Using APEX 20KE CAM for Fast Search
Applications
Technical Brief 56
Altera Corporation
101 Innovation Drive
San Jose, CA 95134
(408) 544-7000
http://www.altera.com
https://websupport.altera.com
August 1999, ver. 1
APEXTM 20KE embedded system blocks (ESBs) support content-addressable memory
(CAM), a parallel processing memory that accelerates applications requiring fast searches
through databases, lists, or patterns. CAM is memory technology developed from RAM.
Most other memory devices store and retrieve data by addressing specific memory
locations. However, in CAM, the system supplies the data and receives the data’s address,
as shown in Figure 1.
Figure 1. APEX Integrated CAM vs. RAM
RAM
CAM
When using CAM, the system supplies
the data, and CAM returns the address.
CAM
2C
Data
Address
A7
0
B3
1
2C
2
4F
3
Address
When using RAM, the system supplies
the address, and RAM returns the data.
RAM
2
2
Match
Flag
High
Address
Data
0
A7
1
B3
2
2C
3
4F
2C
APEX 20KE CAM offers an ideal solution for high-performance applications such as data
compression, network switches, Internet protocol filters, and peripheral component
interconnect (PCI) functions. In addition to performance advantages, integrated CAM in
APEX 20KE devices provides flexible CAM sizes. Figure 2 illustrates typical applications
using various CAM block sizes.
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M-TB-056-01
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TB 56: Using APEX 20KE CAM for Fast Search Applications
Figure 2. APEX CAM Offers Flexible Block Sizes
CAM
Width
APEX 20KE CAM
Discrete CAM (Fixed Sizes)
1,024 × 128
8,192 × 80
256 × 64
512 × 64
2,048 × 64
4,096 × 64
16,384 × 64
4,096 × 32
Small-Sized CAM Applications
n Switch Address Mapping
n Packet Header
Identification
n Pattern Recognition
n Internet Protocol Filter
n Cache Tag
Medium-Sized CAM Applications
n MAC Address Look-Up in
Layer 2 Bridges & Switches
n Address Translation or
Protocol Conversion in
Gateways
n Layer 3 Longest Match
Address Look-up for IPv4,
DECNET & Appletalk
n VPI/VCI Translation in ATM
Switches
Large-Sized CAM Applications
n ARP Cache in Network
Server
n Layer 2/4 Flow Recognition
for Fair Bandwidth Sharing
n Layer 3 Address Caching for
IPv6 Switch
CAM Depth
CAM vs. RAM in Memory Search Applications
Memory applications, which often involve searching, have previously been
implemented in programmable logic devices (PLDs) using RAM. Searching for an item
in RAM can take many clock cycles. The latency of the search depends on the depth of
the RAM block; a 64-word × 32-bit RAM block requires up to 64 clock cycles to find the
data.
Identifying an item stored in memory by its data content rather than its address can be
more efficient. CAM works this way, making it ideal for high-speed search
applications. CAM simultaneously compares the data requested against a list of entries,
providing an order of magnitude reduction in search time over RAM. Other memory
algorithms, such as binary- or tree-based searches, or look-aside tag buffers, perform a
multi-cycle search through the memory space and are much slower than CAM.
To better understand the performance advantages of CAM, compare the total time
required to search an item using both RAM and CAM implementations. Locating an
item in a 32-word × 32-bit RAM block running at 125 MHz requires up to 256 ns
(32 clock cycles × 8-ns clock cycle). In contrast, the total time required to find an item in
a similar-sized CAM block is 4 ns (1 cycle × 4-ns clock cycle). CAM is up to 98% faster
than RAM and has a latency of one clock cycle compared to a maximum of 32 cycles for
RAM.
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Altera Corporation
TB 56: Using APEX 20KE CAM for Fast Search Applications
APEX CAM Integration Enhances Performance
Traditionally, search applications use discrete CAM, where CAM is implemented as a
separate device. A designer must add an individual CAM device to the printed circuit
board (PCB), which increases design time and reduces the amount of usable PCB space.
Discrete CAM also reduces system performance because it introduces additional onand off-chip delays.
