Field-programmable logic devices FPLA circuits –Packaged PLA components with a fuse at every diode in both the AND and OR sections, that can be configured by the user

FPLA Philips PLS100 FPLA

FPLA Realizing functions f 1, f 2 and f 3 with Philips PLS100 FPLA

Programmable Read-Only Memory PROM: fixed AND array and programmable OR array Generates all minterms Can realize k functions

Programmable Read-Only Memory One can directly implement a SOP expression from its minterms using a PROM Is minimization useful with a PROM?

Programmable Read-Only Memory The AND array provides all 2 n minterms –i.e. equivalent to n  2 n decoder Typically no polarity/feedback options No advantage in minimizing the logic function when using a PROM Effective for applications that require most minterms to be used –Code converters –Decoders –Lookup tables

PROM application Gray code generator using a PROM –15 out of the 16 minterms are used in at least one output (which minterm is not used?) –Therefore, a PROM is not a “waste” for this problem

PROM application

Lookup tables

PROM application Lookup table usage example

PROM application Multiplier implementation using PROMs –PROMs 1 to 4 are multipliers

Multiplier elucidation A n-1…n/2 A n/2…0 n bits  low high n/2 bits X n bits +n+1 bits 3n/2 bits 2n bits = 3n/2 + n/2 AHAH ALAL BHBH BLBL W = A L B L X = A H B L Y = A L B H Z = A H B H U = X + Y V = Z*2 n/2 + W H T = U + V R = T*2 n/2 + W L

PROM application Read-Only Memory slightly modified

Programmable Array Logic Fixed OR array, programmable AND array –Limited number of product terms –Product terms cannot be shared –Designer just selects products More compact and cheaper than a PROM

Programmable Array Logic Because of fixed OR, the standard representation for PALs is with OR gates

PAL example

ROM technologies For n inputs and k outputs PROM, what would be the total –Number of diodes? –Number of fuses? Mask programmed ROM –No user-programmable fuses –Custom mask places wires at fabrication time –Compact and fast –Useful only if sold in large volume, otherwise PROM cheaper

ROM technologies EPROM-can be erased by UV light and re- programmed EEPROM -electrically erasable and re- programmable –Both EPROM and EEPROM have relatively short lives: not as permanent as a blown fuse –Complex and slower than ROM –Flexible: can be re-used in iterative design

ROM technologies Answer: –AND array: 2n  2 n diodes –OR array: k  2 n diodes and k  2 n fuses PROMPALFPLA Fixed ANDOR- Programmable ORANDAND, OR FPLA: introduced 1975 –Organized i  p  o –Does FPLA implementation benefit from minimizing “literals”?

PLD design process