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Programmable Logic Array

By Ashik

 Programmable Logic Array (PLA)
A combinational circuit may occasionally have don’t care conditions. When implemented with a ROM, a don’t care condition become an address input that will never occur. The words at don’t care addresses need not to be programmed.


Why PLA??
 
Consider for example, a combinational circuit that converts a 12-bit card code to a 6-bit internal alphanumeric code
 



 
Valid output = 47

If we use ROM size of ROM = 4096X6

The remaining 4096 words of ROM are not used and are thus wasted




 
The number of Programmed links = 2n×k + k × m + m

Whereas that of a ROM is 2n × m

For previous combinational circuit,  n=12  , k=47 , m=6

So, links for ROM = 4096 X 6 = 24576
       links for PLA = 1128+282+6 = 1416



 
F1= AB’+AC    F2= AC+BC

 

 

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