Dynamic Behavior |
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2.5 |
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Out |
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Voltage |
1.5 |
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In & |
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Precharge |
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Time, ns
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VOH |
VOL |
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NMH |
NML |
tpHL |
tpLH |
tp |
6 |
2.5V |
0V |
VTn |
2.5-VTn |
VTn |
110ps |
0ns |
83ps |
CSE477 L# topic.11 |
Irwin&Vijay, PSU, 2001 |
Power Consumption of Dynamic Gate
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Me |
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Power only dissipated when previous Out = 0
CSE477 L# topic.12 |
Irwin&Vijay, PSU, 2001 |
Dynamic Power Consumption is Data Dependent
Dynamic 2-input NOR Gate
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Assume signal probabilities |
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A |
B |
Out |
PA=1 = 1/2 |
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0 |
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PB=1 = 1/2 |
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Then transition probability |
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P0→1 = Pout=0 |
x Pout=1 |
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= 3/4 x |
1 = 3/4 |
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Switching activity always higher in dynamic gates!
P0→1 = Pout=0
CSE477 L# topic.13 |
Irwin&Vijay, PSU, 2001 |
Issues in Dynamic Design 1: Charge Leakage
CLK
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CLK |
3 |
Mp |
Out
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1 |
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CL |
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2 |
Evaluate |
CLK |
VOut |
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Me |
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Precharge |
Leakage sources |
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Minimum clock rate of a few kHz
CSE477 L# topic.14 |
Irwin&Vijay, PSU, 2001 |
Solution to Charge Leakage
Keeper
CLK 
Mp Mkp 
A
!Out
CL
B
CLK 
Me
Same approach as level restorer for pass transistor logic
CSE477 L# topic.15 |
Irwin&Vijay, PSU, 2001 |
Issues in Dynamic Design 2: Charge Sharing
CLK
Mp
Out
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A |
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CL |
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B=0 |
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CA |
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CLK
Me 
CB
Charge stored originally on CL is redistributed (shared) over CL and CA leading to static power consumption by downstream gates and possible circuit malfunction.
CSE477 L# topic.16 |
Irwin&Vijay, PSU, 2001 |
Charge Sharing Example
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Out |
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CL=50fF |
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Ca=15fF |
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B |
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Cb=15fF |
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Cc=10fF |
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!C |
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C |
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Cd=10fF |
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Vout = - VDD (Ca / (Ca + CL ))
CSE477 L# topic.17 |
Irwin&Vijay, PSU, 2001 |
Solution to Charge Redistribution
CLK Mp Mkp |
CLK |
A |
Out |
|
B
CLK
Me
Precharge internal nodes using a clockdriven transistor (at the cost of increased area and power)
CSE477 L# topic.18 |
Irwin&Vijay, PSU, 2001 |
Issues in Dynamic Design 3: Backgate Coupling
CLK |
Mp |
Out1 =1 |
Out2 =0 |
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A=0 |
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CL1 |
In |
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CL2 |
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B=0 |
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CLK |
Me |
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Dynamic NAND |
Static NAND |
CSE477 L# topic.19 |
Irwin&Vijay, PSU, 2001 |
Backgate Coupling Effect
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3 |
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2 |
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Voltage |
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Out1 |
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1 |
CLK |
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0 |
In |
Out2 |
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-1 |
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0 |
2 |
Time, ns |
4 |
6 |
CSE477 |
L# topic.20 |
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Irwin&Vijay, PSU, 2001 |