A programmable logic array (PLA) is a kind of programmable logic device used to implement combinational logiccircuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output. It has 2N AND gates for N input variables, and for M outputs from PLA, there should be M OR gates, each with programmable inputs from all of the AND gates. This layout allows for many logic functions to be synthesized in the sum of products canonical forms.
In 1970, Texas Instruments developed a mask-programmable IC based on the IBM read-only associative memory or ROAM. This device, the TMS2000, was programmed by altering the metal layer during the production of the IC. The TMS2000 had up to 17 inputs and 18 outputs with 8 JK flip-flops for memory. TI coined the term Programmable Logic Array for this device.[1]
Obtain the minimum SOP form to reduce the number of product terms to a minimum.
Decide the input connection of the AND matrix for generating the required product term.
Then decide the input connections of OR matrix to generate the sum terms.
Decide the connections of invert matrix.
Program the PLA.
PLA block diagram:
1ST BLOCK
2ND BLOCK
3RD BLOCK
4TH BLOCK
5TH BLOCK
INPUT BUFFER
AND MATRIX
OR MATRIX
INVERT/ NON INVERT MATRIX
FLIP FLOP OUTPUT BUFFER
Advantages over read-only memory
The desired outputs for each combination of inputs could be programmed into a read-only memory, with the inputs being driven by the address bus and the outputs being read out as data. However, that would require a separate memory location for every possible combination of inputs, including combinations that are never supposed to occur, and also duplicating data for "don't care" conditions (for example, logic like "if input A is 1, then, as far as output X is concerned, we don't care what input B is": in a ROM this would have to be written out twice, once for each possible value of B, and as more "don't care" inputs are added, the duplication grows exponentially); therefore, a programmable logic array can often implement a piece of logic using fewer transistors than the equivalent in read-only memory. This is particularly valuable when it is part of a processing chip where transistors are scarce (for example, the original 6502 chip contained a PLA to direct various operations of the processor[2]).
Applications
One application of a PLA is to implement the control over a datapath. It defines various states in an instruction set, and produces the next state (by conditional branching). [e.g. if the machine is in state 2, and will go to state 4 if the instruction contains an immediate field; then the PLA should define the actions of the control in state 2, will set the next state to be 4 if the instruction contains an immediate field, and will define the actions of the control in state 4]. Programmable logic arrays should correspond to a state diagram for the system.
The earliest Commodore 64 home computers released in 1982 (into early 1983) initially used a programmed Signetics 82S100 PLA, but as the demand increased, MOS Technology / Commodore Semiconductor Group began producing a mask-programmed PLA, which bore part number 906114-01.[3]
. Java Applet. University of Hamburg. Archived from the original on 2013-01-15. Retrieved 2010-04-07.
August 24, 2023
programmable, logic, array, confused, with, programmable, array, logic, programmable, logic, array, kind, programmable, logic, device, used, implement, combinational, logic, circuits, programmable, gate, planes, which, link, programmable, gate, planes, which, . Not to be confused with Programmable Array Logic A programmable logic array PLA is a kind of programmable logic device used to implement combinational logic circuits The PLA has a set of programmable AND gate planes which link to a set of programmable OR gate planes which can then be conditionally complemented to produce an output It has 2N AND gates for N input variables and for M outputs from PLA there should be M OR gates each with programmable inputs from all of the AND gates This layout allows for many logic functions to be synthesized in the sum of products canonical forms PLA schematic examplePLAs differ from programmable array logic devices PALs and GALs in that both the AND and OR gate planes are programmable PAL has programmable AND gates but fixed OR gates clarification needed Contents 1 History 2 Implementation procedure 3 Advantages over read only memory 4 Applications 5 See also 6 References 7 External linksHistory EditIn 1970 Texas Instruments developed a mask programmable IC based on the IBM read only associative memory or ROAM This device the TMS2000 was programmed by altering the metal layer during the production of the IC The TMS2000 had up to 17 inputs and 18 outputs with 8 JK flip flops for memory TI coined the term Programmable Logic Array for this device 1 Implementation procedure EditPreparation in SOP sum of products form Obtain the minimum SOP form to reduce the number of product terms to a minimum Decide the input connection of the AND matrix for generating the required product term Then decide the input connections of OR matrix to generate the sum terms Decide the connections of invert matrix Program the PLA PLA block diagram 1ST BLOCK 2ND BLOCK 3RD BLOCK 4TH BLOCK 5TH BLOCKINPUT BUFFER AND MATRIX OR MATRIX INVERT NON INVERT MATRIX FLIP FLOP OUTPUT BUFFERAdvantages over read only memory EditThe desired outputs for each combination of inputs could be programmed into a read only memory with the inputs being driven by the address bus and the outputs being read out as data However that would require a separate memory location for every possible combination of inputs including combinations that are never supposed to occur and also duplicating data for don t care conditions for example logic like if input A is 1 then as far as output X is concerned we don t care what input B is in a ROM this would have to be written out twice once for each possible value of B and as more don t care inputs are added the duplication grows exponentially therefore a programmable logic array can often implement a piece of logic using fewer transistors than the equivalent in read only memory This is particularly valuable when it is part of a processing chip where transistors are scarce for example the original 6502 chip contained a PLA to direct various operations of the processor 2 Applications EditOne application of a PLA is to implement the control over a datapath It defines various states in an instruction set and produces the next state by conditional branching e g if the machine is in state 2 and will go to state 4 if the instruction contains an immediate field then the PLA should define the actions of the control in state 2 will set the next state to be 4 if the instruction contains an immediate field and will define the actions of the control in state 4 Programmable logic arrays should correspond to a state diagram for the system The earliest Commodore 64 home computers released in 1982 into early 1983 initially used a programmed Signetics 82S100 PLA but as the demand increased MOS Technology Commodore Semiconductor Group began producing a mask programmed PLA which bore part number 906114 01 3 See also EditField programmable gate array Gate array Programmable array logicReferences Edit Andres Kent October 1970 A Texas Instruments Application Report MOS programmable logic arrays Texas Instruments Bulletin CA 158 How MOS 6502 Illegal Opcodes really work arcadecomponents com Commodore 906114 01 64 PLA ICExternal links Edit Programmable Logic Array PLA cmsc311 University of Maryland 2003 Archived from the original on 2017 12 14 PLA programmable logic array Java Applet University of Hamburg Archived from the original on 2013 01 15 Retrieved 2010 04 07 Retrieved from https en wikipedia org w index php title Programmable logic array amp oldid 1129697428, wikipedia, wiki, book, books, library,
