Microsoft Word - Digital Logic Design v_4_6a

(3) Erase ability

Some devices allow the blown fuses to be re-fused by:

(a) Ultraviolet Light (through a small window on the top)

(b) Electrically (typically at much higher voltage and current than normal operation)

”This type is referred to as EE-type.”

There are also universal programming units available that allow for fuse map input/output which

adheres to the Joint Electronic Device Engineering Council (JEDEC) Standard.

Programmable Read Only Memory (PROM)
A typical PROM may have 16 inputs with 2^16 outputs. This device would be called a 64K PROM.
PROM is typically used during development and once the product is in production, Read Only
Memory(ROM) will be used.

ROM is a one-time factory-only programmable device which is the reason why it is called Read

Only Memory. The initial ROM set up is costly but the cost per part is significantly lower than that

of PROM.

The following diagram shows the PROM internal diagram:

PROM Example: Implement F(A 0 ,A 1 )= A 0 .A 1 +A 0 .A 1

Since all the minterms are available, all we have to do to get the desired output is to OR the

appropriate minterms and then blow fuses for all the minterms not needed for the function.

An(MSB) ... A 0 (LSB)

m 0

m 1

m( 2 n+1 -1)

Factory set

Intact Fuse

n-bit input

AND array connections

are set in factory F 0 ... F m

Programmable OR array connections

p 3 p 2 p 1 p 0

p 0 + p 3