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