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 f1, f2 and f3 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 2n minterms– i.e. equivalent to n 2n 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

PROM application

• Lookup tables

PROM application

• Lookup table usage example

PROM application

• Multiplier implementation using PROMs– PROMs 1 to 4

are multipliers

Multiplier elucidation

An-1…n/2 An/2…0

n bits low high n/2 bits

X

n bits

+ n+1 bits

3n/2 bits

3n/2 bits

2n bits = 3n/2 + n/2

AH AL

BH BL

W = ALBL

X = AHBL

Y = ALBH

Z = AHBH

U = X + Y

V = Z*2n/2 + WH

T = U + V

R = T*2n/2 + WL

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: 2n2n diodes– OR array: k2n diodes and k2n fuses

PROM PAL FPLAFixed AND OR -

Programmable OR AND AND, OR

• FPLA: introduced 1975– Organized ipo– Does FPLA implementation benefit from minimizing

“literals”?

PLD design process