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Software Developer’s Manual 301
Register Descriptions
13.4.24 Flow Control Receive Threshold High
FCRTH (02168h; R/W)
This register contains the receive threshold used to determine when to send an XOFF packet. It
counts in units of bytes. Each time the receive FIFO reaches the fullness indicated by FCRTH,
hardware transmits a PAUSE frame if the transmission of flow control frames is enabled
(CTRL.TFCE).
Flow control reception/transmission are negotiated capabilities by the Auto-Negotiation process.
When the Ethernet controller is manually configured, flow control operation is determined by the
CTRL.RFCE & CTRL.TFCE bits.
Table 13-69. FCRTH Register Bit Description
31 30 16 15 3 2 0
XFCE
1
1. 82544GC/EI only.
Reserved RTH 0
Field Bit(s)
Initial
Value
Description
Reserved 2:0 0b
Reserved
Should be written with 0 for future compatibility
Reads as 0
RTH 15:3 0b
Receive Threshold High.
FIFO high water mark for flow control transmission.
Each time the receive FIFO reaches the fullness indicated by RTH,
the Ethernet controller transmits a Pause packet if enabled to do so.
Reserved 31:16 0b
Reserved
Should be written with 0b for future compatibility.
Reads as 0b.
XFCE 31 0
External Flow Control Enabled (82544GC/EI only)
0b = Disabled.
1b = Enabled.
Allows the Ethernet controller to send XOFF and XON frames based
on external pins XOFF and XON. The transmission of pause frames
must be also enabled through the CTRL.TFCE control bit. When the
XOFF signal is asserted high, the Ethernet controller transmits a
single XOFF frame. The assertion of 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.
This behavior also provides a built-in hysteresis mechanism.