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Logical Circuits

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Logical Circuits

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    1. Logical Circuits Philip Gebhardt 3/15/2011

    2. Logic Circuits Negative, Positive, and Complimentary circuits Logic Gates Programmable Logic Devices

    3. Logic Circuits are built from Transistors Transistors act as the switches in circuits Controlled by a logical signal from another logic circuit The most popular transistor used today The metal oxide semiconductor field-effect transistor (MOSFET) Two different types: n-channel (NMOS) and p-channel (PMOS)

    5. NMOS Realization of a NOT gate

    6. NMOS Realization of the NAND gate

    7. NMOS Realization of the AND gate

    8. NMOS Realization of the AND gate

    9. NMOS Realization of the NOR gate

    10. NMOS Realization of the OR gate

    11. Complementary MOS (CMOS)

    12. Logic Devices 7400 Series Standard Chips Programmable Logic Devices Programmable Logic Array (PLA) Programmable Array Logic (PAL) Complex Programmable Logic Devices (CPLDs)

    13. Logic Devices 7400 Series Standard Chips and all other Logic Devices measured in NAND equivalency gates

    14. Logic Devices 7400 Series Standard Chips and all other Logic Devices measured in NAND equivalency gates Ex: The macrocells in PALs are often about 20 equivalent NAND gates

    15. 7400 Series Standard Chips External pins used as input/ouput Two pins used for VDD and GND leads Multiple chips with different logical gates can be connected to realize logical functions

    16. F = X1X2 + X2X3

    17. Not very efficient for operations with large logical capacity Mostly used today as buffers from other logical circuits

    18. Programmable Logic Devices PLDs are general-purpose computer chips designed to implement logical circuits. Devices with a lot of circuitry and logical gates Unlike 7400 series chips, has Non-fixed structure Can be considered a black box with wide range of logical combinations Programmable Logic Array (PLA) Programmable Array Logic (PAL)

    19. PLD as a Black Box

    20. Programmable Logic Arrays (PLAs) PLAs can be broken down into two planes of wires which pass logic gates AND plane: logic variables (and their compliments) pass through the AND gates OR plane: the resultants from the AND gates then pass through the OR gates Switches programmed by the user determine which variable pass through which gates

    21. Generic PLA

    22. PLA Implementation

    23. Programmable Array Logic Unlike PLAs, PALs are not fixed on both logical planes Generally only the AND plane fixed Less flexibility but more cost efficient and better performance Uses extra circuitry after OR gates for more versatility Macrocells Flip-flops, MUXs, more buffers

    24. PAL Structure

    25. Example of macrocell

    26. Benefits Limitations of PLAs and PALs PLAs Efficient for the area needed for implementation Too many switches decreases performance and increases costs for manufacturing PALs Less expensive and easier to manufacture compared to PLAs Macrocells allow for implementation of logical circuits with multiple levels or phases Less flexibility requires extra circuitry