1 Computer Logic, Logic Gates, and Building Circuits Image: Intel Museum

2 Logic and Computers The technology inside of computers (and in fact all logical ideas) is based on several simple logic operations: –AND –OR –NOT These logical operations are related to real world equivalents…

3 What does “AND” mean? When we say “AND,” formally we mean that something is true only when both parts of it are true. If the stove is on AND water’s in the kettle, then we will make steam. If either part isn’t there, then the result won’t be true.

4 What does AND mean in Logic? AND takes two inputs and produces the value TRUE if both inputs are TRUE’s. So, with the inputs: T AND Toutput is: T T AND Foutput is: F F AND Toutput is: F F AND Foutput is: F (Just like in the real world.)

5 A Truth Table A truth table is a convenient way to write down the definition of a logic operation. Let’s use 1’s for True and 0’s for False… Here’s the table for AND: inputinputoutput Output of 1 only when both inputs are 1.

6 Truth Table for OR and NOT ORNOT inputinputoutputinputoutput We only need one of the inputs to be true, but if both are true, that’s ok, too. This just gives the opposite.

7 A Logic Gate A logic gate is a small computer circuit that simulates one of the logic operators we’ve seen. There are one or two wires that go into the gate that represent the input. There is one output wire that represents the result of the logic operation. When the power is on, that means True.

8 Logic Gates When drawing diagrams of logic circuits, we use some traditional shapes for each one. If A and B are on, turn on C.If A or B is on, turn on C. A B A B CC If A is off, turn on C. AC AND OR AND NOT

9 What have we seen so far? We saw what a logic gate is. We saw how to use one. Now we will see how they work inside. But we will return to the LogSim program later to see how we can combine multiple logic gates together to do useful work.

10 But what’s inside the box? How do we build an individual logic gate? “Pattern on the Stone” chapter 1 shows to build each logic gate from tinker toys, water pipes, or wires. But real computers today use logic gates built out of small electronic devices. Image: Tinkertoy Website

11 “OR” Gate from Tinker Toys spring moveable Push either one of these to make the output pole move. Not moveable Pushing a pole to the right means TRUE.

12 Building Gates from Wires AND OR How many switches must you turn on in each circuit in order for the light bulb to light up? Lighting-up means TRUE for output. Flipping a switch means TRUE for input.

13 Real Computers Real computers are not built from tinker toys or any of the other wacky approaches shown in the “Pattern on the Stone.” Computers today use logic gates that are built out of small electronic devices called “transistors.”

14 What’s a Transistor Transistors are miniature electronic switches. Similar to a light switch, transistors have two operating positions, on and off. This on/off functionality is used to build logic gates. Image: Intel Museum

15 Why are transistors special? Transistors are like real switches, but – 1. They have no moving parts. 2. They can be turned on and off by an electric signal. 3. You could hook the output of one switch to the input of another since they both use electricity! So that means – They are small and fast. In fact, as transistors get smaller and faster, so do computers. Image: Intel Museum Control Signal

16 Computers Get Smaller: ENIAC The tremendous size of the first electronic computer, built here at Penn, was because the transistors used to built it were the size of lightbulbs. Modern transistors are microscopic. That’s what we have laptop and handheld computers nowadays. Image: Van Pelt Library ENIAC Exhibit

17 Sizes of Transistors Look at the transistors samples in class. “Moore’s Law” is a famous observation that: Researchers tend to shrink transistor sizes in half every 18 months. Transistors shrink so we can make chips smaller and fit more on a single chip. Image: Intel Research

18 A Transistor A transistor controls whether a switch is on or off. It turns off its switch when it’s given an electric signal. When there is no power on the signal line, the transistor turns the switch on. Transistor signal

19 A Transistor A transistor controls whether a switch is on or off. It turns off its switch when it’s given an electric signal. When there is no power on the signal line, the transistor turns the switch on. Transistor OFF signal ON

20 A Transistor A transistor controls whether a switch is on or off. It turns off its switch when it’s given an electric signal. When there is no power on the signal line, the transistor turns the switch on. Transistor ON signal OFF

21 Logic Gates from Transistors For example, we will build a NOT gate from a transistor. Transistor extra power source Input to NOT gate. Output from NOT gate.

22 Logic Gates from Transistors For example, we will build a NOT gate from a transistor. Transistor OFF extra power source NOT 1  0 Input to NOT gate is ON. Output from NOT gate is OFF.

23 Logic Gates from Transistors For example, we will build a NOT gate from a transistor. Transistor ON extra power source NOT 0  1 Input to NOT gate is OFF. Output from NOT gate is ON.

24 Summary: A Hierarchy of Parts We started with TRANSISTORS. We built some LOGIC GATES. Can we build useful stuff out of logic gates? How can we combine them?

25 Building “Exclusive OR” One or the other but not both. Different than “OR.” _ IN IN OUT “Normal” OR IN IN OUT What we want…

26 Building “Exclusive OR” One or the other but not both. Different than “OR.” Want output TRUE when either: Input 1 is TRUE and Input 2 is FALSE or Input 1 is FALSE and Input 2 is TRUE IN IN OUT “Normal” OR IN IN OUT What we want…

27 Building “Exclusive OR” One or the other but not both. Different than “OR.” Want output TRUE when either: Input 1 is TRUE and Input 2 is FALSE or Input 1 is FALSE and Input 2 is TRUE IN IN OUT “Normal” OR IN IN OUT What we want… Can replace False with NOT TRUE.

28 Building “Exclusive OR” Want output TRUE when either: Input 1 is TRUE and Input 2 is NOT TRUE or Input 1 is NOT TRUE and Input 2 is TRUE OR NOT AND NOT Input 1 Input 2 Output

29 If there’s extra time… Can we build an AND circuit using only NOT and OR gates? IN IN OUT What we want…

30 If there’s extra time… NOT Input 1 Input 2 Output NOT OR AND should be FALSE if either Input 1 is NOT TRUE or Input 2 is NOT TRUE.

31 Credits The Intel Museum: Intel Research Website: Van Pelt Library ENIAC Exhibit: John W. Mauchly and the Development of the ENIAC Computer Tinker Toys Website: