Intel PCI User Manual

174

The contents of the Flow Control Receive Threshold High register (FCRTH) determine at what 
point hardware transmits a PAUSE frame. Hardware monitors the fullness of the receive FIFO and 

compares it with the contents of FCRTH. When the threshold is reached, hardware sends a PAUSE 

frame with its pause time field equal to FCTTV. Once the receive buffer fullness reaches the low 
water mark, hardware sends an XON message (a PAUSE frame with a timer value of 0b). Software 

enables this capability with the XONE field of the FCRTL.

Hardware sends one more PAUSE frames if it has previously sent one and the FIFO overflows (so 

the threshold must not be set greater than the FIFO size). This function is intended to minimize the 
number of packets dropped if the first PAUSE frame does not reach its target.

Transmitting Flow Control frames should only be enabled in full duplex mode per the IEEE 802.3 

standard. Software should ensure that the transmission of flow control packets is disabled when the 

Ethernet controller is operating in half-duplex mode.

The Ethernet controller has the added capability to transmit an XOFF frame through software. This 

function is accomplished by software writing a 1b to the SWXOFF bit of the Transmit Control 
register (TCTL). 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.

The state of the CTRL.TFCE bit or the negotiated flow control configuration does not affect 

software generated PAUSE frame transmission.

Software sends an XON frame by programming a zero in the PAUSE timer field of the FCTTV 
register.

Use of SWXOFF is not recommended due to security concerns.

Transmitting XOFF and XON frames can be triggered by external pins. When enabled through 

FCRTH.XFCE, the XOFF and XON inputs can be used to provide external effective threshold 

information that initiate XOFF and XON transmission, respectively.

When the XOFF signal is asserted high, the device transmits a single XOFF frame. The assertion of 
I_XON (after deassertion of XOFF) initiates an XON frame transmission if enabled by 

FCRTL.XONE. The assertion/deassertion of XON is required between assertions of XOFF in order 

to send another XOFF frame, providing a built-in hysteresis mechanism.

Output signals are also provided from the 82544GC/EI to indicate the device is either above the 
programmed flow control high threshold or below the flow control low threshold (ABV_HI and 

BLW_LOW respectively).

Flow control transmission must also be enabled through CTRL.TFCE.

1.

82544GC/EI only.