2. 11/22/2023 Dynamic Logic Circuits 2
Introduction
•In static circuits the output is connected to
either GND or VDD via a low resistance path.
• fan-in of n requires 2n (n N-type + n P-type) devices
•Dynamic circuits use temporary storage of
signal values on the capacitance of high
impedance nodes.
• requires on n + 2 (n+1 N-type + 1 P-type) transistors
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Advantages and Disadvantages of
Dynamic Logic
• Advantages
• Allow implementation of simple sequential circuits with
memory functions.
• Use of common clock signals throughout the system
enables the synchronization of various circuit blocks.
• Implementation of complex circuits requires a smaller
silicon area than static circuits.
• Often consumes less dynamic power than static
designs, due to smaller parasitic capacitances.
• Operates at higher speed
• Noise Immunity
• Disadvantages
• Requires clock
• Charge Sharing and Charge Leakage
• Can not operate at low frequency
• Complicated design
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Dynamic Latch
• CK = 1: MP turns ON. Cx is charged up or down through MP
depending on the input D voltage level. Q = D.
• CK = 0: MP turns OFF, and Cx is isolated from input D. Q is
determined by charge stored on Cx during previous CK = 1.
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Towards the faster Circuits….
• What makes a circuit fast?
• I = C dV/dt -> tpd (C/I) DV
• low capacitance
• high current
• small swing
• In CMOS
• Speed is less because of high capacitance of pMOS
• In Pseudo-nMOS
• The power is high because of continuous current flow
through pMOS
• Pseudo-nMOS is rationed logic
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A modification to Pseudo-nMOS
• Rather than continuous ON load pMOS, use clocked
pMOS.
1
2
A Y
4/3
2/3
A
Y
1
1
A
Y
Static Pseudo-nMOS Dynamic
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Why the foot?
• What if pulldown network is ON during precharge?
• Use series evaluation transistor to prevent fight.
• Further, input should change during the precharge phase, if they
change during evaluate phase, charge redistribution can corrupt the
output.
A
Y
foot
precharge transistor
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Advantages of PE logic
• Requires n+2 transistors compared to 2n transistors in
CMOS
• Low static power dissipation
• No DC current path to restrict the device sizing
• Pull up time is improved by active switch to Vdd
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Disadvantages
• Output is available only for <50% of time
• Pull down time is degraded because of extra Me
transistor
• Charge sharing problem
• Maximum clock frequency is determined by leakage,
discharge and delays because of C
• Inputs can only change during precharge phase.
Inputs must be stable during evaluate.
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;
• Dynamic gates require monotonically rising inputs
during evaluation
• 0 -> 0
• 0 -> 1
• 1 -> 1
• But not 1 -> 0
A
Precharge Evaluate
Y
Precharge
A
Output should rise but does not
violates monotonicity
during evaluation
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Summary
• Full complementary static logic is best option in
the majority of CMOS circuits.
• Noise-immunity is not sensitive to kn/kp
• Does not involve precharge of nodes
• Dissipate no DC power
• Layout can be automated
• Large fan-in gates lead to complex circuit structures (2N
transistors)
• Larger parasitics
• Slower and higher dynamic power dissipation than
alternatives
• No clock
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Summary (Cont.)
• Pseudo-nMOS static logic finds widest utility in
large fan-in NOR gates.
• Require only N+1 transistors for N fan-in
• Smaller parasitics
• Faster and lower dynamic power dissipation than full
CMOS
• Noise immunity sensitive to kn/kp
• Dissipate DC power when pulled down
• Not well suited for automated layout
• No clock
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Summary (Cont.)
• CMOS domino logic should be used for low-power,
high speed applications
• Require only N+k transistors for N fan-in, size
advantages of pseudo-nMOS.
• Dissipate no DC power
• Noise immunity is not sensitive to kn/kp
• Use of clocks enables synchronous operation
• Rely on storage on soft node
• Require exhaustive simulation at all the process corners
to insure proper operation
• Some of the speed advantage over static gates is
diminished by the required pre-charge (pre-discharge)
time.