GS2985 datasheet(32/45 Pages) SEMTECH

32 / 45 page

GS2985 Multi-Rate SDI Reclocker with Equalization &

De-emphasis

Data Sheet

36663 - 5

July 2012

32 of 45

Proprietary & Confidential

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.

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.

Figure 4-11: Chain Position Decoding

SPI

Master

SCK

SDO

SDI

SPI

Slave

SCK

SDI

SDO

SCK

SDI

SDO

SCK

SDI

SDO

SPI

Slave

SPI

Slave

Chain Position 0

Chain Position 1

Chain Position 2

GS2985

SDI

SDO

Chain Position

-1

A[4:0]

Chain Position

A[4:0]

C[6:0]=N

32 cycles

GS2985

SDI

SDO

Chain Position

-2

32 cycles

C[6:0]=N-1

C[6:0]=N-2

A[4:0]

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