AD7765 datasheet(23/32 Pages) AD

23 / 32 page

AD7765

Rev. 0 | Page 23 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 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 AD7765 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

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 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 AD7765 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 AD7765

device that there are more devices further on in the chain. This

means 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

Figure 36. Writing to 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 ×

tSCO

32 ×

tSCO

32 ×

tSCO

31 ×

tSCO

t10

Figure 37. Daisy-Chain Write Timing Diagram. Writing to Four AD7765 Devices

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