Transcript Programmable logic devices part 2

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
X
AH
AL
BH
BL
n bits  low high n/2 bits
n bits
W = ALBL
+
X = AHBL
Y = ALBH
Z = AHBH
R = T*2n/2 + WL
V = Z*2n/2 + WH
n+1 bits
3n/2 bits
T=U+V
3n/2 bits
U=X+Y
2n bits = 3n/2 + n/2
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 reprogrammed
• EEPROM electrically erasable and reprogrammable
– 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2n diodes
– OR array: k2n diodes and k2n fuses
Fixed
Programmable
PROM
PAL
FPLA
AND
OR
OR
AND
AND, OR
• FPLA: introduced 1975
– Organized ipo
– Does FPLA implementation benefit from minimizing
“literals”?
PLD design process