If the Ethernet MAC has been generated
with PFC functionality included but configured for IEEE 802.3 functionality then the Ethernet
MAC supports XON/XOFF functionality. If, as shown in the following figure, the pause_req signal is asserted, the Ethernet MAC generates a new
802.3 pause frame. If it is subsequently held High for more than one clock cycle, the Ethernet
MAC automatically generates a new pause frame each time the internal quanta count of the
Ethernet MAC reaches the number of quanta specified by the legacy refresh value (in either the
register or the configuration vector). This is shown as an XOFF refresh frame in the following
figure. When the pause_req is de-asserted, an XON frame
(standard pause frame with the pause quanta forced to zero) is automatically generated if the
auto XON feature is enabled. This functionality is shown in the following figure. If auto XON
is not enabled the remaining quanta are left to expire naturally at the link partner.
- To enable an Auto XON after a single pause request, ensure that the spacing between these two pause requests is at least equal to the End of the current frame + IFG + Pause frame duration for a single pause.