This means that the micro-controller is allowed to
send a new command while reading register data.
The new command will be acted upon immediately
and could possibly terminate the first register read.
For example, if a command word is sent to the state
machine to read one of the output registers, then af-
ter the user pulses SCLK for 16-bits of data, a sec-
ond write command word (e.g., to clear the Status
Register) may be pulsed on to the SDI line at the
same time the last 8-bits of data (from the first read
command) are pulsed from the SDO line. As an-
other example, suppose that the user is only inter-
ested in acquiring the 16-most significant bits of
data from the first read. In this case, the user can
begin to strobe a second read command on SDI af-
ter the first 8 data bits have been read from SDO.