2. Overview Programmable Implementation Technologies (section 6.8)
Why Programmable Logic?
Read-Only Memories (ROMs)
Programmable Logic Arrays (PLAs)
Programmable Array Logic (PALs)

3. Why Programmable Logic?
Small and Medium Scale Integration
Up to 200 gates per device/IC
Most common is 74xx series (gates, FF, Decoders …)
Advantages
Easy to understand functions
Exceptional Signal Visibility
Disadvantages
Low logic density means big boards or small designs
Higher power consumption
Failure concern per function

4. Why Programmable Logic?
Large Scale Integration
Ranging from 200 to 200,000 gates per device.
Small memories, programmable logic devices
Advantages
Higher logic density means smaller boards or larger designs.
Many devices can be programmed and reprogrammed, saving expense when changes are made.
Disadvantages
Need to learn how to use and program
Signal visibility is reduced

5. Why Programmable Logic ?
Many programmable logic devices are field- programmable, i. e., can be programmed outside of the manufacturing environment
Most programmable logic devices are erasable and reprogrammable.
Allows “updating” a device or correction of errors
Allows reuse the device for a different design - the ultimate in re-usability!
Ideal for course laboratories

6. Programmable Configurations
Read Only Memory (ROM) - a fixed array of AND gates and a programmable array of OR gates
Programmable Array Logic (PAL) - a programmable array of AND gates feeding a fixed array of OR gates.
Programmable Logic Array (PLA)Ò - a programmable array of AND gates feeding a programmable array of OR gates.
PAL is a registered trademark of Lattice Semiconductor Corp.

7. Gate Symbols
Figure 6-18 Conventional and Array Logic Symbols for OR Gate

8. Read Only Memory
Read Only Memories (ROM) or Programmable Read Only Memories (PROM) are OP logic devices with a fixed AND array and a programmable OR array.
have:
k input lines,
m output lines, and
m = 2k decoded minterms
n OR gates
Usually referred as 2k x n ROM
Can implement n functions
with k inputs
k-to-2k line decoder D7 D6 D5 D4 D3 D2 D1 D0 A2 A1 A0 A B C F0 F1 F2 F3 X

9. Read Only Memory
A program for a ROM or PROM is simply a multiple-output truth table
If a 1 entry, a connection is made to the corresponding minterm for the corresponding output
If a 0, no connection is made
There is no advantage of simplifying the function when using ROM since we need to specify the entire list of minterms.
Can be viewed as a memory with the inputs as addresses of data (output values), hence ROM or PROM names!

10. Read Only Memory Example
Example: A 8 X 4 ROM (N = 3 input lines, M= 4 output lines)
The fixed "AND" array is a “decoder” with 3 inputs and 8 outputs implementing minterms.
The programmable "OR“ array uses a single line to represent all inputs to an OR gate. An “X” in the array corresponds to attaching the minterm to the OR
Read Example: For input (A2,A1,A0) = 001, output is (F3,F2,F1,F0 ) = 0011.
What are the simplified expressions for F3, F2 , F1 and F0 in terms of (A2, A1, A0)? D7 D6 D5 D4 D3 D2 D1 D0 A2 A1 A0 A B C F0 F1 F2 F3 X
F3 = D7 + D5 + D2 = A2 A0 + A2’ A1 A0’
F2 = D7 + D0 = A2 A1 A0 + A2’ A1’ A0’
F1 = D4 + D1 = A1 A1’ A0’ + A2’ A1’ A0
F0 = D7 + D5 + D1 = A2 A0 + A1’ A0

11. Programmable Array Logic (PAL)
PAL is the opposite of ROM; it is PLD that has a programmable AND array and a fixed OR array.
Function has to be reduced into SOP before it is programmed into the PAL
Disadvantage
ROM guaranteed to implement any M functions of N inputs. PAL may have too few inputs to the OR gates.
Advantages
For given internal complexity, a PAL can have larger k (inputs) and n (outputs)
Some PALs have outputs that can be complemented, adding POS functions

12. Figure

13. Programmable Logic Array (PLA)
Compared to a ROM and a PAL, a PLA is the most flexible having a programmable set of ANDs combined with a programmable set of ORs.
A PLA has all of its product terms connectable to all outputs, overcoming the problem of the limited inputs to the PAL OR gates
Some PLAs have outputs that can be complemented (using XOR gates), adding POS functions

14. Programmable Array Logic (PLA)
The PAL is the opposite of the ROM, having a programmable set of ANDs combined with fixed ORs.
Fuse intact
Fuse blown 1 F 2 X A B C 3 4
A B
A C
B C
3-input, 3-output PLA with 4 product terms
Used to produce POS or complement of function

15. ROM, PAL and PLA Summary Fixed Programmable Programmable Inputs
AND array
Outputs
Connections
OR array
(decoder)
(a) Programmable read-only memory (PROM)
Programmable
Programmable
Fixed
Inputs
Outputs
Connections
AND array
OR array
(b) Programmable array logic (PAL) device
Programmable
Programmable
Programmable
Programmable
Inputs
Outputs
Connections
AND array
Connections
OR array
(c) Programmable logic array (PLA) device