Penn ESE370 Fall DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 22: October 31, 2011 Pass Transistor Logic

Teaser What does this do? Penn ESE370 Fall DeHon 2

Previously Penn ESE370 Fall DeHon 3

Elmore Delay For each resistor Ri in path –Compute C Ri = sum of all C’s downstream of Ri –Delay through Ri is Ri×C Ri Penn ESE370 Fall DeHon 4

Capacitance Roundup C GS =C GCS +C O C GD =C GCD +C O C GB =C GCB C SB =C diff C DB =C diff Penn ESE370 Fall DeHon 5 Day 11

Impact on Capacitance Penn ESE370 Fall DeHon 6

Contact/Diffusion Capacitance C j – diffusion depletion C jsw – sidewall capacitance L S – length of diffusion Penn ESE370 Fall DeHon 7 LSLS

Today Elmore Delay with C diff >0 for CMOS Pass Transistor Circuit? –Output levels –Cascading Series pass transistors? Delay –Level Restore Penn ESE370 Fall DeHon 8

CMOS Elmore and Cdiff Penn ESE370 Fall DeHon 9

Identify Function What function? Penn ESE370 Fall DeHon 10

Inverter Delay Delay driving self? –Elmore –Include Cdiff=  Cgate Penn ESE370 Fall DeHon 11 W=1

Identify Capacitances Self output load? Load presented by A input? Capacitance between pd transistors? Penn ESE370 Fall DeHon 12

Identify Capacitances Equivalent RC circuit (output stage) A=B=0? Penn ESE370 Fall DeHon 13

Total Delay Total delay of circuit? Penn ESE370 Fall DeHon 14

Cascade Delay Delay of chain of n of these? (connect Y to A) Penn ESE370 Fall DeHon 15

Pass Transistor Penn ESE370 Fall DeHon 16

Area Compare with CMOS circuit? Penn ESE370 Fall DeHon 17

Output What does output look like (DC transfer)? Penn ESE370 Fall DeHon 18

Pass TR transfer (B=1) Penn ESE370 Fall DeHon 19

CMOS Inverter Transfer Penn ESE370 Fall DeHon 20

Reasonable Input to CMOS Inverter? Penn ESE370 Fall DeHon 21

Pass tr xor2 with inv restore Penn ESE370 Fall DeHon 22

Required to use? What take to use this with CMOS circuit? Penn ESE370 Fall DeHon 23

Restore Output Penn ESE370 Fall DeHon 24

Chain Together Penn ESE370 Fall DeHon 25

Analyze Stage Penn ESE370 Fall DeHon 26

Analyze Stage Equivalent circuit? Delay? Compare area and timing to CMOS Penn ESE370 Fall DeHon 27

Class ended here Penn ESE370 Fall DeHon 28

Cascading Pass Transistors Penn ESE370 Fall DeHon 29

Chain without Inverters What if we did this? Penn ESE370 Fall DeHon 30

Extract key path Penn ESE370 Fall DeHon 31

Voltage of Chain What is voltage at output? Penn ESE370 Fall DeHon 32

How compare Compare Penn ESE370 Fall DeHon 33

DC Analysis Penn ESE370 Fall DeHon 34

DC Analysis – chain of 6 Penn ESE370 Fall DeHon 35

Conclude Can chain any number of pass transistors and only drop a single Vth Penn ESE370 Fall DeHon 36

Capacitance What is Capacitance per stage? Penn ESE370 Fall DeHon 37

Delay Delay as a function of chain length? Penn ESE370 Fall DeHon 38

Compare CMOS Buffered Pass TR Unbuffered Pass TR Delay Area Penn ESE370 Fall DeHon 39

Pass TR Tree What if we did this? Penn ESE370 Fall DeHon 40

Path What’s different about this? Penn ESE370 Fall DeHon 41

Demonstration Circuit Penn ESE370 Fall DeHon 42

SPICE TODO show spice results of voltages Penn ESE370 Fall DeHon 43

Demonstration Chain Penn ESE370 Fall DeHon 44

Spice Penn ESE370 Fall DeHon 45

Conclude Cannot cascade degraded inputs into gates. Penn ESE370 Fall DeHon 46

Admin Project –Due Friday Midterm 2: Nov. 9 th –Week from Wednesday in the evening Penn ESE370 Fall DeHon 47

Idea There are other circuit disciplines Can use pass transistors for logic –Even chains of pass transistors –Sometimes gives area or delay win Do not cascade as easily as CMOS Penn ESE370 Fall DeHon 48