Xilinx V2.1 manual Addressable Shift Register, Block Interface

Xilinx System Generator v2.1 Reference Guide

Addressable Shift Register

The Xilinx Addressable Shift Register block is a variable-length shift register (or delay chain). This block differs from the Xilinx Delay block in that the amount of latency experienced by data from input to block output is variable and depends on the address value.

Data presented to the block will traverse the entire delay chain. The output of the block is not necessarily the output of the last register in the chain, however. Instead, the output of the block is taken from the register pointed to by the address presented on the addr port.

Block Interface

The block interface (inputs and outputs as seen on the Addressable Shift Register icon) are as follows:

Input signals:

d	data input
addr	address
en	enable signal
Output signals:
q	data output

In Simulink, the addr port is given priority over the data (d) port, i.e. on each successive cycle, the addressed data value is read from the register before the shift operation occurs. This order is needed in the Simulink software model to guarantee one clock cycle of latency between the data port and the first register of the delay chain. (If the shift operation were to come first, followed by the read, then there would be no delay, and the hardware would be incorrect.)

26 Xilinx Development System