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AM79C90

CMOS Local Area Network Controller for Ethernet (C-LANCE)

Advanced Micro Devices 

PRELIMINARY

AMD

PROGRAMMING

This section defines the Control and Status Registers

and the memory data structures required to program the

Am79C90 (C-LANCE).

Programming the Am79C90 (C-LANCE)

The Am79C90 (C-LANCE) is designed to operate in an

environment that includes close coupling with local

memory and microprocessor (HOST). The Am79C90

C-LANCE is programmed by a combination of registers

and data structures resident within the C-LANCE and

memory registers. There are four Control and Status

Registers (CSRs) within the C-LANCE which are pro-

grammed by the HOST device. Once enabled, the

C-LANCE has the ability to access memory locations to

acquire additional operating parameters.

The Am79C90 has the ability to do independent buffer

management as well as transfer data packets to and

from the Ethernet. There are three memory structures

accessed by the Chip:

Initialization Block—12 words in contiguous mem-

ory starting on a word boundary. It also contains

the operating parameters necessary for device op-

eration. The initialization block is comprised of:

— Mode of Operation

— Physical Address

— Logical Address Mask

— Location to Receive and Transmit Descriptor

Rings

— Number of Entries in Receive and Transmit

Descriptor Rings

Receive and Transmit Descriptor Rings—Two ring

structures, one for incoming and outgoing packets.

Each entry in the rings is 4 words long and each

entry must start on a quadword boundary. The De-

scriptor Rings are comprised of:

— The address of a data buffer

— The length of that data buffer

— Status information associated with the buffer

Data Buffers—Contiguous portions of memory

reserved for packet buffering. Data buffers may

begin on arbitrary byte boundaries.

In general, the programming sequence of the C-LANCE

may be summarized as:

Program the C-LANCE’s CSRs by a host device to

locate an initialization block in memory. The byte

control, byte address, and address latch enable

modes are also defined here.

The C-LANCE loads itself with the information con-

tained within the initialization block.

The C-LANCE accesses the descriptor rings for

packet handling.

CONTROL AND STATUS REGISTERS

There are four Control and Status Registers (CSRs) on

the chip. The CSRs are accessed through two bus ad-

dressable ports, an address port (RAP) and a data port

(RDP).

Accessing the Control and Status

Registers

The CSRs are read (or written) in a two step operation.

The address of the CSR to be accessed is written into

the RAP during a bus slave transaction. During a subse-

quent bus slave transaction, the data being read from

(or written into) the RDP is read from (or written into) the

CSR selected in the RAP.

Once written, the address in RAP remains unchanged

until rewritten.

To distinguish the data port from the address port, a dis-

crete input pin is provided.

ADR Input Pin

L

H

Port

Register Data Port (RDP)

Register Address Port (RAP)

Register Data Port (RDP)

15

CSR DATA

0

17881B-13

Bit

Name

Description

15:00

CSR Data

Writing data into RDP writes the data

into the CSR selected in RAP. Read-

ing the data from the RDP reads the

data from the CSR selected in RAP.

CSR1, CSR2 and CSR3 are acces-

sible only when the STOP bit of

CSR0 is set.

If the STOP bit is not set while at-

tempting to access CSR1, CSR2 or

CSR3, the C-LANCE will return

READY, but a READ operation will

return undefined data. WRITE op-

eration is ignored.

Am79C90

19

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