Datasheet

AD7765
Rev. A | Page 24 of 32
WRITING DATA IN DAISY-CHAIN MODE
Writing to AD7765 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
Writing data to the AD7765 in daisy-chain mode operates with
the same timing structure as writing to a single device (see
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.
Figure 3). The difference between writing to a single device
and writing to a number of daisy-chained devices is in the
implementation 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 AD7765 devices are being
written to in daisy-chain mode,
FSI
The AD7765 devices can be written to at any time. The falling
edge of
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 64
th
and 96
th
SCO period.
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 AD7765
device that there are more devices further on in the chain. This
means that the AD7765 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.
SYNC
SDI
FSI
SDO
MCLK
AD7765
(D)
FSI
SYNC
MCLK
SYNC
SDI
FSI
SDO
MCLK
AD7765
(C)
SYNC
SDI
FSI
SDO
MCLK
AD7765
(B)
SYNC
SDI
FSI
MCLK
AD7765
(A)
SDO
FSO
SDI
06519-020
Figure 41. Writing to an AD7765 Daisy-Chain Configuration
FSI
SCO
SDI (D)
SDI (C) = SDO (D)
SDI (B) = SDO (C)
SDI (A) = SDO (B)
SDI (D)
SDI (C)
SDI (B)
SDI (A)
32 × t
SCO
32 ×
t
SCO
32 × t
SCO
31 × t
SCO
t
10
06519-021
Figure 42. Daisy-Chain Write Timing Diagram; Writing to Four AD7765 Devices