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174 Software Developer’s Manual
Ethernet Interface
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.
8.7.6 Software Initiated PAUSE Frame Transmission
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.
Caution: Use of SWXOFF is not recommended due to security concerns.
8.7.7 External Control of Flow Control Operation
1
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.