EE365
Adv. Digital Circuit Design
Clarkson University
Lecture #5
Electrical Behavior of Logic Circuits
Topics
• Electrical Characteristics
– Noise & Noise Margins
– Voltage Levels
– Fan-in
– Fan-out
– Output Types
• Timing Characteristics
– Transition Delay
Lect #5
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Logic levels
• Undefined region
is inherent
– digital, not analog
• Switching threshold varies with voltage, temp, process, phase of
the moon
– need “noise margin”
• The more you push the technology, the more “analog” it becomes.
• Logic voltage levels decreasing with process
– 5 -> 3.3 -> 2.5 -> 1.8 V
Lect #5
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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.
Lect #5
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Output Specifications
• Voltage:
– VOLmax and VOHmin
• Current:
– Output sinks current when current flows
into the output - max low state output
current: IOLmax
– Output sources current when current flow
out of the output - max high state output
current: IOHmax
Lect #5
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DC Loading
• An output must
• An output must
sink current from a
source current to a
load when the
load when the
output is in the
output is in the
LOW state.
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.
Lect #5
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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.
– CMOS devices typically have two sets of output
drive specs:
• CMOS loads
• TTL or other resistive loads
Lect #5
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Manufacturer’s data sheet
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Driving Non-Ideals Loads
• Many typical loads may be represented
by a resistive network
• Find the Thevenin equivalent circuit
(review from ES 250) of the load
• Compute the output current and
voltages to determine if they are within
specification
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Example loading calculation
• Need to know “on” and “off” resistances
of output transistors, and know the
characteristics of the load.
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Estimating Values
for Internal Resistances
• Estimate the value of the internal
resistances from the specification for
maximum output current.
– Effective p-channel on resistance is
Rp = [ VDD - VOHmin ] / | IOHmax |
– Effective n-channel on resistance is
Rn = VOLmax / IOLmax
Lect #5
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Example Using
Estimated Values
+5v
1k
=>
2k
+5v
Thevenin equivalent
circuit
667
3.3 v
Rp
DC
667
3.3 v
Output Specifications for CMOS (HC) driving TTL loads
VOHmin = 4.3 v
VOLmax = 0.33 v
IOHmax = - 4.0 ma
IOLmax = 4.0 ma
DC
High State Model
Rp = [5 - 4.3] v / 4 ma = 175. ohms
Rn = 0.33 v / 4 ma
= 82.5 ohms
High State: Iout = - [5 - 3.3] / [175 + 667] = - 2 ma
Vout = 5 - ( 0.002 x 175) = 4.65 v
Lect #5
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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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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.
Lect #5
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Fan-out
The fan-out of a logic gate is the number of
inputs that the gate can drive without
exceeding its worst-case loading specs.
1
Fan-out
N
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Computing Fan-out
• General fan-out is the minimum of
– high state fan-out
– low state fan-out
• In each case, determine max input current of
each expected load and max output current
of the driving device
• Fan-out = max outputDEVICE / max inputLOAD
• Fan-outL = IOLmax/IILmax (for high state L H)
Lect #5
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In-Class Practice Problem
• Find the fan-out of the following device when
connected to identical devices
Lect #5
IOLmax
0.02 mA
IOHmax
-0.03 mA
VOLmax
0.1 V
VOHmax
4.4 V
IILmax
±1.0 μA
IIHmax
±1.0 μA
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In-Class Practice Problem
• Find the fan-out of the following device when
connected to identical devices
IOLmax
0.02 mA
IOHmax
-0.03 mA
VOLmax
0.1 V
VOHmax
4.4 V
IILmax
±1.0 μA
IIHmax
±1.0 μA
-20 μA / ±1.0 μA = 20 (minimum of two states)
Lect #5
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Fan-out Note
• Generally, when driving CMOS devices, fanout is
nearly unlimited because CMOS inputs require
almost no current
• When driving TTL devices, this is not the case
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Fan-in
For a given logic family, the maximum number
of inputs available on any one gate is called
the fan-in.
1
Fan-in
N
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Fan-in
• Limited in practice by the characteristics of a
particular technology.
• For CMOS, limited by the additive “on”
resistance of series transistors in either the
PUN or PDN.
