AD7764
WRITING DATA IN DAISY-CHAIN MODE
Writing to AD7764 devices in daisy-chain mode is similar to
writing to a single device. The serial writing operation is
synchronous to the SCO signal. The status of the frame synchro-
nization input, FSI, is checked on the falling edge of the SCO
signal. If the FSI line is low, then the first data bit on the serial
data in the SDI line is latched in on the next SCO falling edge.
Writing data to the AD7764 in daisy-chain mode operates with
the same timing structure as writing to a single device (see
Figure 3). The difference between writing to a single device and
writing to a number of daisy-chained devices is in the imple-
mentation of the FSI signal. The number of devices that are in
the daisy chain determines the period for which the FSI signal
must remain logic low. To write to n number of devices in the
daisy chain, the period between the falling edge of FSI and the
rising edge of FSI must be between 32 × (n−1) to 32 × n SCO
periods. For example, if three AD7764 devices are being written
to in daisy-chain mode, FSI is logic low for between
32 × (3−1) to 32 × 3 SCO pulses. This means that the rising
edge of FSI must occur between the 64th and 96th SCO period.
The AD7764 devices can be written to at any time. The falling
edge of FSI overrides all attempts to read data from the SDO
pin. In the case of a daisy chain, the FSI signal remaining logic
low for more than 32 SCO periods indicates to the AD7764
device that there are more devices further on in the chain. This
means the AD7764 directs data that is input on the SDI pin to
its SDO pin. This ensures that data is passed to the next device
in the chain.
FSI
SDI
SYNC
MCLK
AD7764
(D)
FSI
SDI
SDO
SYNC
MCLK
AD7764
(C)
FSI
SDI
SDO
SYNC
MCLK
AD7764
(B)
FSI
SDI
SDO
SYNC
MCLK
AD7764
(A)
FSI
FSO
SDI
SDO
SYNC SCO
MCLK
Figure 42. Writing to AD7764 Daisy-Chain Configuration
FSI
SCO
SDI (D)
SDI (C) = SDO (D)
SDI (B) = SDO (C)
SDI (A) = SDO (B)
t10
32 × tSCO
32 × tSCO
32 × tSCO
SDI (D)
SDI (C)
SDI (B)
Figure 43. Daisy-Chain Write Timing Diagram; Writing to Four AD7764 Devices
31 × tSCO
SDI (A)
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