Page 1
CSE477 L07 Pass Transistor Logic.1 Irwin&Vijay, PSU, 2003
CSE477VLSI Digital Circuits
Fall 2003
Lecture 07: Pass Transistor Logic
Mary Jane Irwin ( www.cse.psu.edu/~mji ) www.cse.psu.edu/~cg477
[Adapted from Rabaey’s Digital Integrated Circuits, Second Edition, ©2003 J. Rabaey, A. Chandrakasan, B. Nikolic]
Page 2
CSE477 L07 Pass Transistor Logic.2 Irwin&Vijay, PSU, 2003
Review: Static Complementary CMOS
VDD
F(In1,In2,…InN)
In1In2
InN
In1In2
InN
PUN
PDN
PUN and PDN are dual logic networks
……
High noise margins VOH and VOL are at VDD and
GND, respectively
Low output impedance, high input impedance
No static power consumption Never a direct path between
VDD and GND in steady state
Delay a function of load capacitance and transistor on resistance
Comparable rise and fall times (under the appropriate relative transistor sizing conditions)
Page 3
CSE477 L07 Pass Transistor Logic.3 Irwin&Vijay, PSU, 2003
Review: Static CMOS Full Adder Circuit
B
B B
B B
B
B
B
A
A
A
A
A
AA
A
Cin
Cin
Cin
Cin
Cin
!Cout !Sum
!Cout = !Cin (!A v !B) v !A !B
Cout = Cin (A v B) v A B
!Sum = Cout (!A v !B v !Cin) v !A !B !Cin
Sum = !Cout (A v B v Cin) v A B Cin
Page 4
CSE477 L07 Pass Transistor Logic.4 Irwin&Vijay, PSU, 2003
NMOS Transistors in Series/Parallel
Primary inputs drive both gate and source/drain terminals
NMOS switch closes when the gate input is high
Remember - NMOS transistors pass a strong 0 but a weak 1
A B
X YX = Y if A and B
X Y
A
B X = Y if A or B
Page 5
CSE477 L07 Pass Transistor Logic.5 Irwin&Vijay, PSU, 2003
PMOS Transistors in Series/Parallel
Primary inputs drive both gate and source/drain terminals
PMOS switch closes when the gate input is low
Remember - PMOS transistors pass a strong 1 but a weak 0
A B
X YX = Y if A and B = A + B
X Y
A
B X = Y if A or B = A B
Page 6
CSE477 L07 Pass Transistor Logic.7 Irwin&Vijay, PSU, 2003
Pass Transistor (PT) Logic
AB
FB
0
A
0
B
B= A BF = A B
Gate is static – a low-impedance path exists to both supply rails under all circumstances N transistors instead of 2N No static power consumption Ratioless Bidirectional (versus undirectional)
Page 7
CSE477 L07 Pass Transistor Logic.8 Irwin&Vijay, PSU, 2003
VTC of PT AND Gate
A
0
B
BF= AB
0.5/0.25
0.5/0.25
0.5/0.25
1.5/0.25
0
1
2
0 1 2
B=VDD, A=0VDD
A=VDD, B=0VDD
A=B=0VDDV
out,
V
Vin, V
Pure PT logic is not regenerative - the signal gradually degrades after passing through a number of PTs (can fix with static CMOS inverter insertion)
Page 8
CSE477 L07 Pass Transistor Logic.9 Irwin&Vijay, PSU, 2003
Differential PT Logic (CPL)
A
B
AB
PT NetworkF
A
B
AB
Inverse PT Network F
F
F
F=AB
A
A
B F=AB
B
B B
AND/NAND
A
A
B F=A+B
BF=A+B
BB
OR/NOR
A
A F=AB
F=AB
BB
XOR/XNOR
A
A
Page 9
CSE477 L07 Pass Transistor Logic.10 Irwin&Vijay, PSU, 2003
CPL Properties
Differential so complementary data inputs and outputs are always available (so don’t need extra inverters)
Still static, since the output defining nodes are always tied to VDD or GND through a low resistance path
Design is modular; all gates use the same topology, only the inputs are permuted.
