The Transmit Data FIFO Vacancy Register shown in the figure below is a read-only register that gives the vacancy status of the Transmit Data FIFO. This is an unsigned value and reflects the current snapshot of the number of locations free for data storage in the Transmit Data FIFO. The value reflected (N) in this register tells you that you can perform N writes to Transmit FIFO. The value of this register after reset is C_TX_FIFO_DEPTH-4. The register does not decrement for every Transmit Data FIFO Data Write Port (TDFD) write. It decrements by two for every two write locations.
Figure 1. Transmit Data FIFO Vacancy Register (Offset 0xC)
