1. CSET 4650 Field Programmable Logic Devices
Logic Implementation
Using Programmable ROMs
CSET Field Programmable Logic Devices
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2. Programmable Read Only Memory
A ROM is a memory device that holds a fixed, addressable data set
A PROM may be programmed by the designer
UV erasable
PROM

3. Programmable Read Only Memory
Typical uses include:
Code converters
Character generators
Data storage tables
Program stores
Loaded with tabular data – not Boolean equations
e.g., a truth table

4. Conceptual PROM Architecture
32x8 PROM
5-to-32 decoder
8 OR gates with 32 inputs
32x8 internal programmable connections
k = 5
2k = 32
programmable OR connections
output word (8-bits)

5. Conceptual PROM Architecture
A PROM has a fixed AND array (that decodes the memory address) followed by a programmable OR array (outputs)
For each of a given set of input combinations (address), it generates a multi-bit value which has been programmed into the device

6. Using PROMs as PLDs
The output functions need to be expressed in canonical minterm form for PROM implementation
every input variable appears in each product term in its true or inverted form
Each minterm is used to represent an address
Each address generates a multi-bit output

7. Realistic PROM Architecture
decoder logic
A PROM has a fixed AND array and a programmable OR array
Remember, we are only programming the CONNECTIONS to the OR gates
AND gate connections are all possible combinations

8. Simplified PROM Architecture
Inputs A and B
true and inverted
Outputs Y and Z
e.g., two functions
Programmable connections to OR gates
0 0
0 1
1 0
1 1

9. Example 7-11 from Sandige Binary to hex character generatorOA OC OD OE OB OF OG
Binary to hex character generator
Using a seven-segment display device and the character scheme at right