CS8900A
Crystal LAN™ ISA Ethernet Controller
3.0 FUNCTIONAL DESCRIPTION
3.1 Overview
transmits it onto the network. The second phase be-
gins with the CS8900A transmitting the preamble
and Start-of-Frame delimiter as soon as the proper
number of bytes has been transferred into its trans-
mit buffer (5, 381, 1021 bytes or full frame, de-
pending on configuration). The preamble and Start-
of-Frame delimiter are followed by the Destination
Address, Source Address, Length field and LLC
data (all supplied by the host). If the frame is less
than 64 bytes, including CRC, the CS8900A adds
pad bits if configured to do so. Finally, the
CS8900A appends the proper 32-bit CRC value.
During normal operation, the CS8900A performs
two basic functions: Ethernet packet transmission
and reception. Before transmission or reception is
possible, the CS8900A must be configured.
3.1.1 Configuration
The CS8900A must be configured for packet trans-
mission and reception at power-up or reset. Various
parameters must be written into its internal Config-
uration and Control registers such as Memory Base
Address; Ethernet Physical Address; what frame
types to receive; and which media interface to use.
Configuration data can either be written to the
CS8900A by the host (across the ISA bus), or load-
ed automatically from an external EEPROM. Oper-
ation can begin after configuration is complete.
The Section 5.7 on page 99 provides a detailed de-
scription of packet transmission.
3.1.3 Packet Reception
Like packet transmission, packet reception occurs
in two phases. In the first phase, the CS8900A re-
ceives an Ethernet packet and stores it in on-chip
memory. The first phase of packet reception begins
with the receive frame passing through the analog
front end and Manchester decoder where Manches-
ter data is converted to NRZ data. Next, the pream-
ble and Start-of-Frame delimiter are stripped off
and the receive frame is sent through the address
filter. If the frame’s Destination Address matches
the criteria programmed into the address filter, the
packet is stored in the CS8900A’s internal memo-
ry. The CS8900A then checks the CRC, and de-
pending on the configuration, informs the
processor that a frame has been received.
Section 3.3 on page 19 and Section 3.4 on page 21
describe the configuration process in detail.
Section 4.4 on page 47 provides a detailed descrip-
tion of the bits in the Configuration and Control
Registers.
3.1.2 Packet Transmission
Packet transmission occurs in two phases. In the
first phase, the host moves the Ethernet frame into
the CS8900A’s buffer memory. The first phase be-
gins with the host issuing a Transmit Command.
This informs the CS8900A that a frame is to be
transmitted and tells the chip when to start trans-
mission (i.e. after 5, 381, 1021 or all bytes have
been transferred) and how the frame should be sent
(i.e. with or without CRC, with or without pad bits,
etc.). The Host follows the Transmit Command
with the Transmit Length, indicating how much
buffer space is required. When buffer space is
available, the host writes the Ethernet frame into
the CS8900A’s internal memory, either as a Mem-
ory or I/O space operation.
In the second phase, the host transfers the receive
frame across the ISA bus and into host memory.
Receive frames can be transferred as Memory
space operations, I/O space operations, or as DMA
operations using host DMA. Also, the CS8900A
provides the capability to switch between Memory
or I/O operation and DMA operation by using
Auto-Switch DMA and StreamTransfer.
The Section 5.2 on page 79 through Section 5.6 on
page 96 provide a detailed description of packet re-
ception.
In the second phase of transmission, the CS8900A
converts the frame into an Ethernet packet then
CIRRUS LOGIC PRODUCT DATA SHEET
DS271PP3
17
CIRRUS [ CIRRUS LOGIC ]