APEX CAM provides higher performance than discrete CAM by eliminating on- and
off-chip and PCB board delays. Also, APEX CAM—which is manufactured on a
0.18-µm process—has a much faster access time (4 ns) than typical discrete CAM,
which is generally manufactured on older processes, resulting in a slower access time
(20 ns). Figure 3 illustrates the system performance advantages of APEX CAM over
discrete CAM.
Figure 3. APEX Integrated CAM Provides Superior Performance
Discrete CAM System Performance
APEX CAM System Performance
PLD
APEX 20KE-1
LUT
Discrete CAM
Register
LUT
Register
CAM
Register
t D = 1.0 ns
t LAD = 4.0 ns
tCO = 3.7 ns
Register
t CO = 0.2 ns
t D = 1.0 ns
t SU = 0.7 ns
Delay = 0.2 ns + 4.0 ns + 0.7 ns = 4.9 ns
tACC = 20.0 ns
tSU = 2.5 ns
Delay = 3.7 ns + 1.0 ns + 20.0 ns + 1.0 ns + 2.5 ns = 28.2 ns
In applications that require a CAM block larger than what is available in an APEX 20KE
device, you can use the embedded APEX CAM to cache a larger, discrete CAM block.
This implementation of APEX 20KE CAM accelerates large CAM applications, as
shown in Figure 4. If the APEX CAM that interfaces with the system finds a match in
one clock cycle, the system immediately proceeds. Otherwise, the system waits two or
three clock cycles for the external CAM to find a match, and then proceeds.
Figure 4. APEX CAM as Cache for Large, Discrete CAM
APEX 20KE Device
Large
External CAM
tACC = 20.0 ns
100-MHz System
Data
Data
ESB CAM
tLAD = 4.0 ns
Data
Address
Address
Altera Corporation
Address
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TB 56: Using APEX 20KE CAM for Fast Search Applications
APEX CAM Provides Flexibility
APEX CAM offers variable CAM sizes. One APEX ESB can be configured as a
32-word × 32-bit CAM block, and multiple ESBs can be cascaded together to implement
wider and deeper CAM blocks. You can create any CAM depth or width as long as
there are additional unused ESBs. For example, if all ESBs in an EP20K1500E device are
used for CAM, you can create a 228-Kbit CAM block with varying widths or depths
(e.g., 7,296 words × 32 bits or 3,648 words × 64 bits). Table 1 lists the APEX ESB
resources available.
Table 1. APEX Device Densities & ESB Resources
Feature
EP20K60E EP20K100E EP20K160E EP20K200E EP20K300E EP20K400E EP20K600E EP20K1000E EP20K1500E
Maximum
System Gates
162,000
263,000
404,000
526,000
728,000
Logic Elements
2,560
4,160
6,400
8,320
11,520
16,640
16
26
40
52
72
104
16,384
26,624
40,960
53,248
73,728
106,496
ESBs
Maximum CAM
Bits
1,052,000 1,537,000
1,772,000
2,524,000
24,320
38,400
54,720
152
160
228
155,648
163,840
233,472
Conclusion
The APEX family is the first to offer integrated CAM in a PLD. This advanced feature
provides system performance benefits to designers by simplifying functions that
require searches through lists or data tables. With the additional benefits of enhanced
system performance, effective resource utilization, and inherent configuration
flexibility, APEX devices offer a spectrum of CAM block sizes, providing superior
integration benefits over competing devices.
®
101 Innovation Drive
San Jose, CA 95134
(408) 544-7000
http://www.altera.com
4
Copyright  1999 Altera Corporation. Altera, APEX, APEX 20K, APEX 20KE, EP20K60E, EP20K100E, EP20K160E, EP20K200E, EP20K300E,
EP20K400E, EP20K600E, EP20K1000E, and EP20K1500E are trademarks and/or service marks of Altera Corporation in the United States and
other countries. Other brands or products are trademarks of their respective holders. The specifications contained herein are subject to change
without notice. Altera assumes no responsibility or liability arising out of the application or use of any information, product, or service
described herein except as expressly agreed to in writing by Altera Corporation. Altera customers are advised to obtain the latest version of
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