Transcript Programmable logic devices part 1

Programmable logic devices
• PLA circuits
– Built around homogeneous arrays of elementary
components that can be configured to perform
logical AND and OR operations
– We look at basic structures that can realize AND,
OR functions (in reality PLAs are based on
NAND/NOR functions in CMOS technology)
• PLAs can belong to
– Gate arrays, if interconnect layer is not finished
– PLDs, if fuses are provided along with a full
interconnect layer
• FPGA are PLD
Programmable logic devices
Programmable logic devices
Programmable logic devices
Diode
logic
Diode
logic
Diode logic
( in the compact form, X indicates presence of a diode )
Diode logic
• Multiple
functions
realized by an
AND-OR array
• ab is “shared”
by both outputs
Diode logic
• Example:
• PLA table:
– AND array:
• 0  complement
connected;
• 1  true form
connected;
• X  no connection
– OR array:
• 1  connection;
• 0  no connection
Diode logic
• Resulting PLA:
– The AND and the
OR arrays are
programmable
Diode logic
a figure
How many literals in each prime implicant?
PLA realization of the majority voter
Field-programmable arrays
• Uncommitted AND/OR
arrays with metal fuses
that can be blown.
– Unprogrammed AND
array:
• Programmed AND function A’BC’
and its compact notation
– X means fuse intact (not blown)
• Programmed AND function AB’
and its compact notation
• Fuse-programmable OR
arrays
– Unprogrammed:
– Programmed OR function
P1+P3 and its compact
notation
Other options in programmable devices
• Output
polarity
options
– Programmed
active high
– Programmed
active low
• Apply
transforms
to indirectly
obtain the
POS
formula
Other options in programmable devices
• Bidirectional
pins and
feedback
lines
– Driver
enabled
– Driver
disabled
Other options in programmable devices
• Bidirectional pins and feedback lines
– When driver enabled, sum term Sm available as output
and fed back to the AND array
• This allows multilevel circuits (more than 2 levels)
– When driver disabled, the IOm pin acts as external input
• Example: implement a 2-bit ripple-carry adder
– Use PLA with 4 inputs, 3 outputs and 2 bi-directional pins
– Use one bi-directional pin for input, the other for
output/feedback
Two-bit ripple carry adder implementation
• Question: why are all the fuses left intact on product
term 16?
Commercial PLDs
• Basic PLD organization
Commercial PLDs
• Some commercially available PLDs