EE 261 Introduction to Logic Circuits

1
EE 261 Introduction to Logic Circuits

• Lecture 15
• Agenda
• CMOS Dynamic Behavior (Noise)
• Exam Review
• Announcements
• Exam 1, Wednesday, (10/08/08)- Closed-book,
  closed-notes, you will be allowed a 1-page cheat
  sheet.

2
CMOS Dynamic Behavior

• Simultaneous Switching Noise (SSN)- What
  happens when we switch logic levels with a
  capacitive load?- the
  capacitor looks like a short at AC (or when
  transitioning)

VDD
S
PMOS ON
G
S
D
IL
CL
G
NMOS OFF
S
GND
3
CMOS Dynamic Behavior

• Simultaneous Switching Noise (SSN)- this causes
  a large instantaneous current to be drawn from
  the power supply- the power supply cannot
  supply this sudden change in current because it
  has a much slower response- we can think of
  the power supply as a large capacitor that stores
  our charge. The interconnect that supplies
  this charge to our device tends to be inductive
  and resistive.- when we draw current quickly
  through the interconnect, there is voltage
  drop- as a result, the power supply voltage
  will begin to "droop" as seen by our device

4
CMOS Dynamic Behavior

• Simultaneous Switching Noise (SSN)- "Decoupling
  Capacitors"- we can put decoupling (or bypass)
  capacitors near our device to provide the faster,
  instantaneous current draw due to switching
  logic levels.

RL
VDD
PMOS ON
CDecoupling
S
G
S
D
IL
CL
G
NMOS OFF
S
GND
5
CMOS Dynamic Behavior

• Simultaneous Switching Noise (SSN)- "Ground
  Bounce"- when the interconnect on the IC is
  inductive, we will get a voltage drop when
  instantaneous current is drawn. VL L
  (di/dt)- this causes the VDD (or GND) on-chip
  to be different from the VDD (or GND) in the
  system.- this is called "Ground Bounce" -
  this leads to inadvertent switching of logic
  circuits- this problem becomes more sever when
  multiple signals share the same VDD or GND pin

VDD
L
VL -
PMOS ON
S
G
S
D
IL
CL
G
NMOS OFF
S
GND
6
Exam Review

• Chapters 1 2 Topics- digital vs analog
  (digital revolution)- systems bases-
  system conversions - to from decimal - to
  and from binary/oct/hex- binary arithmetic -
  addition (carries) - subtraction (borrows)-
  negative 's in binary - Signed Magnitude -
  2's Complement - 1's Complement- 2's
  complement arithmetic - complement add -
  range overflow - Codes - BCD, Gray, ASCII,
  Parity

• Chapter 3 Topics- Truth Tables- Logic Gates-
  Simple Logic Circuits- Timing Diagrams-
  MOSFETS - NMOS, PMOS, CMOS- CMOS Inverter-
  CMOS NAND- CMOS NOR- CMOS AND,OR,BUF- CMOS
  Static Behavior - Input Specs - Output Specs
  - Noise Margin - Leakage - Resistive Loads
  - Fan-In / Fan-Out - Latch-Up- CMOS Dynamic
  Behavior - transition time / propagation delay
  - power consumption (SC Load) - Decoupling
  - Ground Bounce / SSN