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AR# 31694

LogiCORE VLYNQ v1.3 - Why does the CMDFIFO_FREE output freeze at 0x07, when the OPB Slave Outbound Command FIFO buffer is full?

Description

Why does the CMDFIFO_FREE output freeze at 0x07, when the OPB Slave Outbound Command FIFO buffer is full?

Solution

In the VLYNQ v1.3 Data Sheet, under Table 4: Control/Status Interface, it states that the CMDFIFO_FREE is an output that tells the number of free entries in the OPB Slave Outbound Command FIFO. The problem is that the CMDFIFO_FREE never drops below 0x07 (7 locations). The reason for this is that the internal FIFO actually has a depth of 31, with a threshold of 24 to match the rest of the control logic. Due to this threshold value, the CMDFIFO_FREE never drops below 0x07. 

 

i.e., 31-24 = 7 

 

This information can be found in the data sheet for v1.4 and beyond. 

 

For a detailed list of LogiCORE VLYNQ Release Notes and Known Issues, see (Xilinx Answer 23509).

AR# 31694
Date Created 09/16/2008
Last Updated 05/21/2014
Status Archive
Type General Article