Penn ESE370 Fall DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 23: October 24, 2014 Pass Transistor Logic: part 2 (Cascading without Buffers)

Previously Penn ESE370 Fall DeHon 2

Two XOR Gates Penn ESE370 Fall DeHon 3

Today Pass Transistor Circuit –Output levels –Cascading Series pass transistors? Delay Start on Distributed RC –Analyzing delay for pass-tr designs Penn ESE370 Fall DeHon 4

Cascading Pass Transistors Penn ESE370 Fall DeHon 5

Chain without Inverters What if we did this? Penn ESE370 Fall DeHon 6

Extract key path Penn ESE370 Fall DeHon 7

t=0 (after Vin transition 1  0) Penn ESE370 Fall DeHon 8

t=4  (after Vin transition 1  0) Penn ESE370 Fall DeHon 9

t=∞ (after Vin transition 1  0) Penn ESE370 Fall DeHon 10

Focus on Pass tr Vgs? Operation mode? Current flow? Penn ESE370 Fall DeHon 11

Voltage of Chain What is voltage at output? Penn ESE370 Fall DeHon 12

How compare Compare Penn ESE370 Fall DeHon 13

DC Analysis Penn ESE370 Fall DeHon 14

DC Analysis – chain of 6 Penn ESE370 Fall DeHon 15

Conclude Can chain any number of pass transistors and only drop a single Vth Penn ESE370 Fall DeHon 16

Transient Penn ESE370 Fall DeHon 17

Closeup Penn ESE370 Fall DeHon 18

Inverter Sense Penn ESE370 Fall DeHon 19

Capacitance What is Capacitance per stage Penn ESE370 Fall DeHon 20

Delay Setup What does RC circuit look like? Penn ESE370 Fall DeHon 21

Pass TR Tree What if we did this? Penn ESE370 Fall DeHon 22

Path What’s different about this? Penn ESE370 Fall DeHon 23

Gate Cascade? What are voltages? Penn ESE370 Fall DeHon 24

Demonstration Circuit Penn ESE370 Fall DeHon 25

SPICE TODO show spice results of voltages Penn ESE370 Fall DeHon 26

Demonstration Chain Penn ESE370 Fall DeHon 27

Spice Penn ESE370 Fall DeHon 28

Transient Response Penn ESE370 Fall DeHon 29

Conclude Cannot cascade degraded inputs into gates. Penn ESE370 Fall DeHon 30

Distribute RC (setup) Time Permitting Penn ESE370 Fall DeHon 31

What is response? Penn ESE370 Fall DeHon 32

What is response? Penn ESE370 Fall DeHon 33

What is response? Penn ESE370 Fall DeHon 34

SPICE Response Penn ESE370 Fall DeHon 35

Intuition Look at series of R’s on path –Must move Q=V(  C) across each R Not as much as if both R’s precede C’s Penn ESE370 Fall DeHon 36

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 37

Admin Project –Milestone 1 in –Will try to get feedback Friday evening/Sat. –Should be working hard on project –Rewarding experience Penn ESE370 Fall DeHon 38