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Electronic Troubleshooting. Chapter 10 Digital Circuits. Digital Circuits. Key Aspects Logic Gates Inverters NAND Gates Specialized Test Equipment MOS Circuits Flip-Flops and Counters. Logic Gates. Characteristics A combinational Logic circuit with two or more inputs and one output

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Electronic troubleshooting

Electronic Troubleshooting

Chapter 10

Digital Circuits

Digital circuits
Digital Circuits

  • Key Aspects

    • Logic Gates

    • Inverters

    • NAND Gates

    • Specialized Test Equipment

    • MOS Circuits

    • Flip-Flops and Counters

Logic gates
Logic Gates

  • Characteristics

    • A combinational Logic circuit with two or more inputs and one output

      • OR Gates

      • And Gates

      • Exclusive OR Gates

      • etc.

    • Inputs are limited too two values

      • High –Logic 1

        • Often assumed to be +5V

      • Low – Logic 0

        • Often assumed to be 0V

Logic gates1
Logic Gates

  • Characteristics

    • Inputs are limited too two values

      • Possible combinations

        • 2-inputs with 2-possible values => 4 permutations

        • Permutations 2n , n= number of inputs

  • OR Gate

Logic gates2
Logic Gates

AND Gate

Logic gates3
Logic Gates

Boolean algebra

  • Exclusive OR Gate

    • The XOR gate (sometimes EOR gate) is a digital logic gate that implements exclusive disjunction - it behaves according to the truth table

      • A HIGH output (1) results if one, and only one, of the inputs to the gate is HIGH (1).

      • If both inputs are LOW (0) or both are HIGH (1), a LOW output (0) results.

Logic gates4
Logic Gates

  • Sample Gate Application

    • AND Gate

    • What would the Output be with: OR Gate, XOR Gate


  • Characteristics

    • Changes one logic level to the other

      • Often needed in digital circuits

        • Chapter 9 page 248

          • The “R” input to the flip-flop has an

            invert on it


Notice the line over BURST. It is called BURST NOT

  • Sample Application

Nand gates
NAND Gates

  • Key Aspects

    • Can be built with the gates already covered

      • An AND Gate followed by an Inverter

      • So commonly used construction –

        are available monolithic implementations

  • Characteristics

In class review
In Class Review

  • Combination Logic and Truth Tables

    • Problem 10-2 on page 293

    • Problem 10-5 on page 293

  • Simple Troubleshooting

    • Problem 10-3 on page 293

Actual gate considerations
Actual Gate Considerations

  • Key Aspects

    • Will use NAND Gates as a sub fro all gates

  • Simplified /Improved Component Count

    • Two emitters almost as easy in manufacturing as one

      • Accomplished when artwork for the IC is made

      • Less components

      • Q1 functions like the three diodes it replaced, not as an amplifier

Actual gate considerations1
Actual Gate Considerations

  • Rise Time Problems

    • Caused by the input capacitance of gates driven high

      • TTL gates typically have a Fan out of 10

      • Thus the parallel connection the gate’s input capacitance is significant

      • Rise time will decrease if R2 was made smaller

        • However significant current would flow when Q2 was turned on

Actual gate considerations2
Actual Gate Considerations

  • Rise Time Problems

    • Solution

      • Use Totem Pole Output

    • Totem Pole Operation

      • When at least on input is low

        • Q2 is off, No current in R3 and Q4 is off

        • Q3 is on and R4 can be small and minimize the time constant for the output to go high

          • With a Low out Q3 is off

      • When both inputs are High

        • Reverse currents supply base of Q4

Actual gate considerations3
Actual Gate Considerations

  • Rise Time Problems

    • Totem Pole Operation

      • When both inputs are High

        • Reverse currents supply base of Q2, Q2 conducts

        • Base of Q4 goes high and Q4 conducts

        • Output is Low

      • Much faster Rise times

        • Since Q3 only conducts when the output is high, R3 can be sized to minimize the time constant and not cause a heat and efficiency problem

