Transcript ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 22: October 31, 2012 Pass Transistor Logic Penn ESE370 Fall2012 -- DeHon.

ESE370:
Circuit-Level
Modeling, Design, and Optimization
for Digital Systems
Day 22: October 31, 2012
Pass Transistor Logic
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Penn ESE370 Fall2012 -- DeHon
Teaser
• What does this do?
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Previously
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Elmore Delay
• For each resistor Ri in path
– Compute CRi = sum of all C’s downstream of Ri
– Delay through Ri is Ri×CRi



Delay  
R

C

j 
 i
p a th
path
i 
 j 
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Impact of Capacitance
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Contact/Diffusion Capacitance
• Cj – diffusion depletion
• Cjsw – sidewall capacitance
• LS – length of diffusion
Cdiff  Cdiff 0W
LS
Cdiff  C j LSW  C jsw 2LS  W 
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Today
• Pass Transistor Circuits
• Elmore Delay with Cdiff>0 for CMOS
– Payoff: compare pass transistor to CMOS
– Other benefit: exercise Elmore delay calc.
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Pass Transistor
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Identify Function
• What function?
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Area
• Compare with CMOS circuit?
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Output
• What does output look like (DC transfer)?
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Pass TR transfer
(B=1)
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CMOS Inverter Transfer
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Reasonable Input to CMOS
Inverter?
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Pass tr xor2 with inv restore
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Required to use?
• What take to use this with CMOS
circuit?
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Restore Output
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Restore Output
• Area? (compare to CMOS)
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Chain Together
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Analyze Stage
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Inverter Delay
• Delay driving another inverter?
– Elmore
– Include Cdiff=gCgate
W=1
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Analyze Stage
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Delay B=1?
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Delay B=0?
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CMOS Elmore and Cdiff
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Identify Capacitances
• Self output
load?
• Load presented
by A input?
• Capacitance
between pd
transistors?
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Identify Capacitances
• Which
transistors on?
• Equivalent RC
circuit (output
stage) A=B=0?
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Total Delay
• Total delay of
circuit?
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Cascade Delay
• Delay of chain
of n of these?
(connect Y to A)
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Compare
• Delay?
• Area?
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Idea
• There are other circuit disciplines
• Can use pass transistors for logic
– Sometimes gives area or delay win
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Admin
• Project
– Due: latest Saturday midnight
• Udit office hour today
• Q: need for extra office hours on
Thursday?
• Midterm 2: Wednesday, Nov. 7th
– Say more on Friday
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