# PLAs Programmable Logic Arrays

## Presentation on theme: "PLAs Programmable Logic Arrays"— Presentation transcript:

PLAs Programmable Logic Arrays
ECEn 224

PLAs Objectives Explain the operation of a PLA
Use a PLA to implement a logic network Given a PLA design, determine the logic functions implemented ECEn 224

Programmable Logic Array (PLA)
n Input Lines . AND Array OR Array . ... k Word Lines m Outputs Lines ECEn 224

PLA AND-OR Equivalent OR AND Array Array F1 = AB’+ C F2 = A’C’ + BC
F3 = AB’ + A’C’ F4 = A’C’ + BC + AB’C Note: Not every minterm is available ECEn 224

PLA Table Generation F1 = AB’+ C F2 = A’C’ + BC F3 = AB’ + A’C’
F4 = A’C’ + BC + AB’C ECEn 224

Internal PLA Structure
Inputs Outputs ECEn 224

Internal PLA Structure
Inputs The AND plane lines get pulled up to +V Outputs ECEn 224

Internal PLA Structure
Inputs The AND plane lines stay at +V unless one of the connected inputs pulls it low. Outputs ECEn 224

Internal PLA Structure
Inputs 1 The AND plane lines stay at +V unless one of the connected inputs pulls it low. Outputs ECEn 224

Internal PLA Structure
Inputs The OR plane lines get pulled down to Ground Outputs ECEn 224

Internal PLA Structure
Inputs The OR plane lines get pulled down to Ground Outputs ECEn 224

Internal PLA Structure
Inputs The OR plane lines stay at GND unless one of the AND plane lines pulls it high. Outputs ECEn 224

Internal PLA Structure
Inputs The OR plane lines stay at GND unless one of the AND plane lines pulls it high. Outputs ECEn 224

Internal PLA Structure
Inputs The OR plane lines stay at GND unless one of the AND plane lines pulls it high. Outputs ECEn 224

Internal PLA Structure
Inputs The OR plane lines stay at GND unless one of the AND plane lines pulls it high. Outputs ECEn 224

Another PLA Representation
Inputs Outputs ECEn 224

Programmable Logic Array (PLA)
All inputs and inverted inputs available Limited number of AND functions This is the biggest difference between a ROM and a PLA Complete choice of inputs and inverted inputs for each AND function OR function for each output Complete choice of AND functions for each OR function (output) ECEn 224

Implement these equations: X = ABC + B’D’ + AB’D + C’D’ Y = BC + D’
PLA Example Implement these equations: X = ABC + B’D’ + AB’D + C’D’ Y = BC + D’ Z = CD + B’D’ + A’BC in this PLA: 8 terms 1 2 3 4 5 6 7 8 6 terms or AND plane lines How can we implement 8 product terms with 6 AND plane lines? ECEn 224

PLA Example Implement these equations: X = ABC + B’D’ + AB’D + C’D’
Y = BC + D’ Z = CD + B’D’ + A’BC ECEn 224

PLA Example Implement these equations: X = ABC + B’D’ + AB’D + C’D’
Y = BC + D’ Z = CD + B’D’ + A’BC ECEn 224

PLA Example Implement these equations: X = ABC + B’D’ + AB’D + C’D’
Y = BC + D’ Z = CD + B’D’ + A’BC ECEn 224

PLA Example Implement these equations: X = ABC + B’D’ + AB’D + C’D’
Y = BC + D’ Z = CD + B’D’ + A’BC ECEn 224

PLA Example C’D’ is in X and Y and looks useful ECEn 224

PLA Example C’D’ is in X and Y and looks useful
B’D’ is in all three functions ECEn 224

PLA Example C’D’ is in X and Y and looks useful
B’D’ is in all three functions A’BC and ABC cover a lot of minterms ECEn 224

PLA Example C’D’ is in X and Y and looks useful
B’D’ is in all three functions A’BC and ABC cover a lot of minterms The only ones left are AB’ and CD ECEn 224

PLA Example X = C’D’ + B’D’ + AB’ + ABC Y = C’D’ + B’D’ + ABC + A’BC
Z = B’D’ + CD + A’BC All of the functions are covered using only 6 product terms How is this possible? ECEn 224

PLA Example X = C’D’ + B’D’ + AB’ + ABC Y = C’D’ + B’D’ + ABC + A’BC
Z = B’D’ + CD + A’BC ECEn 224

PLA Example X = C’D’ + B’D’ + AB’ + ABC Y = C’D’ + B’D’ + ABC + A’BC
Z = B’D’ + CD + A’BC ECEn 224

PLA Example X = C’D’ + B’D’ + AB’ + ABC Y = C’D’ + B’D’ + ABC + A’BC
Z = B’D’ + CD + A’BC ECEn 224

PLA Example X = C’D’ + B’D’ + AB’ + ABC
Y = C’D’ + B’D’ + ABC + A’BC Z = B’D’ + CD + A’BC ECEn 224

Summary Explain the operation of a ROM
Use a ROM to implement a logic network Explain the operation of a PLA Use a PLA to implement a logic network Given a PLA design, determine the logic functions implemented ECEn 224