CS8900 데이터 시트보기 (PDF) - Cirrus Logic

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CS8900

Highly-Integrated ISA Ethernet Controller

Cirrus Logic 

CS8900

5.5 Auto-Switch DMA

Overview

The CS8900 supports a unique feature, Auto-

Switch DMA, that allows it to switch between

Memory or I/O mode and Receive DMA auto-

matically. Auto-Switch DMA allows the CS8900

to realize the performance advantages of Mem-

ory or I/O mode while minimizing the number of

missed frames that could result due to slow proc-

essing by the host.

Configuring the CS8900 for Auto-Switch DMA

Auto-Switch DMA mode requires the same con-

figuration as Receive-DMA-only mode, with one

exception: the AutoRxDMAE bit (Register 3,

RxCFG, Bit A) must be set, and the

RxDMAonly bit (Register 3, RxCFG, Bit 9)

must be clear (see Section 5.4, Configuring the

CS8900 for DMA Operation). In Auto-Switch

DMA mode, the CS8900 operates in non-DMA

mode if possible, only switching to slave DMA

if necessary.

Note that if the AutoRxDMAE bit and the

RxDMAonly bit (Register 3, RxCFG, bit 9) are

both set, the CS8900 uses DMA for all receive

frames.

Auto-Switch DMA Operation

Whenever a frame begins to be received in Auto-

Switch DMA mode, the CS8900 checks to see if

there is enough on-chip buffer space to store a

maximum length frame. If there is, the incoming

frame is pre-processed and buffered as normal. If

there isn’t, the CS8900’s MAC engine compares

the frame’s Destination Address (DA) to the cri-

teria programmed into the DA filter. If the

incoming DA fails the DA filter, the frame is dis-

carded. If the DA passes the DA filter, the

CS8900 automatically switches to DMA mode

and starts transferring the frame(s) currently be-

ing held in the on-chip buffer into host memory.

94

This frees up buffer space for the incoming

frame.

Figure 5.8 shows the steps the CS8900 goes

through in determining when to automatically

switch to DMA.

Whenever the CS8900 automatically enters

DMA, at least one complete frame is already

stored in the on-chip buffer. Because frames are

transferred to the host in the same order as re-

ceived (first in, first out), the beginning of the

received frame that triggered the switch to DMA

is not the first frame to be transferred. Instead,

the oldest non-committed frame in the on-chip

buffer is the first frame to use DMA. When

DMA begins, any pending RxEvent reports in

Packet Received

Frame

Passed the No

DA filter?

Yes

Frame

Discarded

RxDMA only Yes

Bit=1

No

All Frames

use DMA

More

Buffer Space

Yes

Available?

Frame Buffered

in On-chip RAM

No

AutoRxDMA No

Bit=1?

Auto-Switch

DMA Disabled

Yes

Auto-Switch to DMA

Figure 5.8. Conditions for Switching to DMA

DS150PP2

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