You can request a pause packet to be transmitted using the
ctl_tx_pause_req[8:0]
and ctl_tx_pause_enable[8:0]
input
buses. Bit [8] corresponds to global pause packets and bits [7:0] correspond to priority
pause packets.
Each bit of this bus must be held at a steady state for a minimum of 16 cycles before the next transition.
The contents of the pause packet are determined using the following input pins.
ctl_tx_da_gpp[47:0]
ctl_tx_sa_gpp[47:0]
ctl_tx_ethertype_gpp[15:0]
ctl_tx_opcode_gpp[15:0]
ctl_tx_pause_quanta8[15:0]
Priority pause packets:
ctl_tx_da_ppp[47:0]
ctl_tx_sa_ppp[47:0]
ctl_tx_ethertype_ppp[15:0]
ctl_tx_opcode_ppp[15:0]
ctl_tx_pause_quanta0[15:0]
ctl_tx_pause_quanta1[15:0]
ctl_tx_pause_quanta2[15:0]
ctl_tx_pause_quanta3[15:0]
ctl_tx_pause_quanta4[15:0]
ctl_tx_pause_quanta5[15:0]
ctl_tx_pause_quanta6[15:0]
ctl_tx_pause_quanta7[15:0]
The USXGMII core automatically calculates and adds the FCS
to the packet. For priority pause packets the USXGMII core also automatically generates the
enable vector based on the priorities that are requested.To request a pause packet, you must set the corresponding bit of
the ctl_tx_pause_req[8:0]
and ctl_tx_pause_enable[8:0]
bus
to a 1 and keep it at 1 for the duration of the pause request (that is, if these inputs are
set to 0, all pending pause packets are canceled). The USXGMII core transmits the pause
packet immediately after the current packet in flight is completed.
To retransmit pause packets, the USXGMII core maintains a total
of nine independent timers; one for each priority and one for global pause. These timers are
loaded with the value of the corresponding input buses. After a pause packet is transmitted
the corresponding timer is loaded with the corresponding value of the
ctl_tx_pause_refresh_timer[8:0]
input bus. When a timer times out,
another packet for that priority (or global) is transmitted as soon as the current packet in
flight is completed. Additionally, you can manually force the timers to 0, and therefore
force a retransmission, by setting the ctl_tx_resend_pause
input to 1 for
one clock cycle.
To reduce the number of pause packets for priority mode operation, a timer is considered timed out if any of the other timers time out. Additionally, while waiting for the current packet in flight to be completed, any new timer that times out or any new requests are merged into a single pause frame. For example, if two timers are counting down, and you send a request for a third priority, the two timers are forced to be timed out and a pause packet for all three priorities is sent as soon as the current in-flight packet (if any) is transmitted. Similarly, if one of the two timers times out without an additional request, both timers are forced to be timed out and a pause packet for both priorities is sent as soon as the current in-flight packet (if any) is transmitted.
You can stop pause packet generation by setting the appropriate
bits of ctl_tx_pause_req[8:0]
or ctl_tx_pause_enable[8:0]
to 0.