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**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

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**Programmable logic devices**

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**Programmable logic devices**

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**Programmable logic devices**

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Diode logic

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Diode logic

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Diode logic ( in the compact form, X indicates presence of a diode )

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**Diode logic Multiple functions realized by an AND-OR array**

ab is “shared” by both outputs

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**Diode logic Example: PLA table: AND array: OR array:**

0 complement connected; 1 true form connected; X no connection OR array: 1 connection; 0 no connection

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Diode logic Resulting PLA: The AND and the OR arrays are programmable

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Diode logic a figure How many literals in each prime implicant?

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**PLA realization of the majority voter**

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**Field-programmable arrays**

Uncommitted AND/OR arrays with metal fuses that can be blown. Unprogrammed AND array:

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**Programmed AND function A’BC’ and its compact notation**

X means fuse intact (not blown)

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**Programmed AND function AB’ and its compact notation**

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**Fuse-programmable OR arrays**

Unprogrammed: Programmed OR function P1+P3 and its compact notation

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**Other options in programmable devices**

Output polarity options Programmed active high Programmed active low

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**Apply transforms to indirectly obtain the POS formula**

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**Other options in programmable devices**

Bidirectional pins and feedback lines Driver enabled Driver disabled

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**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

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**Two-bit ripple carry adder implementation**

Question: why are all the fuses left intact on product term 16?

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Commercial PLDs Basic PLD organization

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Commercial PLDs Some commercially available PLDs

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