Electronic Troubleshooting
Chapter 10
Digital Circuits
Digital Circuits
• Key Aspects
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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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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 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 Gates
• AND Gate
Logic Gates
• 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.
Boolean algebra
Logic Gates
• Sample Gate Application
• AND Gate
• What would the Output be with: OR Gate, XOR Gate
Inverters
• 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
Inverters
• Sample Application
Notice the line
over BURST. It is
called BURST
NOT
• Key Aspects
NAND Gates
• Can be built with the gates already covered
• An AND Gate followed by an Inverter
• So commonly used construction –
are available monolithic implementations
• Characteristics
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
• Three components less
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 we decrease if
R2 was made smaller
• However significant
current would flow when
Q2 was turned on
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
» Whit a Low out Q3 is off
• When both inputs are High
• Reverse currents supply
base of Q4
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 Considerations
• Typical TTL parameters
Actual Gate Considerations
• Typical TTL part - 7400
Actual Gate Considerations
• Standard 74 series TTL has evolved into other
series:
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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
Specialized Test Equipment
• Logic Probe
• Example: Instek GLP-1A Logic Probe
Specialized Test Equipment
• Digital Pulser
• Digital Pulser (SJ-1)
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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 Equipment
• Logic Analyzere
Specialized Test Equipment
• Testing a gate in a Live Circuit
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 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 at ground – Logic 1
• Q1 conducts and appears as a
1000 Ω resistor
• Q2 is off and appears as an open
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 turn on and conduct
• Q3 and Q4 are open and not conducting
• - VDD appears at the Output
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 Circuits
• CMOS Characteristics
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
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RS Flip-Flops
D Flip-Flops
J-K Flip-Flops
Binary and Decade Counters
7-Segment displays
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 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 Counters
• RS Flip-Flops
• NAND Gate version
• Alternate Logic symbol drawing
• Also – Pull-Up Resistor
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 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 Counters
• Binary Counter using J-K Flip-Flops
• Q output acts as the clock input to the next Flip-Flop
Walk through the
circuit and timing
diagram
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 Counters
• Sample Monolithic
Counter
• 7490
• Decade counter
• Counts 0 – 9 and can
be reset to zero
• Has 4 outputs
• Reference
http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS006533.PDF
Flip-Flops and Counters
• BCD Displays
• A common Anode version is shown
• Common Cathode versions are also available
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