CS8900A datasheet(95/128 Pages) CIRRUS

95 / 128 page

DS271PP3

CS8900A

Crystal LAN™ ISA Ethernet Controller

CIRRUS LOGIC PRODUCT DATA SHEET

Figure 26 shows the steps the CS8900A goes

through in determining when to automatically

switch to DMA.

Whenever the CS8900A automatically enters

DMA, at least one complete frame is already stored

in the on-chip buffer. Because frames are trans-

ferred to the host in the same order as received (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 noncom-

mitted frame in the on-chip buffer is the first frame

to use DMA. When DMA begins, any pending Rx-

Event reports in the Interrupt Status Queue are dis-

carded because the host cannot process those

events until the corresponding frames have been

completely DMAed.

Auto-Switch DMA works only on entire received

frames. The CS8900A does not use Auto-Switch

DMA to transfer partial frames. Also, when a frame

has been committed (see Section 5.2.5 on page 84),

the CS8900A will not switch to DMA mode until

the committed frame has been transferred com-

pletely or skipped.

After a complete frame has been moved to host

memory, the CS8900A updates the DMA Start-of-

Frame register (PacketPage base + 0126h), the

DMA Frame Count register (PacketPage base +

0128h), and the DMA Byte Count register, then

sets the RxDMAFrame bit (Register C, BufEvent,

bit 7). If RxDMAiE (Register B, BufCFG, bit 7) is

set, a corresponding interrupt occurs.

5.5.4 DMA Channel Speed vs. Missed Frames

When the CS8900A starts DMA, the entire oldest,

noncommitted frame must be placed in host mem-

ory before on-chip buffer space will be freed for the

next incoming frame. If the oldest frame is relative-

ly large, and the next incoming frame also large,

the incoming frame may be missed, depending on

the speed of the DMA channel. If this happens, the

CS8900A will increment the RxMiss counter (Reg-

ister 10) and clear any event reports (RxEvent and

BufEvent) associated with the missed frame.

5.5.5 Exit From DMA

When the CS8900A has activated receive DMA, it

remains in DMA mode until all of the following are

true:

•

The host processes all RxEvent and BufEvent

reports pending in the ISQ.

All Frames

use DMA

Yes

Frame

Discarded

Frame Buffered

in On-chip RAM

Auto-Switch

DMA Disabled

Packet Received

Auto-Switch to DMA

Frame

Passed the

DA filter?

RxDMA only

Bit=1

Buffer Space

Available?

AutoRxDMA

Bit=1?

Figure 26. Conditions for Switching to DMA

CS8900A Datasheet(PDF) 95 Page - Cirrus Logic

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