# CMOS gates Electrical characteristics and timing TTL gates

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CMOS gates Electrical characteristics and timing TTL gates
EE 365 CMOS gates Electrical characteristics and timing TTL gates

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

CMOS NAND -- switch model

CMOS NAND -- more inputs (3)

Inherent inversion. Non-inverting buffer:

2-input AND gate:

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

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

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

Fancy stuff CMOS AND-OR-INVERT gate

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)

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.

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.

Need to know “on” and “off” resistances of output transistors, and know the characteristics of the load.

Calculate for LOW and HIGH state

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

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.

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.)

Manufacturer’s data sheet

TTL Electrical Characteristics

TTL LOW-State Behavior

TTL HIGH-State Behavior

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

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

Fig 3-84

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).

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.

Transition times

Circuit for transition-time analysis

HIGH-to-LOW transition

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

LOW-to-HIGH transition

Exponential rise time

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

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

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

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

Important Tables in Chapter 3