AD7792/AD7793
Rev. B | Page 21 of 32
0
–10
–20
–30
–40
–50
–60
0 10000900080007000600050004000300020001000
04855-021
FREQUENCY (Hz)
(dB)
Figure 16. Filter Response at 470 Hz Update Rate
DIGITAL INTERFACE
The programmable functions of the AD7792/AD7793 are
controlled using a set of on-chip registers. Data is written to
these registers via the serial interface of the device; read access
to the on-chip registers is also provided by this interface. All
communications with the device must start with a write to the
communications register. After power-on or reset, the device
expects a write to its communications register. The data written
to this register determines whether the next operation is a read
operation or a write operation and determines to which register
this read or write operation occurs. Therefore, write access to
any of the other registers on the part begins with a write
operation to the communications 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 communications register followed by a read
operation from the selected register.
The serial interfaces of the AD7792/AD7793 consist of four
signals:
CS
, DIN, SCLK, and DOUT/
RDY
. The DIN line is used
to transfer data into the on-chip registers, and 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 operates as a data-ready signal also, the line
going 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. It also goes high prior to the updating of
the data register to indicate when not to read from the device, to
ensure 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 AD7792/AD7793 in systems where several
components are connected to the serial bus.
Figure 3 and Figure 4 show timing diagrams for interfacing to
the AD7792/AD7793 with
CS
being used to decode the part.
Figure 3 shows the timing for a read operation from the
AD7792/AD7793 output shift register, and 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 AD7792/AD7793. The end of the
conversion can be monitored using the
RDY
bit in the status
register. This scheme is suitable for interfacing to microcon-
trollers. 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 AD7792/AD7793 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
would normally occur 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 AD7792/AD7793 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 some glitch in
the system. Reset returns the interface to the state in which it is
expecting a write to the communications register. This opera-
tion 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 AD7792/AD7793 can be configured to continuously
convert or to perform a single conversion. See
Figure 17
through
Figure 19.