Simple XOR makes it attractive for structures like adders
Fast (assuming number of transistors in series is small)
Additional routing overhead for complementary signals
Still have static power dissipation problems
Page 10
CSE477 L07 Pass Transistor Logic.12 Irwin&Vijay, PSU, 2003
CPL Full Adder
A
A
BB CinCin
!Sum
Sum
Cout
!CoutA
A
B
B
B
B Cin Cin
Cin
Cin
Page 11
CSE477 L07 Pass Transistor Logic.13 Irwin&Vijay, PSU, 2003
NMOS Only PT Driving an Inverter
Vx does not pull up to VDD, but VDD – VTn
In = VDD
A = VDD
Vx = VDD-VTn
M1
M2
BSD
Threshold voltage drop causes static power consumption (M2 may be weakly conducting forming a path from VDD to GND)
Notice VTn increases for pass transistor due to body effect (VSB)
VGS
Page 12
CSE477 L07 Pass Transistor Logic.14 Irwin&Vijay, PSU, 2003
Voltage Swing of PT Driving an Inverter
Body effect – large VSB at x - when pulling high (B is tied to GND and S charged up close to VDD)
So the voltage drop is even worse
Vx = VDD - (VTn0 + ((|2f| + Vx) - |2f|))
In = 0 VDD
VDD
xOut
0.5/0.25
0.5/0.25
1.5/0.25
0
1
2
3
0 0.5 1 1.5 2Time, ns
Vol
tage
, V
In
Out
x = 1.8VD
S
B
Page 13
CSE477 L07 Pass Transistor Logic.15 Irwin&Vijay, PSU, 2003
Cascaded NMOS Only PTs
B = VDD
Out
M1
yM2
Swing on y = VDD - VTn1 - VTn2
xM1
B = VDD
OutyM2
Swing on y = VDD - VTn1
C = VDD
A = VDD
C = VDD
A = VDD
Pass transistor gates should never be cascaded as on the left
Logic on the right suffers from static power dissipation and reduced noise margins
x = VDD - VTn1
G
S
G
S
Page 14
CSE477 L07 Pass Transistor Logic.16 Irwin&Vijay, PSU, 2003
Solution 1: Level Restorer
For correct operation Mr must be sized correctly (ratioed)
Level Restorer
M1
M2
A=0 Mn
Mr
x
B
Out =1
off
= 0A=1 Out=0
on
1
Full swing on x (due to Level Restorer) so no static power consumption by inverter
No static backward current path through Level Restorer and PT since Restorer is only active when A is high
Page 15
CSE477 L07 Pass Transistor Logic.17 Irwin&Vijay, PSU, 2003
Transient Level Restorer Circuit Response
0
1
2
3
0 100 200 300 400 500
Vol
tage
, V
Time, ps
W/Lr=1.75/0.25
W/Lr=1.50/0.25
W/Lr=1.25/0.25W/Lr=1.0/0.25
W/Ln=0.50/0.25
W/L2=1.50/0.25
W/L1=0.50/0.25
node x never goes below VM of inverter so output never switches
Restorer has speed and power impacts: increases the capacitance at x, slowing down the gate; increases tr (but decreases tf)
Page 16
CSE477 L07 Pass Transistor Logic.18 Irwin&Vijay, PSU, 2003
Solution 2: Multiple VT Transistors Technology solution: Use (near) zero VT devices for the
NMOS PTs to eliminate most of the threshold drop (body effect still in force preventing full swing to VDD)
Impacts static power consumption due to subthreshold currents flowing through the PTs (even if VGS is below VT)
Out
In2 = 0V
In1 = 2.5V
A = 2.5V
B = 0V
low VT transistors
sneak path
on
off but leaking
Page 17
CSE477 L07 Pass Transistor Logic.20 Irwin&Vijay, PSU, 2003
Solution 3: Transmission Gates (TGs)
Full swing bidirectional switch controlled by the gate signal C, A = B if C = 1
A B
C
C
A B
C
C
B
C = VDD
C = GND
A = VDD B
C = VDD
C = GND
A = GND
Most widely used solution
Page 18
CSE477 L07 Pass Transistor Logic.21 Irwin&Vijay, PSU, 2003
TG Multiplexer
GND
VDD
In1 In2S S
S S
S
S
S
In2
In1
F
F
F = !(In1 S + In2 S)
Page 19
CSE477 L07 Pass Transistor Logic.23 Irwin&Vijay, PSU, 2003
Transmission Gate XOR
B
A A B
1
off
off
an inverter
B !A
0
on
on
weak 0 if !A
weak 1 if A
A !B
Page 20
CSE477 L07 Pass Transistor Logic.24 Irwin&Vijay, PSU, 2003
TG Full Adder
Sum
Cout
A
B
Cin
Page 21
CSE477 L07 Pass Transistor Logic.25 Irwin&Vijay, PSU, 2003
Differential TG Logic (DPL)
A
A
B
B
B
AND/NAND
F=AB
F=AB
XOR/XNOR
A
A
B
B
A
A
B
F=AB
F=AB
A
A
B
B A B A
GND
GND
VDD
VDD
B
Page 22
CSE477 L07 Pass Transistor Logic.26 Irwin&Vijay, PSU, 2003
Next Time: The MOS Transistor
MOS transistor dynamic behavior (R and C)
Wire capacitance
Page 23
CSE477 L07 Pass Transistor Logic.27 Irwin&Vijay, PSU, 2003
Next Lecture and Reminders Next lecture
MOS transistor dynamic behavior- Reading assignment – Rabaey, et al, 3.2.3 & 3.3.3-3.3.5
Wiring capacitance- Reading assignment – Rabaey, et al, 4.1-4.3.1
Reminders HW#2 due September 30th (Tuesday) Project specs due (on-line) October 9th Evening midterm exam scheduled
- Monday, October 20th , 20:15 to 22:15, Location TBD
- Only one midterm conflict scheduled