MU9C4320L ATMCAM
General Description
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.
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,
register read and write, data move, comparison, validity
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.
The ATMCAM simultaneously compares the VPI/VCI in
the CAM array as well as the VPI in a separate VP table.
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.
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.
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.
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.
ATM Data Stream
Switch
Control
and
CAM
Control
Data
ATM Controller
VPI/VCI
Data
Switch
Fabric
Address
RAM
ATMCAM
Figure 2: Switch Block Diagram
2
Rev. 3
MUSIC [ MUSIC SEMICONDUCTORS ]
