Data Sheet AD7796/AD7797
Rev. B | Page 17 of 24
DIGITAL INTERFACE
As outlined in the On-Chip Registers section, the AD7796/
AD7797 programmable functions are controlled by a set of
on-chip registers. Data is written to these registers via the serial
interface of the device and read access to the on-chip registers is
also provided by this interface. All communication with the device
must start with a write to the communication register. After power-
on or reset, the device expects a write to its communication
register. The data written to this register determines whether
the next operation is a read or a write operation, and determines
the register where this operation occurs. Therefore, write access
to any of the other registers on the device begins with a write
operation to the communication register followed by a write to
the selected register. A read operation from any other register
(except when continuous read mode is selected) starts with a
write to the communication register followed by a read
operation from the selected register.
The serial interface of the AD7796/AD7797 consists of four
signals:
CS
, DIN, SCLK, and DOUT/
RDY
. The DIN line is used
to transfer data into the on-chip registers, while DOUT/
RDY
is
used for accessing from the on-chip registers. SCLK is the serial
clock input for the device, and all data transfers (either on DIN
or DOUT/
RDY
) occur with respect to the SCLK signal. The
DOUT/
RDY
pin also operates as a data-ready signal, that is, the
line goes low when a new data-word is available in the output
register. It is reset high when a read operation from the data
register is complete. DOUT/
RDY
also goes high prior to the
data register update to indicate when not to read from the
device. This ensures that a data read is not attempted while the
register is being updated.
CS
is used to select a device. It can be
used to decode the AD7796/AD7797 in systems where several
components are connected to the serial bus.
Figure 3 and Figure 4 show timing diagrams for interfacing to
the AD7796/AD7797 with
CS
being used to decode the device.
Figure 3 shows the timing for a read operation from the AD7796/
AD7797 output shift register, while Figure 4 shows the timing
for a write operation to the input shift register. It is possible to
read the same word from the data register several times, even
though the DOUT/
RDY
line returns high after the first read
operation. However, care must be taken to ensure that the read
operations have been completed before the next output update
occurs. In continuous read mode, the data register can be read
only once.
The serial interface can operate in 3-wire mode by tying
CS
low.
In this case, the SCLK, DIN, and DOUT/
RDY
lines are used
to communicate with the
AD7796/AD7797. The end of the
conversion can be monitored using the
RDY
bit in the status
register. This scheme is suitable for interfacing to micro-
controllers. If
CS
is required as a decoding signal, it can be
generated from a port pin. For microcontroller interfaces, it is
recommended that SCLK idle high between data transfers.
The AD7796/AD7797 can be operated with
CS
being used as a
frame synchronization signal. This scheme is useful for DSP
interfaces. In this case, the first bit (MSB) is effectively clocked
out by
CS
because
CS
normally occurs after the falling edge of
SCLK in DSPs. The SCLK can continue to run between data
transfers, provided the timing numbers are obeyed.
The serial interface can be reset by writing a series of 1s on the
DIN input. If a Logic 1 is written to the AD7796/AD7797 DIN
line for at least 32 serial clock cycles, the serial interface is reset.
This ensures that the interface can be reset to a known state if
the interface gets lost due to a software error or glitch in the
system. Reset returns the interface to the state where it is expecting
a write to the communication register. This operation resets the
contents of all registers to their power-on values. Following a
reset, the user should allow a period of 500 µs before addressing
the serial interface.
The AD7796/AD7797 can be configured to continuously
convert or to perform a single conversion. See Figure 14
through Figure 16.
