Data Sheet

Ethernet Controller I210 —Interconnects
104
The I210 sends a PAUSE frame when Rx packet buffer is full above the high threshold defined in the
Flow Control Receive Threshold High (FCRT0.RTH) register field. When the threshold is reached, the
I210 sends a PAUSE frame with its pause time field equal to FCTTV. The threshold should be large
enough to overcome the worst case latency from the time that crossing the threshold is sensed until
packets are not received from the link partner. The Flow Control Receive Threshold High value should
be calculated as follows:
Flow Control Receive Threshold High = Internal Rx Buffer Size - (Threshold Cross to XOFF Transmission
+ Round-trip Latency + XOFF Reception to Link Partner response)
Parameter values to be used for calculating the FCRT0.RTH value are listed in Table 3-28.
Table 3-28. Flow Control Receive Threshold High (FCRTH0.RTH) Value Calculation
Note: When DMA Coalescing is enabled (DMACR.DMAC_EN = 1b),the value placed in the
FCRTC.RTH_Coal field should be equal or lower than:
FCRTC.RTH_Coal = FCRTH0.RTH + Max packet size * 1.25
The FCRTC.RTH_Coal is used as the high watermark to generate XOFF flow control packets when the
internal Tx buffer is empty and the I210 is executing DMA coalescing. In this case, no delay to
transmission of flow control packet exists so its possible to increase level of watermark before issuing a
XOFF flow control frame.
After transmitting a PAUSE frame, the I210 activates an internal shadow counter that reflects the link
partner pause timeout counter. When the counter reaches the value indicated in the FCRTV register,
then, if the PAUSE condition is still valid (meaning that the buffer fullness is still above the high
watermark), a XOFF message is sent again.
Once the receive buffer fullness reaches the low water mark, the I210 sends a XON message (a PAUSE
frame with a timer value of zero). Software enables this capability with the XONE field of FCRTL.
The I210 sends an additional PAUSE frame if it has previously sent one and the packet buffer overflows.
This is intended to minimize the amount of packets dropped if the first PAUSE frame did not reach its
target.
3.7.5.3.2 Software Initiated PAUSE Frame Transmission
The I210 has the added capability to transmit an XOFF frame via software. This is accomplished by
software writing a 1b to the SWXOFF bit of the Transmit Control register. Once this bit is set, hardware
initiates the transmission of a PAUSE frame in a manner similar to that automatically generated by
hardware.
The SWXOFF bit is self-clearing after the PAUSE frame has been transmitted.
Latency Parameter Affected by Parameter Value
Internal Rx Buffer Size Internal Tx buffer size.
60 KB - Internal Tx Buffer
Size.
Threshold Cross to XOFF
Transmission
Max packet size. Max packet size * 1.25.
XOFF Reception to Link Partner
response
Max packet size. Max packet size.
Round trip latency
The latencies on the wire and the LAN devices at both
sides of the wire.
320-byte (for 1000Base-T
operation).