AM79C976 datasheet(141/309 Pages) AMD

141 / 309 page

8/01/00

Am79C976

141

P R E L I M INARY

17-16

XMTSP

Transmit Start Point. XMTSP controls the point at which preamble transmission attempts to

commence in relation to the number of bytes written to the MAC Transmit FIFO for the current

transmit frame. When the entire frame is in the MAC Transmit FIFO, transmission will start

regardless of the value in XMTSP.

If any of REX_UFLO, REX_RTRY, or RTRY_LCOL are set, no frame data will be overwritten until

the frame has been transmitted or discarded. Otherwise, no data will be overwritten until at least

64 bytes have been transmitted.

Note that when the No Underflow (NOUFLO) bit (BCR18, bit 11) is set to 1 or XMTSP = 11b,

transmission will not start until the complete frame has been copied into the Transmit FIFO. This

mode is useful in a system where high latencies cannot be avoided.

The default value for XMTSP[1:0] is 01b (64 bytes).

This field is an alias of CSR80, bits 11:10.

15-10

RES

Reserved locations. Written as zeros and read as undefined.

9-8

XMTFW

Transmit FIFO Watermark. XMTFW specifies the point at which transmit DMA is requested, based

upon the number of bytes that could be written to the Transmit FIFO without FIFO overflow.

Transmit DMA is requested when the number of bytes specified by XMTFW could be written to the

FIFO without causing Transmit FIFO overflow, and the internal state machine has reached a point

where the Transmit FIFO is checked to determine if DMA servicing is required.

The default value for XMTFW[1:0] is 00b (16 bytes).

This field is an alias of CSR80, bits 9:8.

Bit

Name

Description

XMTSP[1:0]

NOUFLO

Bytes Written

128

Full Frame

XMTFW[1:0]

Bytes Available

128

256

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