CSC Intro. to Computing Lecture 5: Gates, Circuits, & Transistors

Announcements Homework available on Blackboard  Homework due Friday at 4:00PM Hand to me or put in my box in Wehle 207  Quiz on this material next Thursday Quiz last week went fairly well  Top was 100; Mean was 77 (σ = 20.7)  Scores available on Blackboard Come to my office to pick up your quiz

Announcements CSC tutors are now available  Hours posted outside Wehle 208  Also come to my office hours, make an appointment, or call/ me questions

Circuit Design In your group, complete work with circuits  Compute the truth tables for circuits  Draw the diagrams for these equations

Truth Table

Truth Table

Circuit Design

Circuit Design

Algebraic Properties Law of Double Negation: a’’ = a

Improving Circuit Design CircuitProperty Used in this Step Identity Commutative Distributive

Improving Circuit Design CircuitProperty Used in this Step Identity DeMorgan’s Law Associativity Commutativity Distributive Identity Distributive Identity

Circuit Propagation Delay Time taken for signal to get through circuit  Important measure when building processor  Gate cannot generate results until it has all of its inputs Each gate starts at the time of the latest input  Each gate requires a set amount of time to complete Could be specific amount of time (e.g., 10 ps) Or state result as multiple of gate delays

How are these improved? How long will this circuit need to complete?

How are these improved? How long will it take for the signal to propagate through?

Circuit Delay Propagation What is the propagation delay for this circuit?

Circuit Delay Propagation What about this circuit?

Transistors Transistors used to implement gates  Uses a semiconductive material Material can serve as both conductor and insulator Silicon is the preferred semiconductor because of cost. Why is it so cheap?

Transistors Originally invented by Bell Labs in 1947  Have been improved since then… Can switch on-and-off in nanoseconds Each transistor dissipates energy  Why is this be a problem?

My View of Transistor Source

My View of Transistor Ground

My View of Transistor Output

My View of Circuit Input: Franklin “off” flying a kite

My View of Circuit Input: Franklin “on” poking key

Engineer’s View of a Circuit Source connects system power  Always at +5V (e.g. “high” state or 1) Ground drains transistor’s energy  Leaves transistor at +1V (e.g. 0) When V in controls “base”  Acts like on-off switch  When on, source drains into ground  When off, source signal sent to V out

Transistor Design Turns out NOT, NAND, and NOR are easiest gates to turn into transistors  How do these work?

Transistor Design Apple wanted NAND-based memory (rather than NOR-based) for iPod Nano. Why?

Combinatorial Circuits So far, all circuits have been combinatorial  Output is determined only by input values  Why would we need other circuits?

Sequential Circuits Sequential circuits include another feature  Output determine by inputs AND current state  Used when current state is important detail E.g., Memory

S-R Latch S-R latch stores single binary digit (1 or 0)  Result is value of X Inputs stand for Set and Reset Could also be implemented with NOR gates Adapted from Computer Science Illuminated, Dale and Lewis, p. 112 X’

S-R Latch X’ Normally, S & R = 1  Maintains value of X S = 1, R = 0  X = 0  Called the “set state” S = 0, R = 1  X = 1  Called the “reset state” Latches also called “flip-flop”s

For next lecture Start doing the homework Start reading Section 5 Be ready to discuss:  What Individual Computer Component Descriptions Mean Sizes Disks Speed