AM79C976 datasheet(142/309 Pages) AMD

142 / 309 page

142

Am79C976

8/01/00

P R E L I M INARY

7-2

RES

Reserved locations. Written as zeros and read as undefined.

1-0

RCVFW

Receive FIFO Watermark. RCVFW controls the point at which receive DMA is requested in relation

to the number of received bytes in the Receive FIFO. RCVFW specifies the number of bytes which

must be present (once the frame has been verified as a non-runt) before receive DMA is requested.

Note, however, that if the network interface is operating in half-duplex mode, in order for receive

DMA to be performed for a new frame, at least 64 bytes must have been received. This effectively

avoids having to react to receive frames which are runts or suffer a collision during the slot time

(512 bit times). If the Runt Packet Accept feature is enabled or if the network interface is operating

in full-duplex mode, receive DMA will be requested as soon as either the RCVFW threshold is

reached, or a complete valid receive frame is detected (regardless of length). When the Full Duplex

Runt Packet Accept Disable (FDRPAD) bit in CMD2 is set and the Am79C976 controller is in full-

duplex mode, in order for receive DMA to be performed for a new frame, at least 64 bytes must

have been received. This effectively disables the runt packet accept feature in full-duplex mode.

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

This field is an alias of CSR80, bits 13:12.

Bit

Name

Description

RCVFW[1:0]

Bytes Received

128

256

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