CS8900A datasheet(90/128 Pages) CIRRUS

90 / 128 page

DS271PP3

CS8900A

Crystal LAN™ ISA Ethernet Controller

CIRRUS LOGIC PRODUCT DATA SHEET

5.4 Receive DMA

5.4.1 Overview

The CS8900A supports a direct interface to the host

DMA controller allowing it to transfer receive

frames to host memory via slave DMA. The DMA

option applies only to receive frames, and not

transmit operation. The CS8900A offers three pos-

sible Receive DMA modes:

1) Receive-DMA-only mode: All receive frames

are transferred via DMA.

2) Auto-Switch DMA: DMA is used only when

needed to help prevent missed frames.

3) StreamTransfer: DMA is used to minimize the

number of interrupts to the host.

This section provides a description of Receive-

DMA-only mode. Section 5.5 on page 93 describes

Auto-Switch DMA and Section 5.6 on page 96 de-

scribes StreamTransfer.

5.4.2 Configuring the CS8900A for DMA Opera-

tion

The CS8900A interfaces to the host DMA control-

ler through one pair of the DMA request/acknowl-

edge pins (see Section 3.2 on page 18 for a

description of the CS8900A’s DMA interface).

Four 16-bit registers are used for DMA operation.

These are described in Table 26.

Receive-DMA-only mode is enabled by setting the

RxDMAonly bit (Register 3, RxCFG, Bit 9).

Note: If the RxDMAonly bit and the AutoRxD-

MAE bit (Register 3, RxCFG, Bit A) are both set,

then RxDMAonly takes precedence, and the

CS8900A is in DMA mode for all receive frames.

5.4.3 DMA Receive Buffer Size

In receive DMA mode, the CS8900A stores re-

ceived frames (along with their status and length)

in a circular buffer located in host memory space.

Address

Type of

Received

Frame

Erred

Frame?

Passes

Hash

Filter?

Contents of RxEvent

Bits F-A

Bit 9

Hashed

Bit 8

RxOK

Bit 6

IAHash

Individual

Address

yes

Hash Table Index

ExtraData Runt CRC Error Broadcast Individual Adr

yes

don’t care ExtraData Runt CRC Error Broadcast Individual Adr

Multicast

Address

yes

Hash table index

ExtraData Runt CRC Error Broadcast Individual Adr

yes

don’t care ExtraData Runt CRC Error Broadcast Individual Adr

Broad-

cast

Address

yes

(Note 6)

ExtraData Runt CRC Error Broadcast Individual Adr

(actual value X00010)

yes

(Note 7)

ExtraData Runt CRC Error Broadcast Individual Adr

yes

don’t care ExtraData Runt CRC Error Broadcast Individual Adr

Notes: 6. Broadcast frames are accepted as Multicast frames if and only if all the following conditions are met

simultaneously:

a) the Logical Address Filter is programmed as: (MSB) 0000 8000 0000 0000h (LSB). Note that this

LAF value corresponds to a Multicast Addresses of both all 1s and 03-00-00-00-00-01.

b) the Rx Control Register (register 5) is programmed to accept IndividualA, MulticastA, RxOK-only,

and the following address filters were enabled: IAHashA and BroadcastA.

7. NOT (Note 1).

Table 25. Contents of RxEvent Upon Various Conditions

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