PROGRAMMABL
E LOGIC ARRAY
Programmable Logic Array
• Programmable Logic Array is a type of PLD, which has
both programmable AND array and programmable OR
array.
• In a combinational circuit, because of don’t care
conditions, not all the minterms are used. In the case of
PROM, where the AND matrix is fixed, all the product
terms are not used. This makes the PROM PLD to poor
utilization.
Programmable Logic Array
• If the number of don’t care conditions is more, then
PROM will be not suitable for efficient usage.
Hence, it is very necessary to switch to a more
economical device, called Programmable Logic
Array(PLA).
Programmable Logic Array
• PLA is somewhat similar to the PROM. Both have a
programmable OR array, but in PLA, there is a
modification in the AND array. PLA does not
generate all the minterms, rather depending on the
function, the required product terms can be
obtained.
Block Diagram of Programmable Logic
Array
The following figure shows the block diagram of programmable logic array.
• It consists of n-inputs, inverters, input buffer, moutputs and an output buffer. Both AND and OR
array have fuses at the inputs, which makes them
to be programmable.
Block Diagram of Programmable Logic
Array
The following figure shows the block diagram of programmable logic array.
• The AND matrix or array produces product terms,
through the AND gates, which are ORed to get the
required function.
Block Diagram of Programmable Logic
Array
The following figure shows the block diagram of programmable logic array.
• In PLA, the input buffers are used to limit the
loading of the sources that drive the inputs. The
driving capacity of PLA device is increased with the
help of output buffers.
An example of PLA circuit
of a combinational circuit
is shown below. As you
can observe from the
circuit diagram, it is similar
to the PROM circuit,
except that the AND array
consists of fuses, to
program according to the
user requirements.
• The sequential circuits can also
be implemented with PLA. For
such implementation, flip-flops
are also provided at each output
of the OR matrix.
Programmable Logic Array
• The programmable logic array (PLA) is similar in
concept to the ROM, except that the PLA does not
provide full decoding of the variables and does not
generate all the minterms. The decoder is replaced by
an array of AND gates that can be programmed to
generate product terms of the input variables. The
product terms are then selectively connected to OR
gates to provide the sum of products for the required
Boolean functions
Programmable Logic Array
•Suppose we have 3 inputs A,B and C
Programmable Logic Array