GS2985 Datasheet, PDF (32/45 Pages) Gennum Corporation – Uses standard 27MHz crystal

4.15.8 Daisy Chain Operation
For applications with multiple GS2985 devices, it is possible to daisy-chain up to 127 parts in
serial. In this configuration, the first device SDI should be connected to the SPI Master SDO. The
serial data output of each device is then connected to the serial data input of the following
device, and so on. The last device's SDO connects to the Master's SDI. Connecting devices in
serial reduces the number of I/O ports required by the master by removing the need for
additional chip select lines.
SPI
Master
SCK
SDO
SDI
CS
SCK
SDI
SDO
CS
SPI
Slave
Chain Position 0
SCK
SDI
SDO
CS
SPI
Slave ^{Chain Position 1}
SCK
SDI
SDO
CS
SPI
Slave
Chain Position 2
Figure 4-10: Daisy Chained SPI Bus
The position of each GS2985 device in the serial chain is referred to as its Chain Position, with 0
corresponding to the first device. The Chain Position in the SPI command word is decoded by
each slave to know which device the master is talking to.
Each GS2985 slave is designed to output a replica of what it receives at its input after a delay of
32 cycles. The Chain Position part of the command is decremented by one in the duplicated
command word at the output. Each device in the chain will only execute the issued command
if it verifies that the current chain position is set to 0.
A[4:0]
C[6:0]=N
Chain Position
32 cycles
SDI SDO
GS2985
A[4:0] C[6:0]=N-1
Chain Position
-1
32 cycles
SDI SDO
GS2985
A[4:0] C[6:0]=N-2
Chain Position
-2
Figure 4-11: Chain Position Decoding
GS2985 Multi-Rate SDI Reclocker with Equalization &
De-emphasis
Data Sheet
36663 - 5
July 2012
32 of 45
Proprietary & Confidential