Flow Control
BCM5718 Programmer’s Guide
Broadcom
®
January 29, 2016 • 5718-PG108-R
Page 224
As soon as PAUSE frame is transmitted, any incoming packet can be dropped, and the ifInDiscard counter in
statistics will increase. When packet size is small (64 bytes) with 1000 Mbps, more frames can be dropped. Even
if the PAUSE frame is transmitted, Pause frames cannot inhibit MAC control frames.
Low Water Mark Maximum Receive Frames register (see
“Low Watermark Maximum Receive Frame Register
) control the number of good frames to receive after the RX MBUF Low Water Mark
has been reached. After the RX MAC receives this number of frames, it will drop subsequent incoming frames
until the MBUF High Water Mark is reached.
The IEEE 802.3 pause control frame contains a pause_time field. The Ethernet controller inserts a time quanta
into the pause_time field. Software should set the Enable_Long_Pause bit in the Transmit_MAC_Mode register
to configure long pause quanta. Clearing the Enable_Long_Pause bit will default the pause_time back to the
shorter quanta.
shows the pause quanta based on the Enable_Long_Pause bit setting.
Receive MAC
The Ethernet controller receive MAC’s link partner may want to inhibit frame transmission until upstream
resources become available. The receive MAC must be configured to accept IEEE 802.3x pause frames (see
). Software should set the Enable_Flow_Control bit in the Receive_MAC_Mode_Control register to
enable automatic processing of flow control frames. If software clears the Enable_Flow_Control bit, IEEE 802.3x
pause frames will be discarded. The Keep_Pause bit in the Receive_MAC_Mode_Control register will instruct
the RX engine to forward pause frames to host memory. Software may be interested in setting this bit for
debugging or promiscuous/sniffer configurations. Passing pause frames to the host will increase DMA and
protocol processing and consume available host buffers. The receive MAC will filter pause control frames when
the Keep_Pause bit is disabled.
Table 83: Transmit MAC Watermark Recommendation
Register
Recommended Value
MAC_RX_MBUF_Low_Water_Mark
24
Table 84: Pause Quanta
Enable_Long_Pause Bit
Pause_Time
DISABLED (0)
0x1FFF
ENABLED (1)
0xFFFF
Table 85: Keep_Pause Recommended Value
Register.Bit
Recommended Value
Receive_MAC_Mode_Control.Keep_Pause
DISABLED