Actual gate considerations4
Actual Gate Considerations

Typical TTL parameters

Actual gate considerations5
Actual Gate Considerations

Typical TTL part - 7400

Actual gate considerations6
Actual Gate Considerations

  • Standard 74 series TTL has evolved into other series:

    • Standard TTL, 74 series

    • Schottky TTL, 74S series, Low power Schottky TTL, 74LS series (LS-TTL), Advanced Schottky TTL, 74AS series (AS-TTL), Advanced low power Schottky TTL, 74ALS series

    • 74F fast TTL

  • CMOS series

    • CD 4000, incompatible with TTL voltage levels

    • Low power CMOS 74 series pin compatible, 74LS series

    • Low power CMOS that is TTL level and pin compatable – 74HCT,

    • Improved versions, e.g., LV, LVT, ALVT, AC/ACT (A is for advanced, T is for TTL compatable)

Specialized test equipment
Specialized Test Equipment

  • Logic Probe

    • Example: Instek GLP-1A Logic Probe

Specialized test equipment1
Specialized Test Equipment

  • Digital Pulser

    • Digital Pulser (SJ-1)

      • Accurate Timebase Generator

      • Output: Open Collector

      • (Interfaceable with any Logic Circuits)

      • Supply: 4.5V-18VDC

      • 9 Selectable Output Frequencies: 16MHz (crystal osc. output), 8MHz, 1MHz, 100KHz, 10KHz, 1KHz, 100Hz, 10Hz & 1Hz.

Specialized test equipment3
Specialized Test Equipment

Testing a gate in a Live Circuit

Mos circuits
MOS Circuits

  • Characteristics

    • Most common type is CMOS – Complementary MOS

      • Circuits use both P-Channel and N-Channel devices in the same circuit

      • CMOS Circuits consume very little power

    • Most of the TTL logic gates have been implemented in CMOS

    • Typical Gates covered

      • Inverters and NOR gates

  • Inverter

    • Same logic symbol as for the TTL version

    • Same truth table

Mos circuits1
MOS Circuits

  • Inverter

    • Circuit Operation

      • With the input at ground – Logic 0

        • 0V Gate-source on the N-Channel device (Q2) and it is off

        • -Vdd Gate to-Source on Q1 and it is on and acts like a 1000 Ω resistor

        • Vdd on the output

      • With the input high – Logic 1

        • Q2 conducts and appears as a 1000 Ω resistor

        • Q1 is off and appears as an open

Mos circuits2
MOS Circuits

  • NOR Gate

    • Characteristics

      • Refer to Figure 10-16 on page 281 of the textbook

      • Logic table

        • Logic 1 out only with all logic 0s on the input

      • Construction

        • Two P channel MOSFETs connected to the inputs and connected in series with the VDD and the output

        • Two N channel MOSFETs connected to the inputs and in parallel between the output and ground

    • Circuit Operation

      • With both inputs at ground – A & B at Logic 0

        • Q1 and Q2 (P channel devices) turn on and conduct

        • Q3 and Q4 (N channel devices) are open and not conducting

        • - VDD appears at the Output

Mos circuits3
MOS Circuits

  • NOR Gate

    • Circuit Operation

      • With both inputs, A & B at Logic 0

        • Q1 and Q2 turn on and conduct

        • Q3 and Q4 are open and not conducting

        • - VDD appears at the Output

      • With either or both A & B at Logic 1

        • Either Q1 or Q2, or both are turned off and not conducting much

        • Either Q3, Q4, or both are turned on and conducting

        • Logic 0 appears at the Output

  • CMOS Characteristics

    • See chart on the next slide

    • Handling Precautions – see top of page 283

Mos circuits4
MOS Circuits

CMOS Characteristics

In class review1
In Class Review

  • Logic Levels

    • Logic Level References

    • Problem 10-1 on page 292 using TTL levels

    • Problem 10-1 on page 292 using 5V CMOS

  • Open Inputs

    • Problem 10-13 on page 295

  • Fan Outs

    • Problem 10-15 on page 295

    • Problem 10-16 on page 295

    • Problem 10-17 on page 296

    • Problem 10-18 on page 296

Flip flops and counters
Flip-Flops and Counters

  • Characteristics

    • Used to make sequential logic circuits

      • Outputs depend upon:

