ECE 2020 Digital Design

Prof. Matthieu Bloch

Monday, September 22, 2025 (v1.0) - Switches

Last time

  • Last time
    • How do we implement boolean operations in cicuits?
    • Switches offer a solution
    • We discussed
  • To be effectively prepared for today you should have:
    • Relaxed a bit after the exam and the test
    • Started reviewing the textbook and your notes in preparation for the exam
    • Possibly looked at the problems in the textbook for extra practice
  • Important notes
    • Homework 3 assigned on Wednesday September 24, 2025 (we need to cover a bit more)
    • Breanna's office hours Thursdays 2-3pm in TSRB 530
    • Dr. Bloch office hours Wednesday 8:30am-9:30am
  • Today
    • We will continue talking about the implementation of boolean algebra
  • Be ready!
    • I expect you to take notes
    • We will take a quizz on canvas for attendance

Last time: controllable switches

  • Our logic needs to be programmable to be useful
  • How do we implement a controllable switch?
    • MOSFETs: Metal Oxide Semiconductors Field Effect Transistors
    • These constitute imperfect switches
  • nMOS allow us to create a switch, but…
    • they are not perfect
    • when closed, the switch is very good a propagating 0 but not good at propagating 1
  • pMOS comes to the rescue!

Transmission gate

  • Can we build a perfect switch using both nMOS and pMOS?
  • We can create a composite switch
  • We can create logic families

High impedance

CMOS

  • CMOS logic: Complementary MOSFET logic family such that
    • pMOSes propagate 1s
    • nMOSEs propagate 0s
  • How do we implement boolean expressions in CMOS logic?
  • Example: inverter

General approach to implement CMOS Logic

  • Step 1: separate the variables using DeMorgan's law as needed
  • Step 2: draw the Pull Up Network (PUN)
    • OR: pMos in parallel
    • AND: pMOS in series
    • connect gates to inverse of variables
  • Step 3: draw the Pull Down Network (PDN) for the dual function
    • OR: nMOS in parallel
    • AND: nMOS in series
    • connect gates to inverse of variables

Example: NAND

Example: NOR

Example: more complicated boolean expression

  • Example 1: \(D=A\cdot\overline{C}+B\)

  • Example 2: \(F = E\cdot(\overline{A}\cdot D+\overline{B}\cdot(A+\overline{C}))\)

Until next time

  • To be effectively prepared for Wednesday September 24, 2025, you should:
    • Read your notes and review the examples