CSET 4650 Field Programmable Logic Devices

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

2. UV erasable PROM 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

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

4. programmable OR connections Conceptual PROM Architecture k = 5 2k = 32 • 32x8 PROM • 5-to-32 decoder • 8 OR gates with 32 inputs • 32x8 internal programmable 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. decoder logic 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

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

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