DP8390D datasheet(15/56 Pages) NSC

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90 Remote DMA (Continued)

Maximum Bus Latency for Byte Mode

TLF8582 – 98

Maximum Bus Latency for Word Mode

TLF8582 – 99

byte has entered the FIFO thus with an 8 byte threshold

the NIC issues Bus Request (BREQ) when the 9th byte has

entered the FIFO For Word Mode BREQ is not generated

until the na2 bytes have entered the FIFO Thus with a 4

word threshold (equivalent to 8 byte threshold) BREQ is

issued when the 10th byte has entered the FIFO The two

graphs the figures above indicate the maximum allowable

bus latency for Word and Byte transfer modes

The FIFO at the Beginning of Transmit

Before transmitting the NIC performs a prefetch from mem-

ory to load the FIFO The number of bytes prefetched is the

programmed FIFO threshold The next BREQ is not issued

until after the NIC actually begins trasmitting data ie after

SFD The Transmit Prefetch diagram illustrates this process

SEND PACKET COMMAND

The Remote DMA channel can be automatically initialized

to transfer a single packet from the Receive Buffer Ring

The CPU begins this transfer by issuing a ‘‘Send Packet’’

Command The DMA will be initialized to the value of the

Boundary Pointer Register and the Remote Byte Count

of the Receive Byte Count fields found in the Buffer Header

of each packet After the data is transferred the Boundary

Pointer is advanced to allow the buffers to be used for new

receive packets The Remote Read will terminate when the

Byte Count equals zero The Remote DMA is then prepared

to read the next packet from the Receive Buffer Ring If the

DMA pointer crosses the Page Stop Register it is reset to

the Page Start Address This allows the Remote DMA to

remove packets that have wrapped around to the top of the

Receive Buffer Ring

Note 1

In order for the NIC to correctly execute the Send Packet Com-

mand the upper Remote Byte Count Register (RBCR1) must first

be loaded with 0FH

Note 2

The Send Packet command cannot be used with 68000 type proc-

essors

Transmit Prefetch Timing

TLF8582 – A0

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