AM79C976 Advanced Micro Devices, AM79C976 Datasheet - Page 142

• Manufacturer Part Number: AM79C976
• Description: PCnet-PRO 10/100 Mbps PCI Ethernet Controller
• Manufacturer: Advanced Micro Devices

142
7-2
1-0
Bit
RCVFW
Name
RES
Reserved locations. Written as zeros and read as undefined.
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.
P R E L I M I N A R Y
Am79C976
RCVFW[1:0]
00
01
10
11
Description
Bytes Received
128
256
48
64
8/01/00