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AR# 21141

Virtex-4 RocketIO - Why do I see data on the upper bytes of RXDATA with a 1 or 2 byte fabric interface width?

Description

General Description:  

When using 1 or 2 byte fabric interface width, the upper 2,3,4 or 3,4 bytes respectively appear to have data on them. Why?

Solution

This behavior is inherent in the functionality of the Virtex-4 RocketIO. Below is an explanation of this behavior. 

 

8-byte fabric interface width (User only accesses all 8 bytes) 

USRCLK2 cycle 0 RXDATA[63:0] 

USRCLK2 cycle 1 RXDATA[63:0] 

USRCLK2 cycle 2 Same as cycle 0 

USRCLK2 cycle 3 Same as cycle 1 ............ 

 

4-byte fabric interface width (User only accesses lower 4 bytes) 

USRCLK2 cycle 0 32'h00000000, RXDATA[31:0] 

USRCLK2 cycle 1 32'h00000000, RXDATA[31:0] 

USRCLK2 cycle 2 Same as cycle 0 

USRCLK2 cycle 3 Same as cycle 1 ............ 

 

2-byte fabric interface width (User only accesses lower 2 bytes ) 

USRCLK2 cycle 0 32'h00000000, RXDATA[31:16], RXDATA[15:0] 

USRCLK2 cycle 1 32'h00000000, 16'h0000, RXDATA[31:16], 

USRCLK2 cycle 2 Same as cycle 0 

USRCLK2 cycle 3 Same as cycle 1 ............ 

 

1-byte fabric interface width (User only accesses lower 1 byte) 

USRCLK2 cycle 0 32'h00000000, RXDATA[31:24], RXDATA[23:16], RXDATA[15:8], RXDATA[7:0] 

USRCLK2 cycle 1 32'h00000000, 8'h00, RXDATA[31:24], RXDATA[23:16], RXDATA[15:8] 

USRCLK2 cycle 2 32'h00000000, 8'h00, 8'h00, RXDATA[31:24], RXDATA[23:16] 

USRCLK2 cycle 3 32'h00000000, 8'h00, 8'h00, 8'h00, RXDATA[31:24] 

USRCLK2 cycle 4 same as cycle 0 

USRCLK2 cycle 5 same as cycle 1...........

AR# 21141
Date Created 09/04/2007
Last Updated 05/19/2014
Status Archive
Type General Article