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Understanding CMOS and TTL Gates for Digital Circuit Design | Electrical Characteristics and Timing Analysis

Learn about the principles of CMOS and TTL gates, including their electrical characteristics, timing analysis, input and output loading, transition times, and more for designing efficient digital circuits. Discover how to calculate limitations on DC load, output-drive specs, input-loading, and transition-time considerations. Explore the differences between CMOS and TTL levels, their logic levels, noise margins, and AC loading implications.

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Understanding CMOS and TTL Gates for Digital Circuit Design | Electrical Characteristics and Timing Analysis

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  1. EE121 John Wakerly Lecture #2 CMOS gatesElectrical characteristics and timingTTL gates

  2. CMOS NAND Gates • Use 2n transistors for n-input gate

  3. CMOS NAND -- switch model

  4. CMOS NAND -- more inputs (3)

  5. Inherent inversion. • Non-inverting buffer:

  6. 2-input AND gate:

  7. CMOS NOR Gates • Like NAND -- 2n transistors for n-input gate

  8. NAND NOR NAND vs. NOR • For a given silicon area, PMOS transistors are “weaker” than NMOS transistors. • Result: NAND gates are preferred in CMOS.

  9. Limited # of inputs in one gate • 8-input CMOS NAND

  10. Fancy stuff • CMOS AND-OR-INVERT gate

  11. CMOS Electrical Characteristics • Digital analysis works only if circuits are operated in spec: • Power supply voltage • Temperature • Input-signal quality • Output loading • Must do some “analog” analysis to prove that circuits are operated in spec. • Fanout specs • Timing analysis (setup and hold times)

  12. DC Loading • An output must sink current from a load when the output is in the LOW state. • An output must source current to a load when the output is in the HIGH state.

  13. Output-voltage drops • Resistance of “off” transistor is > 1 Megohm, but resistance of “on” transistor is nonzero, • Voltage drops across “on” transistor, V = IR • For “CMOS” loads, current and voltage drop are negligible. • For TTL inputs, LEDs, terminations, or other resistive loads, current and voltage drop are significant and must be calculated.

  14. Example loading calculation • Need to know “on” and “off” resistances of output transistors, and know the characteristics of the load.

  15. Calculate for LOW and HIGH state

  16. Limitation on DC load • If too much load, output voltage will go outside of valid logic-voltage range. • VOHmin, VIHmin • VOLmax, VILmax

  17. Output-drive specs • VOLmax and VOHmin are specified for certain output-current values, IOLmax and IOHmax. • No need to know details about the output circuit, only the load.

  18. Input-loading specs • Each gate input requires a certain amount of current to drive it in the LOW state and in the HIGH state. • IIL and IIH • These amounts are specified by the manufacturer. • Fanout calculation • (LOW state) The sum of the IIL values of the driven inputs may not exceed IOLmax of the driving output. • (HIGH state) The sum of the IIH values of the driven inputs may not exceed IOHmax of the driving output. • Need to do Thevenin-equivalent calculation for non-gate loads (LEDs, termination resistors, etc.)

  19. Manufacturer’s data sheet

  20. TTL Electrical Characteristics

  21. TTL LOW-State Behavior

  22. TTL HIGH-State Behavior

  23. TTL Logic Levels and Noise Margins • Asymmetric, unlike CMOS • CMOS can be made compatible with TTL • “T” CMOS logic families

  24. TTL levels CMOS with TTL Levels -- HCT, FCT, VHCT, etc. CMOS vs. TTL Levels CMOS levels

  25. TTL differences from CMOS • Asymmetric input and output characteristics. • Inputs source significant current in the LOW state, leakage current in the HIGH state. • Output can handle much more current in the LOW state (saturated transistor). • Output can source only limited current in the HIGH state (resistor plus partially-on transistor). • TTL has difficulty driving “pure” CMOS inputs because VOH = 2.4 V (except “T” CMOS).

  26. AC Loading • AC loading has become a critical design factor as industry has moved to pure CMOS systems. • CMOS inputs have very high impedance, DC loading is negligible. • CMOS inputs and related packaging and wiring have significant capacitance. • Time to charge and discharge capacitance is a major component of delay.

  27. Transition times

  28. Circuit for transition-time analysis

  29. HIGH-to-LOW transition

  30. Exponential rise time

  31. LOW-to-HIGH transition

  32. Exponential fall time t = RC time constant exponential formulas, e-t/RC

  33. Transition-time considerations • Higher capacitance ==> more delay • Higher on-resistance ==> more delay • Lower on-resistance requires bigger transistors • Slower transition times ==> more power dissipation (output stage partially shorted) • Faster transition times ==> worse transmission-line effects (Chapter 11) • Higher capacitance ==> more power dissipation (CV2f power), regardless of rise and fall time

  34. Open-drain outputs • No PMOS transistor, use resistor pull-up

  35. What good is it? • Open-drain bus • Problem -- really bad rise time

  36. Open-drain transition times • Pull-up resistance is larger than a PMOS transistor’s “on” resistance. • Can reduce rise time by reducing pull-up resistor value • But not too much

  37. Additional topics to read about • Weird logic • Wired Logic • LEDs • Fighting outputs

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