advertisement

Digital Circuits A key element of electrical engineering sources: http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/EEIndex.html http://www.jhu.edu/~virtlab/logic/log_cir.htm Foundations of Electrical Engineering Electrophysics Maxwell's Information (Communications) Theory Data Equations, Circuit theory rate vs Signal Strength, Error correction Digital Logic Using Logic Gates to build up complex ccts 2 Lecture 1 EE Subdisciplines Power Systems Electromagnetics Solid State Communication/Signal Processing Controls Analog/Digital Design 3 Lecture 1 What is Voltage? V = “Electrical pressure” - measured in volts. H2O High Pressure Low Pressure Figure 1.1 4 A battery in an electrical circuit plays the same role as a pump in a water system. 5 Where can I find a digital circuit? • Digital Circuits and Digital Logic are the buiding blocks for – computers – certain control systems (robots, factories) – LED displays • Digital Circuits are built up from Analog circuits – analog: continuous varying, digital: discrete steps – electricity has many characteristics of logic (+/-, True/False, etc) Digital Logic is based on Binary • Two possible states – True/False – 1/0 – Yes/No – On/Off – Light/Dark Logic Signals • Different systems for representing binary info in the physical world: – A voltage signal: • Zero (0) corresponding to 0 volts • One (1) corresponding to five volts. – A sinewave: • Zero corresponding to some frequency • One corresponding to some other frequency. – Switches: • OPEN for "0" and CLOSED for "1". – (And there are more ways!) Characteristics of Logic Signals • Change in time – Initially we look at unchanging values • Voltage signal takes on two values – corresponding to 0 and 1. – for example, 0 Volts and 5 volts • We can associate a variable with a logic signal – assign a symbol to represent that variable – like the symbol A. Think Binary! • Examples: – Cell phone converts voice into series of ones and zeros – Thermostat: 1 when temp is too low, 0 when too high • Logic signal, A, takes on values of 0 (FALSE, OFF) or 1 (TRUE, ON). – might be a voltage, a switch closure, etc. – More generally, think in terms of zeros and ones • not in terms of the values of the voltage or switches. Operations on Logic Circuits • Suppose there are 2 logic signals. A and B OR GATE • We often want to derive a 3rd signal C based on the values of A and B – if it is dark outside (A) OR raining (B) • Turn on lights (C) Logic Gates • Logic Gates are digital circuit packages that perform logic operations on logic signals – AND gate: both must be true for true output – OR gate: only one must be true for true output – NOT gate: true in means false out and vice versa – NAND gate: the opposite of AND • AND followed by NOT Truth Tables and Logic Gates • Truth tables use 1/0 instead of True/False • OR AND NOT (inverter) NAND GATE = NOT AND • An AND gate – followed by NOT A B • Truth Table • • C Usually drawn as: A B Any logic gate can be built from 1 or more NAND gates! C Lab: LS7400 Quad Nand Gate IC • Pinout shows which pins are connected to what • Vcc = +5 V, -Vcc = 0V • Verify the notch location!! Breadboard a NAND gate • a breadboard is a quick way to build circuits without soldering • 5 pin rows are connected by metal clips • vertical column on sides are connected. Connect Power Supply to Breadboard Black to Ground Red to +5V Ground +5V Add signal wires for A and B Add LEDs to show value of A,B (1- 470 Ohm Resistor for each LED) The shorter Wire on LED goes to GROUND When both are connected, turn on power (A, B are '1') Move brown wire to change B to '0' Add LED to output pin 3 Are these showing the NAND truth table? How about these? A NAND Inverter • Connecting the two inputs A – A and B are both at A – Truth table is the same as Inverter C NAND + NAND = AND • Taking NOT-NOT AND A B Try it! Add wires from Pin 3 to 4, 4 to 5 move your output Resistor to pin 6 C FLIP FLOP – another 2 NAND cct • Output of 1 (P) – feeds back to input of 2 • Output of 2 (Q) – feeds back to input of 1 MEMORY!! previous state was X=1, Y=0 previous state was X=0, Y=1 Flip Flop Wiring And each labelled pin A,B,P,Q have resistors and LED to ground for monitoring Flip Flop in Action A=0 B=0 A=0 B=1 Flip Flop in Action A=0 B=1 A=1, B=1 A=1 B=0 A=1, B=1 3 NANDs make an OR A C B Use the digital circuit simulator to try out: http://www.jhu.edu/~virtlab/logic/logic.htm Team Challenges (simulate or wire up) Using only NAND gates: A . Design, construct and test a three input AND. B. Design, construct and test a three input NAND. C. Design a four input NAND. Hint: If wiring, use an LED indicator to show the inputs and/or the outputs in your circuits. Use a current-limiting resistor for each LED. Use the circuit builder http://www.jhu.edu/~virtlab/logic/logic.htm D. Using only AND, NOT, and OR produce a three-input OR circuit, ie., the output is 1 if any of the inputs is 1. E. Create a two-input "adder" with two outputs: the one-digit result of the add, and the value of a "carry" bit. In binary arithmetic, 1+1=0 with a carry=1. Super Bowl Problem • At the beginning of halftime during the Superbowl, – 35 million toilets are flushed almost simultaneously. – Resulting loss of water pressure wreaks havoc on many municipal water systems. • Make a controller to solve the problem for a "three toilet" system. – Devise a logic circuit whose "1" inputs represent "flushes" – and whose "1" outputs represent opened water valves – If no more than one toilet is flushed, then that toilet's water valve opens, • the others remaining closed. – If more than one toilet is flushed, then all the water valves remain closed.