Programmable Logic Array (PLA) and Programmable Array Logic (PAL) Guide
1. Programmable Array Logic and Programmable Logic Array
Programmable Logic Array
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.
Logic expressions for content information to be stored in PLA must be obtained first,
then minimized, and finally programmed into PLA.
Number of input buffer = number of variables
Numer of programmable AND gates = number of minterms (minterms should not be
repeated)
Number of programmable OR gates = number of function
X = fusible/programmable = fixed
Block Diagram of PLA:
2. Application:
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.
Note that the use of the word "programmable" does not indicate that all PLAs are field-
programmable; in fact many are mask-programmed during manufacture in the same manner as
a mask ROM. This is particularly true of PLAs that are embedded in more complex and
numerous integrated circuits such as microprocessors. PLAs that can be programmed after
manufacture are called FPGA (Field-programmable gate array), or less frequently FPLA (Field-
programmable logic array)..
The Commodore 64 home computer released in 1982 used a "906114-01 PLA" to handle system
signals.
Example 1:
Given the truth table, design the combinational circuit using PLA.
A B C Y1 Y2
0 0 0 0 0
0 0 1 0 0
0 1 0 0 0
0 1 1 0 1
1 0 0 1 0
1 0 1 1 1
1 1 0 0 0
1 1 1 1 1
Solution:
Y1:
3. 0 0 0 0
1 1 1 0
Y1= AB + AC
Y2:
0 0 1 0
0 1 1 0
Y2= AC + BC
Example 2:
Given:
Y1 = ABC + ABC + ABC
Y2 = AB + AC + ABC
Y3 = ∈ 0 ,2 , 6 ,7
Implement the given function into PLA.
Y3:
5. Programmable Array Logic (PAL)
The PAL architecture consisted of a programmable AND array and a fixed OR array so
that each output is the sum of a specific set of product terms.
Commonly used type of PLD
Only the connected inputs to AND are programmable, OR gates are fixed
Unlike the PLA, a product term cannot be shared among two or more OR gates.
Therefore, each function can be simplified by itself without regard to common product
terms.
Standard architecture used to implement combinational circuits
Block Diagram:
9. Example 1:
Given the outputs X, Y, Z and its minterms are specified.
X (A B C) = €m (2, 3, 5, 7)
Y (A B C) = €m (0, 1, 5)
Z (A B C) = €m (0, 2, 3, 5)
Solution:
X:
0 0 1 1
0 1 1 0
X= AC + A’B + AB = 3 minterms
Y:
1 1 0 0
0 1 0 0
Y = A’B’ + B’C = 2 minterms
Z:
1 0 1 1
0 1 0 0
Z= AB’C + A’C’ + A’B = 3 minterms
Time to program using PAL.
1. Looking for the number of variable. The number of variables is equal to the input buffer.
An input buffer is a combination of NOT gate that gives you two outputs, the
complemented and non-complemented.
We have 3 variablessowe have 3
inputbuffer.
10. 2. The number of AND arrays is equal to the number of output times to the number of
maximum min-terms.
3. The number of the outputs are 3 so we have 3 OR gates the (X, Y, and Z).
We have 9 AND gate because we
have 3 outputsandhas 3 minterms.
We are goingto program the
Programmable ArrayLogic(PAL) now.
11. 4. The programmed PLA.
Example 2. Full adder using PAL.
In this example we are doing almost the same as we did to the example 1 but
some changes occurred.
A B Cin Sum Carry
0 0 0 0 0
0 0 1 1 0
0 1 0 1 0
0 1 1 0 1
1 0 0 1 0
1 0 1 0 1
1 1 0 0 1
1 1 1 1 1
12. K-Mapping
Sum
0 1 0 1
1 0 1 0
Sum = AB’Cin’ + A’B’Cin + ABCin + A’BCin’ = 4 minterms
Carry
0 0 1 0
0 1 1 1
Carry = ACin + BCin + AB = 3 minters
Programming the PAL
13. Example 3. Given the function W, X, Y, and Z.
W (A B C D) = Σm(2, 12, 13)
X (A B C D) = Σm(7, 8, 9, 10, 11, 12, 13, 14, 15)
Y (A B C D) = Σm(0, 2, 3, 4, 5, 6, 7, 8, 10, 11, 15)
Z (A B C D) = Σm(1, 2, 8, 12, 13)
K-Mapping
W
0 0 0 1
0 0 0 0
1 1 0 0
0 0 0 0
W= ABC’ + A’B’CD’
X
0 0 0 0
0 0 1 0
1 1 1 1
1 1 1 1
X = A + BCD
Y
1 0 1 1
1 1 1 1
0 0 1 0
1 0 1 1
Y = A’B + CD + B’D
Z
0 1 0 1
0 0 0 0
1 1 0 0
1 0 0 0
Z = A’B’CD’ + ABC’ + A’B’C’D + AC’D’
Z = W + A’B’C’D + AC’D’
As you can see, we used the output of the other function to minimize the used of terms.
![Application:
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.
Note that the use of the word "programmable" does not indicate that all PLAs are field-
programmable; in fact many are mask-programmed during manufacture in the same manner as
a mask ROM. This is particularly true of PLAs that are embedded in more complex and
numerous integrated circuits such as microprocessors. PLAs that can be programmed after
manufacture are called FPGA (Field-programmable gate array), or less frequently FPLA (Field-
programmable logic array)..
The Commodore 64 home computer released in 1982 used a "906114-01 PLA" to handle system
signals.
Example 1:
Given the truth table, design the combinational circuit using PLA.
A B C Y1 Y2
0 0 0 0 0
0 0 1 0 0
0 1 0 0 0
0 1 1 0 1
1 0 0 1 0
1 0 1 1 1
1 1 0 0 0
1 1 1 1 1
Solution:
Y1:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)