MU9C4320L datasheet(2/32 Pages) MUSIC

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MU9C4320L ATMCAM

General Description

Rev. 3

GENERAL DESCRIPTION

The MU9C4320L ATMCAM is a 4K x 32 Content

Addressable Memory (CAM) with a 32-bit wide data

interface. The device is designed for use in ATM switches

and routers to provide very high throughput VPI/VCI

translation through lookup tables held in external RAM.

VPI/VCI fields from the ATM cell header are compared

against a list of current connections stored in the CAM

array. As a result of the comparison, the ATMCAM

generates an address that is used to access an external

RAM where VPI/VCI mapping data and other information

associated with the connection are stored.

The ATMCAM simultaneously compares the VPI/VCI in

the CAM array as well as the VPI in a separate VP table.

This capability provides support for both VPCs and VCCs,

and halves the number of cycles needed to compare VPI

and VCI information. A set of control states provides a

powerful and flexible control interface to the ATMCAM.

This control structure allows memory read and write,

control, addressing control, and initialization operations.

The ATMCAM architecture uses a non-multiplexed data

bus, direct hardware control of the device, and an

independent bus for returning match results. Software

control is also supported for systems where optimum

performance is not needed.

OPERATIONAL OVERVIEW

The ATMCAM is designed to act as an address translator

for lookup tables in ATM switch and router systems. It

takes incoming VP/VC identifiers in ATM cell headers

and generates addresses that access data in an external

RAM. The RAM holds translated VPI/VCI information

and other data relating to the particular connection. Refer

to Figure 2 for a simplified block diagram of an ATM

switch.

When a new connection is set up, the VPI/VCI of the

connection is written to the ATMCAM. The write cycle

can be to either a specific address within the device, or to

the next free address. The address at which the write takes

place is driven onto the Output Address bus, so VPI/VCI

mapping data can be written simultaneously into the

external RAM at the correct address.

With the connection established, the controller strips the

VPI/VCI information off an arriving cell to form the

Comparand, which is then compared against the contents

of the ATMCAM. The ATMCAM generates an address

that is used to access the VPI/VCI mapping data in the

external RAM. The controller reads the data from the

RAM and prefixes the translated header to the cell

payload.

If the switch operates on VPI only, as well as on full VPI/

VCI, the ATMCAM can compare both the VPI and VPI/

VCI fields simultaneously. The option to test VPI fields is

selected in the Configuration register of the ATMCAM.

When selected, the VPI field is used to form an address

that accesses a single-bit RAM array in the ATMCAM.

This area of memory is referred to as the VP Table, and

holds a single bit of information indicating the validity of

a VP value.

Figure 2: Switch Block Diagram

ATMCAM

Switch

Fabric

ATM Data Stream

ATM Controller

CAM

Control

Switch

Control

and

Data

VPI/VCI

Address

RAM

Data

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