Operation - 9.0 English

Tri-Mode Ethernet MAC LogiCORE IP Product Guide (PG051)

Document ID
Release Date
9.0 English

The following figure shows the Priority 0 FIFO occupancy over time.

Figure 1. Priority Flow Control Implementation Triggered from FIFO Occupancy
  1. The FIFO occupancy is maintained at a low level as the client logic is able to service the frames. At point A, the client logic is unable to service the FIFO and the occupancy increases. At point B, the FIFO has reached the threshold of 7/8 occupancy. This triggers the XOFF request assertion, a PFC frame is generated and requested priority 0, traffic is stopped.
  2. Upon receiving the PFC frame, the right-hand priority 0 FIFO ceases transmission.
  3. The client logic remains unable to service the priority 0 FIFO for an extended duration, and subsequent PFC frames are automatically generated at C to ensure the Priority 0 FIFO on the right does not restart.
  4. The client begins to service the priority 0 FIFO at point D, and the FIFO empties. The occupancy falls to the second threshold of ¾ occupancy at point E. This triggers the XON PFC frame request.
  5. Upon receiving this PFC frame, the right-hand MAC de-asserts the rx_pfc_p0_tvalid and the priority 0 FIFO resumes transmission.
  6. Normal operation resumes.