DP83901AV datasheet(42/62 Pages) NSC

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130 Bus Arbitration and Timing (Continued)

REMOTE DMA-BIDIRECTIONAL PORT CONTROL

The Remote DMA transfers data between the local buffer

memory and a bidirectional port (memory to IO transfer)

This transfer is arbited on a byte by byte basis versus the

burst transfer used for Local DMA transfers This bidirec-

tional port is also readwritten by the host All transfers

through this port are asynchronous At any one time trans-

fers are limited to one direction either from the port to local

buffer memory (Remote Write) or from local buffer memory

to the port (Remote Read)

Bus Handshake Signals for Remote DMA Transfers

TLF10469 – 27

REMOTE READ TIMING

1 The DMA reads byteword from local buffer memory and

writes byteword into latch increments the DMA address

and decrements the byte count (RBCR0 1)

2 A Request Line (PRQ) is asserted to inform the system

that a byte is available

3 The system reads the port the read strobe (RACK)is

used as an acknowledge by the Remote DMA and it goes

back to step 1

Steps 1 – 3 are repeated until the remote DMA is complete

Note that in order for the Remote DMA to transfer a byte

from memory to the latch it must arbitrate access to the

local bus via a BREQ BACK handshake After each byte or

word is transferred to the latch BREQ is dropped If a Local

DMA is in progress the Remote DMA is held off until the

local DMA is complete

TLF10469 – 28

DP83901AV датащи(PDF) 42 Page - National Semiconductor (TI)