ECE 331 – Digital System Design

1. ECE 331 – Digital System Design Power Dissipation and Additional Design Constraints (Lecture #14) The slides included herein were taken from the materials accompanying Fundamentals of Logic Design, 6th Edition, by Roth and Kinney, and were used with permission from Cengage Learning.

2. ECE 331 - Digital System Design Material to be covered … Supplemental Chapter 8: Sections 1 – 5

3. ECE 331 - Digital System Design Power Dissipation

4. ECE 331 - Digital System Design Power Dissipation Each integrated circuit (IC) dissipates power PT = PS + PD PT = total power dissipated by IC PS = static or quiescent power dissipation PD = dynamic power dissipation

5. ECE 331 - Digital System Design Static Power Dissipation PS = VCC * ICC VCC = supply voltage ICC = quiescent supply current PS = static power consumption ICC and VCC are specified in the datasheet for the integrated circuit (IC). For CMOS devices, PS is very small.

6. ECE 331 - Digital System Design 74LS00 Datasheet

7. ECE 331 - Digital System Design Static Power Dissipation Example: 74LS00 (Quad 2-input NAND) Supply voltage 4.75 V <= VCC <= 5.25 V Supply current High output: ICCmax = 1.6 mA Low output: ICCmax = 4.4 mA Maximum static power dissipation High output: PS = 8.4 mW Low output: PS = 23.1 mW

8. ECE 331 - Digital System Design Static Power Dissipation Duty Cycle Clock signal typically has 50% duty cycle PS = PS_high * thigh + PS_low * tlow PS_high = 8.4 mW PS_low = 23.1 mW Assume 50% duty cycle (high / low half the time) PS = 8.4 mW * 0.5 + 23.1 mW * 0.5 = 15.8 mW Assume 60% duty cycle (high 60% of the time) PS = 8.4 mW * 0.6 + 23.1 mW * 0.4 = 14.28 mW

9. ECE 331 - Digital System Design Dynamic Power Dissipation For TTL devices, PD is negligible compared to PS. Assume PS = 0 For CMOS devices, PD dominates PT. PD >> PS PD in CMOS circuits arises from the movement of charge into and out of the device capacitance.

10. ECE 331 - Digital System Design Dynamic Power Dissipation In CMOS devices, charge is stored in the CPD = power dissipation capacitance (internal) CL = capacitance of the load and wires (external) These capacitors are in parallel CT = CPD + CL The stored charge (on these capacitors) is QT = CT * VDD = (CPD + CL) * VDD

11. ECE 331 - Digital System Design Dynamic Power Dissipation The charge moves into and out of the capacitors on every transition of the output. Low → High High → Low Current = movement of charge IAVG = (CPD + CL) * VDD * fT Where fT = output frequency PD = IAVG * VDD = (CPD + CL) * V2DD * fT

12. ECE 331 - Digital System Design 74HC00 Datasheet

13. ECE 331 - Digital System Design Dynamic Power Dissipation Example: 74HC00 (Quad 2-input NAND) VDD = 5V CPD = 20 pF, CL = 50 pF PD = (20 + 50 pF) * (5V)2 * fT fT (Hz) PD 1K 1.8 mW 1M 1.8 mW 100M 180 mW

14. ECE 331 - Digital System Design 74HC00 Datasheet

15. ECE 331 - Digital System Design Total Power Dissipation For the 74HC00, PS is determined as follows VCC = 5V ICC = 20 mA PS = VCC * ICC = 5V * 20 mA = 100 mA The PT is then determined from PT = PS + PD where PD is a function of fT

16. ECE 331 - Digital System Design Total Power Dissipation PT = PS + PD Compare PT for Quad 2-input NAND (74xx00) 0 Hz 1 MHz 100 MHz TTL 15.8 mW 15.8 mW 15.8 mW CMOS 100 W 1.805 mW 180 mW Compare TTL and CMOS TTL CMOS PS VCC * ICC VDD * IDD PD ~ 0 W (CPD + CL) * V2DD * fT

17. ECE 331 - Digital System Design Hazards

18. ECE 331 - Digital System Design Hazards When the input to a combinational logic circuit changes, unwanted switching transients may appear on the output. These transients occur when different paths from input to output have different propagation delays.

19. ECE 331 - Digital System Design Hazards

20. ECE 331 - Digital System Design Hazards When analyzing combinational logic circuits for hazards we will consider the case where only one input changes at a time. Under this condition, a static 1-hazard occurs when the input change causes one product term (in a SOP expression) to transition from 1 to 0 and another product term to transition from 0 to 1. Both product terms can be transiently 0, resulting in the static 1-hazard.

21. ECE 331 - Digital System Design Hazards Under the same condition, a static 0-hazard occurs when the input change causes one sum term (in a POS expression) to transition from 0 to 1 and another sum term to transition from 1 to 0. Both sum terms can be transiently 1, resulting in the static 0-hazard.

22. ECE 331 - Digital System Design Detecting Static 1-Hazards We can detect hazards in a two-level AND-OR circuit using the following procedure: Write down the sum-of-products expression for the circuit. Plot each term on the map and loop it. If any two adjacent 1′s are not covered by the same loop, a 1-hazard exists for the transition between the two 1′s. For an n-variable map, this transition occurs when one variable changes and the other n – 1 variables are held constant.

23. ECE 331 - Digital System Design Detecting Static 1-Hazards

24. ECE 331 - Digital System Design Removing Static 1-Hazards redundant, but necessary to remove hazard

25. ECE 331 - Digital System Design Detecting Static 0-Hazards We can detect hazards in a two-level OR-AND circuit using the following procedure: Write down the product-of-sums expression for the circuit. Plot each sum term on the map and loop the zeros. If any two adjacent 0′s are not covered by the same loop, a 0-hazard exists for the transition between the two 0′s. For an n-variable map, this transition occurs when one variable changes and the other n – 1 variables are held constant.

26. ECE 331 - Digital System Design Detecting Static 0-Hazards

27. ECE 331 - Digital System Design Removing Static 0-Hazards How many redundant gates are necessary to remove the 0-hazards?

28. ECE 331 - Digital System Design Hazards Exercise: Design a hazard-free combinational logic circuit to implement the following logic function F(A,B,C) = A'.C' + A.D + B.C.D'

29. ECE 331 - Digital System Design Hazards Exercise: Design a hazard-free combinational logic circuit to implement the following logic function F(A,B,C) = (A'+C').(A+D).(B+C+D')

30. ECE 331 - Digital System Design Hazards Two-level AND-OR circuits (SOP) cannot have static 1-Hazards. Why? Two-level OR-AND circuits (POS) cannot have static 0-Hazards. Why?

31. ECE 331 - Digital System Design Questions?