ECE 3110: Introduction to Digital Systems

Chapter 5 Combinational Logic Design

Practices

Programmable Logic Devices

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Previous… Timing Circuits

Timing diagrams Single value delay Min:typical:max delay specifications Propogation delays LOW-to-HIGH and HIGH-to-LOW

delays Worst-case delay

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Programmable Logic Devices (PLDs) PLDs can implement wide functions

efficiently (functions with many input variables).

PLDs can implement multiple functions of different variables efficiently.

The logic in PLDs is programmable -- it can be defined by the user and programmed on the desktop Most PLDs can be erased and reprogrammed

many times.

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PLD types

There are MANY different types of PLDs. Densities ranges from from 10’s of gates

to 100’s of thousands of gates. We will look at PLAs(Programmabe Logic

Arrays) and PALs (Programmable Array Logic devices).

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Programmable Logic Arrays (PLAs) Any combinational logic function can be

realized as a sum of products. Idea: Build a large AND-OR array with lots of

inputs and product terms, and programmable connections. n variables

AND gates have 2n inputs -- true and complement of each variable.

m outputs, driven by large OR gates Each AND gate is programmably connected to each

output’s OR gate. p AND gates (p<<2n)

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Example: 4x3 PLA, 6 product terms

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Compact representation

Actually, closer to physical layout (“wired logic”).

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Some product terms

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PLA Electrical Design See Section 5.3.5 -- wired-AND

logic

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Programmable Array Logic (PALs)

PLAs Both AND and OR arrays are programmable Product terms can be shared by OR gates

PALs ==> fixed OR array Each AND gate is permanently connected to a

certain OR gate. AND arrays are programmable, while OR arrays

are not. Product terms cannot be shared by OR gates

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D J

P

Understanding the Diagram

Vertical Lines indicate a product term. Horizontal lines provide True and Complemented forms of external inputs.

Even though a product term looks like it has only one input, it actually has 2 * N inputs, where N is the number of external inputs.

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Product Term

This looks like an AND gate with one input. Is actually:

B B’A A’ C C’ D D’I I’ J J’ E E’ F F’ K K’GG’H H’

BB’AA’CC’

H’H

Only drawn with a single line to save space.

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D J

P

Fuse Points

A cross over of a Vertical input line and a horizontal product term line is a FUSE LOCATION. When the PAL is in its blank or erased state, all FUSES are connected. This means that each product term implements the equation:

( A A’ B B’ C C’……. KK’) will be ‘0’! This means that the output will be high!

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D J

P

PAL Programming

To program, will want to BLOW most of the fuses (break the vertical/horizontal crossover connection). To indicate a logic function, will use a ‘ X ‘ over a fuse that I want to KEEP INTACT.

Will mark Intact fuse location.

When a fuse is blown, that product term input acts as a ‘1’ so that the input no longer effects the product term.

D J

P

P’ = D + J’

When implementing an equation, sometimes will not want to use all available product terms. If ALL fuses along product term are left intact, then product term value will be ‘0’ and will not affect equation. Mark unused PT’s by placing an X over them.

Note that P’ must be implemented!

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Example Product Term AC’H’

The connections will be:

B B’A A’ C C’ D D’I I’ J J’ E E’ F F’ K K’GG’H H’

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A11C’

H’1

Fuse blown

Fuse blown

Fuse intactFuse blown

Fuse blownFuse intact

Fuse blown

Fuse intact

Actually, fuses are not ‘blown’ in eraseable PLDs - the connection is broken in a non-destructive way for eraseable PLDs.

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Another Example

ABCD

GHIJ

P

P’ = A’BGH’ + CD’ + HIJ + BG’H

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10 primary inputs 8 outputs, with 7 ANDs

per output 1 AND for 3-state enable 6 outputs available as

inputs more inputs, at expense

of outputs two-pass logic, helper

terms Note inversion on

outputs output is complement of

sum-of-products newer PALs have

selectable inversion

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Next… HW #9

Building blocks Decoders Encoders

Reading Wakerly CH-5.4-5.5