Principles of Computer Engineering: Lecture 9: Sequential Logic Circuits

Introduction  Digital Timing Circuits  Sequential Logic  Flip-Flops  Traffic Light Problem  Combinational Logic Required  Summary

Digital Timing Circuits (NE555)  All sequential digital circuits require a master clock signal  A “555” timer is often used for this purpose  Need to design our timer to give a frequency of 0.5 Hz  f = 1/(1.4RC)  Given that C = 1  F  What value for R = ? +5V

Sequential Logic: State Machine  Can construct a “state machine” using JK Flip Flops  Can use combination logic to control state changes  Simplest state machine is a counter (prebuilt JK FFs cascaded)  We will use a 3-bit counter (000  111) 8-states

Traffic Light Problem  Red, amber and green, LEDs to indicate the traffic lights  The LEDs are turned on by logic ‘0’ inverse logic (don’t use the Hex inverter package this time as logic simpler).  In this way, the decoder can be designed using a maximum of just one 74LS00 quad NAND chip.

Traffic Light Problem T/sQ0Q1Q2Q  (Not) Red = ?  (Not) Amber = ?  (Not) Green = ?

(Not) Red = (Not) Amber = (Not) Green = Combinational NAND Logic Required 8

Summary  Timing circuits required for sequential logic  555 timer often used for this purpose  Traffic light problem requires sequential logic  Simple to use a binary counter  Combinational NAND logic required to drive LEDs  Questions?

Formal Report Semester 1  You are each required to submit a formal report based on one experiment, either;  Exp 4: The Wheatstone Bridge OR  Exp 7: Transformers and Power Supplies  Details and guidelines on VLE please read!  The report is due on Wednesday the 7 th of January 2015 to the Faculty Office T313 by 4:00pm  You must submit your logbooks at the same time  Report worth 20% of module  Logbook worth 10% of module