CS8900-CQ View Datasheet(PDF) - Cirrus Logic

Part Name

Description

MFG CO.

CS8900-CQ

Highly-Integrated ISA Ethernet Controller

Cirrus Logic 

CS8900

5.2 Basic Receive Operation

Overview

Once an incoming packet has passed through the

analog front end and Manchester decoder, it goes

through the following three-step receive process:

1. Pre-Processing

2. Temporary Buffering

3. Transfer to Host

Figure 5.2 shows the steps in frame reception.

As shown in the figure, all receive frames go

through the same pre-processing and temporary

buffing phases, regardless of transfer method.

Packet Received

Preamble and

Start-of-Frame

Delimiter Removed

Frame Pre-

Processed

Frame

Temporarily

Buffered

No Use Yes

DMA?

Frame Held

On Chip

Frame DMAed

to Host Memory

Host Reads

Frame from

CS8900 Memory

Host Reads

Frame from

Host Memory

Figure 5.2. Frame Reception

Once a frame has been pre-processed and buff-

ered, it can be accessed by the host in either

Memory or I/O space. In addition, the CS8900

can transfer receive frames to host memory via

host DMA. This section describes receive frame

pre-processing and Memory and I/O space re-

ceive operation. Sections 5.4 through 5.5

describe DMA operation.

5.2.1 Terminology: Packet, Frame, and

Transfer

The terms Packet, Frame, and Transfer are used

extensively in the following sections. They are

defined below for clarity:

Packet: The term "packet" refers to the entire se-

rial string of bits transmitted over an Ethernet

network. This includes the preamble, Start-of-

Frame Delimiter (SFD), Destination Address

(DA), Source Address (SA), Length field, Data

field, pad bits (if necessary), and Frame Check

Sequence (FCS, also called CRC). Figure 3.6

shows the format of a packet.

Frame: The term "frame" refers to the portion of

a packet from the DA to the FCS. This includes

the Destination Address (DA), Source Address

(SA), Length field, Data field, pad bits (if neces-

sary), and Frame Check Sequence (FCS, also

called CRC). Figure 3.6 shows the format of a

frame. The term "frame data" refers to all the

data from the DA to the FCS that is to be trans-

mitted, or that has been received.

Transfer: The term "transfer" refers to moving

data across the ISA bus, to and from the

CS8900. During receive operations, only frame

data are transferred from the CS8900 to the host

(the preamble and SFD are stripped off by the

CS8900’s MAC engine). The FCS may or may

not be transferred, depending on the configura-

tion. All transfers to and from the CS8900 are

counted in bytes, but may be padded for double

word alignment.

DS150PP2

79

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