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

How many literals in each prime implicant?

a figure

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