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Software Developer’s Manual 307
Register Descriptions
Table 13-76. TCTL Register Bit Description
31 26 25 22 21 12 11 4 3 0
Reserved CNTL Bits COLD CT CNTL Bits
Field Bit(s)
Initial
Value
Description
Reserved 0 0b
Reserved
Write as 0b for future compatibility.
EN 1 0b
Transmit Enable
The transmitter is enabled when this bit is set to 1b. Writing 0b to
this bit stops transmission after any in progress packets are sent.
Data remains in the transmit FIFO until the device is re-enabled.
Software should combine this operation with reset if the packets in
the TX FIFO should be flushed.
Reserved 2 0b
Reserved
Reads as 0b.
Should be written to 0b for future compatibility.
PSP 3 0b
Pad Short Packets
0b = Do not pad.
1b = Pad short packets.
Padding makes the packet 64 bytes long. The padding content is
data.
When the Pad Short Packet feature is disabled, the minimum
packet size the Ethernet controller can transfer to the host is 32
bytes long.
This feature is not the same as Minimum Collision Distance
(TCTL.COLD).
CT 11:4 0b
Collision Threshold
This determines the number of attempts at re-transmission prior to
giving up on the packet. The Ethernet back–off algorithm is
implemented and clamps to the maximum value after 16 retries. It
only has meaning in half-duplex operation. Recommended value –
0Fh.
COLD 21:12 0b
Collision Distance
Specifies the minimum number of byte times that must elapse for
proper CSMA/CD operation. Packets are padded with special
symbols, not valid data bytes. Hardware checks this value and
padded packets even in full-duplex operation.
Recommended value:
Half-Duplex – 512-byte time (200h)
Full-Duplex – 64-byte time (40h)
Note: 10/100 half-duplex - 64 - 68 (40h to 44h) byte times for the
82541xx and 82547GI/EI only.
SWXOFF 22 0b
Software XOFF Transmission
When set to 1b, the Ethernet controller schedules the transmission
of an XOFF (PAUSE) frame using the current value of the PAUSE
timer (FCTTV.TTV). This bit self-clears upon transmission of the
XOFF frame. This bit is valid only in Full-Duplex mode of
operation. Software should not set this bit while the Ethernet
controller is configured for half-duplex operation.