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)

4. Negative (NMOS) Positive (PMOS) VxVx VxVx X = LOWX = HIGH X = LOW

5. NMOS Realization of a NOT gate VxVx V DD VfVf XVfVf onoff on V DD V f = 0 V DD V f = V DD onoff

6. NMOS Realization of the NAND gate VyVy V DD VfVf VxVx VxVx VyVy VfVf 001 011 101 110

7. NMOS Realization of the AND gate VyVy V DD VxVx VfVf VxVx VyVy V f’ VfVf 0010 0110 1010 1101

8. NMOS Realization of the AND gate VyVy V DD VxVx VfVf NOT Gate NAND Gate

9. NMOS Realization of the NOR gate VxVx VyVy V DD VfVf

10. NMOS Realization of the OR gate VxVx VyVy V DD VfVf

11. Complementary MOS (CMOS) VyVy VxVx VfVf V DD VfVf VyVy VxVx CMOS NAND gate CMOS NOR gate

12. Logic Devices 7400 Series Standard Chips Programmable Logic Devices Programmable Logic Array (PLA) Programmable Array Logic (PAL) Complex Programmable Logic Devices (CPLDs) VyVy V DD VfVf VxVx

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 V DD and GND leads Multiple chips with different logical gates can be connected to realize logical functions

16. F = X 1 X 2 + X 2 ’X 3 X1X1 X2X2 X3X3 V DD F

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............ Logic Gates And Switches Input Variables Output Values (function values)

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...... Input (buffers & inverters)............ AND plane … OR plane XnXn fmfm

22. PLA Implementation X 1, X’ 1, … X n, X’ n f 1 … f m

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 Programmable switches determine AND gate results… Fixed OR gates connect to macrocells

25. Example of macrocell D Q CLK MUX 2:1 Back to AND plane fifi PAL block

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