EE121
John Wakerly
Lecture #2
CMOS gates
Electrical characteristics and timing
TTL gates
CMOS NAND Gates
• Use 2n transistors for n-input gate
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• CMOS NAND -- switch model
• CMOS NAND -- more inputs (3)
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• Inherent inversion.
• Non-inverting buffer:
• 2-input AND gate:
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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.
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Limited # of inputs in one gate
• 8-input CMOS NAND
Fancy stuff
• CMOS AND-ORINVERT gate
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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.
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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.
Example loading calculation
• Need to know “on” and “off” resistances of
output transistors, and know the
characteristics of the load.
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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
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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 nongate loads (LEDs, termination resistors, etc.)
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Manufacturer’s data sheet
TTL Electrical Characteristics
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TTL LOW-State Behavior
TTL HIGH-State Behavior
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TTL Logic Levels and Noise Margins
• Asymmetric, unlike CMOS
• CMOS can be made compatible with TTL
– “T” CMOS logic families
CMOS vs. TTL Levels
CMOS levels
TTL levels
CMOS with TTL Levels
-- HCT, FCT, VHCT, etc.
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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.
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Transition times
Circuit for transition-time analysis
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HIGH-to-LOW transition
Exponential rise time
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LOW-to-HIGH transition
Exponential fall time
t = RC time constant
exponential formulas, e-t/RC
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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
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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
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Additional topics to read about
• Wired
Weird logic
Logic
• LEDs
• Fighting outputs
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