DP83901AV datasheet(9/62 Pages) NSC

9 / 62 page

60 Direct Memory Access Control (DMA)

The DMA capabilities of the SNIC greatly simplify the use of

the DP83901A in typical configurations The local DMA

channel transfers data between the FIFO and memory On

transmission the packet is DMA’d from memory to the FIFO

in bursts Should a collision occur (up to 15 times) the pack-

et is retransmitted with no processor intervention On recep-

tion packets are DMAed from the FIFO to the receive buffer

ring (as explained below)

A remote DMA channel is also provided on the SNIC to

accomplish transfers between a buffer memory and system

memory The two DMA channels can alternatively be com-

bined to form a single 32-bit address with 8- or 16-bit data

DUAL DMA CONFIGURATION

An example configuration using both the local and remote

DMA channels is shown below Network activity is isolated

on a local bus where the SNIC’s local DMA channel per-

forms burst transfers between the buffer memory and the

SNIC’s FIFO The Remote DMA transfers data between the

buffer memory and the host memory via a bidirectional IO

port The Remote DMA provides local addressing capability

and is used as a slave DMA by the host Host side address-

ing must be provided by a host DMA or the CPU The SNIC

allows Local and Remote DMA operations to be interleaved

SINGLE CHANNEL DMA OPERATION

If desirable the two DMA channels can be combined to

provide a 32-bit DMA address The upper 16 bits of the

32-bit address are static and are used to point to a 64 kbyte

(or 32k word) page of memory where packets are to be

received and transmitted

Dual Bus System

TLF10469 – 6

DP83901AV датащи(PDF) 9 Page - National Semiconductor (TI)