• Typical values are 4 for NOR gates and 6 for
NAND gates.
• No need to calculate
Lect #5
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Fan-in
• Cascade Structure for Large Inputs
works around the fan-in limitation
– 8-input CMOS NAND:
Lect #5
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Non-Ideal Inputs
• What happens if the gate input is not
nearly zero or nearly +5 v ?
– Can occur if driven by devices of another
logic family (e.g. TTL drives CMOS)
– Inputs may still meet VIHmin or VILmax
Lect #5
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Non-Ideal Inputs
V DD
T
V in
p-channel
V out
n-channel
Lect #5
Output High State
If Vin = 0 v. =>
p-channel device conducts
n-channel device is off
As Vin increases
n-channel device begins to turn-on
Output Low State
If Vin = 5 v =>
p-channel device is off
n-channel device conducts
As Vin decreases
p-channel device begins to turn-on
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Non-Ideal Inputs
• Example: suppose input voltage is ~ 1.3 v
instead of 0 v.
– The p-transistors will have increased resistance
– The n-transistors will have decreased resistance
• Result: output voltage will be reduced, but still
above VOHmin
• Also increased current flow from VDD to
ground
• Result: increased power consumption
Lect #5
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Unused Inputs
• A three NAND is available, but you only
need a two input NAND - what about
the unused input?
• Logically, an unused input should be:
– a constant logic 1 for a NAND gate
– a constant logic 0 for a NOR gate
– identical to any one of the other inputs
Lect #5
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Unused Inputs
A
(A B)'
B
A
B
(A B)'

+5v
Lect #5
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Unused Inputs - CMOS
• Electrically, must NOT be left unconnected
• Very high input impedance => any noise can
cause the apparent input value to change
between a logic 0 and a logic 1.
• Highly susceptible to electrostatic discharge
(ESD)
• Loose devices easily destroyed on a winter’s
day in Potsdam !
Lect #5
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Unused Inputs - TTL
• May be left “open” (appears as logic 1)
• Can be changed by noise
• If pulled high or low by resistor, must
carefully compute resistor value since
input current not negligible
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Dynamics
• Fanout also limited by dynamic
considerations
• Switching from low to high state or high to
low cannot happen instantly - why not ?
• If the load has any capacitive effects,
what do you know about voltage across a
capacitor?
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Dynamics
+5v
Vout
0v
Vout = VDD e -t/ Rn C
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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.
Lect #5
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Transition times
Lect #5
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Circuit for transition-time
analysis
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HIGH-to-LOW transition
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Exponential rise time
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LOW-to-HIGH transition
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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
Lect #5
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Open-drain outputs
• No PMOS transistor, use resistor pull-up
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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
Lect #5
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Power Consumption
• Static power dissipation - no signal
transitions
• Dynamic power dissipation - signal
transitions
– P = [ CPD + CL ] VDD2 f , where f is the
frequency of signal transitions
Lect #5
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Comparison of Signal Levels
CMOS
(HC, AC)
CMOS
(HCT, ACT)
5v
VOH
4.4 v
VIH
3.5 v
VIL
VOL
Lect #5
TTL
(S, LS, AL, ALS, F)
5v
5v
VOH
2.4 v
VOH
2.4 v
VIH
2.0 v
VIH
2.0 v
VIL
VOL
0.8 v
0.4 v
0v
VIL
VOL
0.8 v
0.4 v
0v
1.5 v
0.5 v
0v
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TTL Input Specifications
• Unlike CMOS, TTL gates sink or source
current at the input
• Fanout must examine input currents
and output currents
• Low state input sources current ( it flows
out of the device)
• High state input sinks current
• LS-TTL: IILmax= -0.4 mA; IIHmax= 20 A
Lect #5
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TTL Output Specifications
•
•
•
•
Similar to CMOS
LS-TTL: IOLmax = 8 mA; IOHmax = -400 A
High state fanout = low state fanout = 20
But note that TTL has very asymmetric output
drive capability
– low state output can sink much more than high
state output
– moderate current loads only in low state
Lect #5
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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).
Lect #5
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Next Class
• Timing Considerations
• Propagation Delay
• Hazards
Lect #5
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