Logical Circuits

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. X = LOW X = HIGH Vx Negative (NMOS) X = HIGH X = LOW Vx Positive (PMOS)

5. NMOS Realization of a NOT gate VDD VDD VDD Vf = 0 Vf = VDD Vf Vx on off

6. NMOS Realization of the NAND gate VDD Vf Vy Vx

7. NMOS Realization of the AND gate VDD Vf VDD Vf ’ Vy Vx

8. NMOS Realization of the AND gate VDD Vf VDD Vy NOT Gate Vx NAND Gate

9. NMOS Realization of the NOR gate VDD Vf Vx Vy

10. NMOS Realization of the OR gate VDD VDD Vf Vx Vy

11. Complementary MOS (CMOS) VDD VDD Vy Vf Vx Vy Vf Vx CMOS NOR gate CMOS NAND gate

12. Logic Devices • 7400 Series Standard Chips • Programmable Logic Devices • Programmable Logic Array (PLA) • Programmable Array Logic (PAL) • Complex Programmable Logic Devices (CPLDs) VDD Vf Vy Vx

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 + X2’X3 VDD X1 X2 X3 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 . . . . . . Output Values (function values) Input Variables

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) OR plane AND plane Xn … fm

22. PLA Implementation X1,X’1, … Xn, X’n f1 … fm

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

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

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