Download Digital Logic Circuits 1

Basic Digital Troubleshooting 1
©Paul Godin
Updated August 2013
prgodin @ gmail.com
TS 1.1
Digital Circuit Faults
◊ Even the best designed digital electronic
circuits are susceptible to faults and failures.
◊ Good troubleshooting techniques are important
for isolating the fault.
◊ Use of the proper tools and techniques can
determine faults in digital circuits.
TS 1.2
Common Basic Faults
Internal errors:
◊ Open circuit
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Floating output
Open input
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Vcc
Ground
Another pin
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Logic errors
Mislabeled or wrong device
Defective device
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Short Circuit to:
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General malfunction
External Errors
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A float is neither
high (Vcc) nor low
(ground).
Open (no connection)
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Trace
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Pad
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Poor solder connection
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Wire
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Corrosion
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Other factors
Short (to Vcc, GND or other
conductor)
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Solder bridge
Wire or other conductor
Other factors
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Attached circuit fault
Power supply
Environmental
Design (example: loading)
Other
TS 1.3
Troubleshooting Philosophy
◊ Accept only that which you are sure of
◊ Divide the problem into smaller, more basic
forms
◊ Solve the easiest first, working toward the
more difficult
Rene Descartes
TS 1.4
Troubleshooting
◊ Troubleshooting a circuit goes hand-in-hand
with designing and building circuits.
◊ Troubleshooting can be frustrating at times,
but there are some simple approaches that can
be taken to improve success.
◊ A circuit or logic diagram is vital for
troubleshooting.
TS 1.5
Troubleshooting Process part 1
◊ Check for the obvious, common, basic
problem. Example: check the fuse, power switch, plug,
connectors, IC part number, etc... .
◊ Check those things that are the quickest.
Example: a high temperature on the regulator often means a
short circuit. Probe contacts on the ICs, not the breadboard.
◊ Rule of Halves.
Find the junction between functional and
non-functional circuit. Where possible, split the circuit in half.
Perform tests that will eliminate the greatest quantity of
circuit.
TS 1.6
Troubleshooting Process part 2
◊ Think logically based on the measured values.
Gather evidence. Evaluate the inputs and outputs and think
what may be the cause of the fault, or where the fault may
originate.
◊ Visually inspect the components and the
wiring. Look for clues. Are all pin connections accounted for.
Look at points that contain multiple connections. Check to
ensure there are no floating inputs.
◊ Substitute a suspected faulty device.
TS 1.7
Top student circuit problems in Lab
◊ Power/Ground:
◊ Power switch in off position
◊ Board not plugged into the wall
◊ Loose connection between the Vulcan board and the
breadboard
◊ Connection to each component missing
◊
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Miswired (adjacent pin or wrong spec sheet)
Switch Configuration
Asynchronous inputs left floating
Function Generator not configured properly
Wrong IC
TS 1.8
Short between pins
◊
◊
Pin-to-pin shorts are sometimes difficult to detect. The
circuit appears to function properly if both gates are
providing the same logic.
If the outputs are in opposite states, the output usually
appears as either a float state or a logic low.
Logic Probe
TS 1.9
Question 1: Logic Probes
◊ The following circuit has an error condition.
◊ What is the error?
◊ What are the possible sources?
◊ Next steps?
1
1
0
Place probe tip on
the pin of the IC.
Careful not to short
the pins with the
probe tip.
1
TS 1.10
Question 2: Logic Probes
◊ The following circuit has an error condition.
◊ What is the error?
◊ What are the possible sources?
◊ Next steps?
0
1
1
1
TS 1.11
Question 3: Logic Probes
◊ The following circuit has a “float” condition.
◊ What does this mean?
◊ What are the possible sources of error?
◊ What is the next step?
1
Float
1
1
TS 1.12
Problems in Digital Circuits
◊ Digital circuits are often among the easier
circuits to troubleshoot; the output is either
right or wrong.
◊ The difficult problems are often those that
involve timing.
◊ High frequency operation of digital circuits also
brings electrical and timing effects that are
sometimes difficult to sense or anticipate.
TS 1.13
Power Supply Problems
◊ Digital circuits are sensitive to power supply
problems such as:
◊ Noise
◊ Voltage drops and surges
◊ Grounds
TS 1.14
Drops and Surges
◊ If the supply voltage has momentary voltage
drops or surges, this may have the effect of
faulty logic, clocking circuits or even resetting
the circuit.
◊ These drops and surges are often caused when
logic is switching (transient load).
◊ May be caused by other devices sharing the
supply.
TS 1.15
Power Supply Drops and Surges
◊ Recommended resolutions:
◊ Select a power supply with low internal resistance
◊ Configure voltage regulators with transient loads in
mind
◊ Select conductors with low resistance (shorten or
thicken to reduce R)
◊ Dedicate the power supply solely to the digital circuit
and not share it
◊ Filter the power supply conductors
◊ Utilize decoupling capacitors (see next slide)
TS 1.16
Decoupling Capacitors
0.01μF (blue)
◊ Rule of thumb:
◊ 0.01μF for each IC
◊ 0.1μF for every 5 ICs
0.1μF (Yellow)
TS 1.17
Other effects
◊ Many of the following design considerations will
be addressed in semester 2, including:
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Bus design considerations
Effective grounding
Interface and interconnection considerations
Noise
Electrical design requirements
Limitations in IC design
TS 1.18
END
©prgodin @ gmail.com
TS 1.19