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PROGRAMMABLE LOGIC DEVICES [PLDs]
A programmable Logic Device is a device whose logic characteristics can be configured by the user.
A. ONE TIME CONFIGURABLE PLDs [SPLDs]
- PROGRAMMABLE ARRAY LOGIC [PALs]
PALs consists of an array of AND and OR gates. The AND array is configurable while the OR array is fixed. Variants with a registered o/p are available.
- PROGRAMMABLE LOGIC ARRAY [PLAs]
PLAs consists of an array of AND and OR gates both of which are configurable.
B. RE-CONFIGURABLE PLDs
- COMPLEX PROGRAMMABLE LOGIC DEVICES [CPLDs]
CPLDs are made up of Macro cells, which are configurable. CPLDs consist of multiple PALS like function blocks that can be interconnected through a switch matrix. Configuration data in these devices is stored in FLASH; EPROM or E2PROM memory cells and thus remains even when power is switched off. These devices can be re-configured almost indefinitely.
- FIELD PROGRAMMABLE GATE ARRAY [FPGAs]
These are programmable logic devices that support the implementation of relatively large logic circuits. FPGAs are different from SPLDs and CPLDs in that they provide logic blocks instead of AND, OR planes for implementation of the required functions. Configuration data in these devices is stored in static random access memory (SRAM) cells, which being volatile, loose their information when power supply is turned off. These devices however can be re-configured indefinitely.
C. APPLICATION SPECIFIC INTEGRATED CIRCUITS [ASICs]
These are chips designed for a specific application. They provide a higher logic density than the chips described above, lower power consumption, lower cost and higher performance. They are however not user configurable, require longer design cycles, have a much higher Non-recurring Engineering (NRE) cost and require order quantities in millions of pieces.
D. FPGAs vs. ASICs
ASICs are perfectly suitable for the mass production situations whereas FPGAs are suited for prototyping, low volume and higher value addition situations. Having said this the gap between ASICs and FPGAs is fast narrowing with FPGAs and masked FPGAs being used in situations never seen before. In a time of increasing design complexity along with shortened time to market requirements, FPGAs are providing a timely solution.
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