        • A previous event

        • Combinational logic inputs

    • The circuits remember what has happened

    • Covered topics

      • RS Flip-Flops

      • D Flip-Flops

      • J-K Flip-Flops

      • Binary and Decade Counters

      • 7-Segment displays

Flip flops and counters1
Flip-Flops and Counters

  • RS Flip-Flops

    • Can be implemented using NAND, NOR, AND, OR, and Inverters

    • NOR gate Implementation

      • Lower right drawing

        • Used ½ of a 7402 IC

      • Inputs are Active Highs

        • A high input will change the state of the Gate

    • NAND gate Implementation

      • Used ½ of a 7400 IC

Flip flops and counters2
Flip-Flops and Counters

  • RS Flip-Flops

    • NAND gate Implementation

      • Notice on the circuit and the logic symbol – Active Low inputs

        • A Low input will change the state of the Gate

        • A High input will not effect the output

    • Other implementations use

      • AND & OR gates with inverters

        • See NAND Gates below

Flip flops and counters3
Flip-Flops and Counters

  • RS Flip-Flops

    • NAND Gate version

      • Alternate Logic symbol drawing

      • Also – Pull-Up Resistor

Flip flops and counters4
Flip-Flops and Counters

  • D Flip-Flops

    • Operation

      • Logic symbol arrows indicate I/O

      • PR and CLR act like the Set (S) and Reset (R) inputs on a NAND Gate

        R-S Filip-Flop

      • Q and Q are always in opposite states

      • The input CK (clock) on a positive transition causes Q to go either high or low depending on the D input

        • Q’s state will match the state of D at that time

Flip flops and counters5
Flip-Flops and Counters

  • J-K Flip-Flops

    • Operation

      • Has same PR and CLR as type D

      • Has two inputs J and K instead of

        the D input

        • See the truth Table

        • Has an additional MODE of

          operation – Toggle

          • Outputs will toggle when a new clock pulse arrives at the CK pin

        • Bubble on the CK indicates that negative transition is active

Flip flops and counters6
Flip-Flops and Counters

Walk through the circuit and timing diagram

  • Binary Counter using J-K Flip-Flops

    • Q output acts as the clock input to the next Flip-Flop

Flip flops and counters7
Flip-Flops and Counters

  • Sample Monolithic Counter

    • 7493

      • Can be a 3-bit or 4-bit counter

        • Wire QA output to input B for 4-bit

          • MOD 16 counter

        • Otherwise use input B

          • MOD 8 counter

      • 14 – pin DIP

Flip flops and counters8
Flip-Flops and Counters


  • Sample Monolithic Counter

    • 7490

      • Decade counter

        • Counts 0 – 9 and can be reset to zero

        • Has 4 outputs

      • Reference

Flip flops and counters9
Flip-Flops and Counters

  • BCD Displays

    • A common Anode version is shown

    • Common Cathode versions are also available

Flip flops and counters10
Flip-Flops and Counters

  • Interface Circuit

    • The BCD counters output binary that resets after 9

    • The 7 segment display with decimal point has eight inputs that cause numbers 0-9 to display

    • The 7447 is a seven segment display driver that translates binary counts into a seven segment inputs

    • See pages 291 and 292

In class review2
In Class Review

  • Timing

    • Problem 10-19 on page 296

  • Identifying problems in Digital Circuits

    • Problem 10-21 on page 297

    • Problem 10-22 on page 298

    • Problem 10-23 on page 298

    • Problem 10-27 on page 298