Data Sheet

Interconnects—Ethernet Controller I210
101
The implementation of asymmetric flow control allows for one link partner to send flow control packets
while being allowed to ignore their reception. For example, not required to respond to PAUSE frames.
The following registers are defined for the implementation of flow control:
• CTRL.RFCE field is used to enable reception of legacy flow control packets and reaction to them
• CTRL.TFCE field is used to enable transmission of legacy flow control packets
• Flow Control Address Low, High (FCAL/H) - 6-byte flow control multicast address
• Flow Control Type (FCT) 16-bit field to indicate flow control type
• Flow Control bits in Device Control (CTRL) register - Enables flow control modes
• Discard PAUSE Frames (DPF) and Pass MAC Control Frames (PMCF) in RCTL - controls the
forwarding of control packets to the host
• Flow Control Receive Threshold High (FCRTH0) - A 13-bit high watermark indicating receive buffer
fullness. A single watermark is used in link FC mode.
• DMA Coalescing Receive Threshold High (FCRTC) - A 13-bit high watermark indicating receive
buffer fullness when in DMA coalescing and Tx buffer is empty. The value in this register can be
higher than value placed in the FCRTH0 register since the watermark needs to be set to allow for
only receiving a maximum sized Rx packet before XOFF flow control takes effect and reception is
stopped (refer to Tabl e 3-28 for information on flow control threshold calculation).
• Flow Control Receive Threshold Low (FCRTL0) - A 13-bit low watermark indicating receive buffer
emptiness. A single watermark is used in link FC mode.
• Flow Control Transmit Timer Value (FCTTV) - a set of 16-bit timer values to include in transmitted
PAUSE frame. A single timer is used in Link FC mode
• Flow Control Refresh Threshold Value (FCRTV) - 16-bit PAUSE refresh threshold value
• RXPBSIZE.Rxpbsize field is used to control the size of the receive packet buffer
3.7.5.1 MAC Control Frames and Receiving Flow Control Packets
3.7.5.1.1 Structure of 802.3X FC Packets
Three comparisons are used to determine the validity of a flow control frame:
1. A match on the 6-byte multicast address for MAC control frames or to the station address of the
I210 (Receive Address Register 0).
2. A match on the type field.
3. A comparison of the MAC Control Op-Code field.
The 802.3x standard defines the MAC control frame multicast address as 01-80-C2-00-00-01.
The Type field in the FC packet is compared against an IEEE reserved value of 0x8808.
The final check for a valid PAUSE frame is the MAC control op-code. At this time only the PAUSE control
frame op-code is defined. It has a value of 0x0001.
Frame-based flow control differentiates XOFF from XON based on the value of the PAUSE timer field.
Non-zero values constitute XOFF frames while a value of zero constitutes an XON frame. Values in the
Timer field are in units of pause quantum (slot time). A pause quantum lasts 64 byte times, which is
converted in absolute time duration according to the line speed.
Note: XON frame signals the cancellation of the pause from initiated by an XOFF frame - pause for
zero pause quantum.