Logic Implementation Using Programmable ROMs

Logic Implementation
Using Programmable ROMs
CSET 4650
Field Programmable Logic Devices
Dan Solarek

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
2

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
3

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

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 multi bit value which has been
programmed into the device
5

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 multi bit output
6

Realistic PROM Architecture
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
decoder logic
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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
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Example 7-11 7 11 from Sandige
Binary to hex character
generator
Using a seven-segment
seven segment
display device and the
character scheme at right
OE
OF
OC
OA
OG
OD OB
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