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There is no API to get the number of bytes (or free bytes) in the TX FIFO. Even if there was one, the value returned will be invalid as soon as it is read from the HW block because the HW runs its own TX state machine and will stream the bytes out as long as the FIFO is non-empty. So any value read by such API will be stale. The only steady-state value is FIFO empty. Or the interrupt that it has reached the set watermark (which can be between 0 and 15 both included). See discussion MichaelF_56 linked to above.