EP1M350 datasheet(43/86 Pages) ALTERA

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Altera Corporation

Mercury Programmable Logic Device Family Data Sheet

CAM is used for high-speed search operations. When searching for data

within a RAM block, the search is performed serially. Thus, finding a

particular data word can take many cycles. CAM searches all addresses in

parallel and outputs the address storing a particular word. When a match

is found, a match flag is set high. CAM is ideally suited for applications

such as Ethernet address lookup, data compression, pattern recognition,

cache tags, fast routing table lookup, and high-bandwidth address

filtering. Figure 22 shows the CAM block diagram.

Figure 22. CAM Block Diagram

The Mercury on-chip CAM provides faster system performance than

traditional discrete CAM. Integrating CAM and logic into the Mercury

device eliminates off-chip and on-chip delays, improving system

performance.

When in CAM mode, the ESB implements a 32-word, 32-bit CAM. Wider

or deeper CAM, such as a 32-word, 64-bit or 128-word, 32-bit block, can

be implemented by combining multiple CAM blocks with some ancillary

logic implemented in LEs. The Quartus II software automatically

combines ESBs and LEs to create larger CAM blocks.

CAM supports writing “don’t care” bits into words of the memory. The

don’t-care bit can be used as a mask for CAM comparisons; any bit set to

don’t-care has no effect on matches.

CAM can generate outputs in three different modes: single-match mode,

multiple-match mode, and fast multiple-match mode. In each mode, the

ESB outputs the matched data’s location as an encoded or unencoded

address. When encoded, the ESB outputs an encoded address of the data’s

location. For instance, if the data is located in address 12, the ESB output

is 12. When unencoded, each ESB port uses its 16 outputs to show the

location of the data over two clock cycles. In this case, if the data is located

in address 12, the 12th output line goes high. Figures 22 and 23 show the

encoded CAM outputs and unencoded CAM outputs, respectively.

data[]

wraddress[]

wren

inclock

inclocken

inaclr

data_address[]

match

outclock

outclocken

outaclr

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