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Basic Digital Troubleshooting 1

©Paul GodinUpdated August 2013prgodin @ gmail.com

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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.

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Common Basic Faults

Internal errors:◊ Open circuit

◊ Floating output◊ Open input

◊ Short Circuit to:◊ Vcc◊ Ground◊ Another pin

◊ General malfunction◊ Logic errors◊ Mislabeled or wrong device◊ Defective device

External Errors◊ Open (no connection)

◊ Trace◊ Pad◊ Poor solder connection◊ Wire◊ Corrosion ◊ Other factors

◊ Short (to Vcc, GND or other conductor)◊ Solder bridge◊ Wire or other conductor◊ Other factors

◊ Other◊ Attached circuit fault◊ Power supply◊ Environmental◊ Design (example: loading)

A float is neither high (Vcc) nor low

(ground).

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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

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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.

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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.

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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.

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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

◊ Miswired (adjacent pin or wrong spec sheet)◊ Switch Configuration◊ Asynchronous inputs left floating◊ Function Generator not configured properly◊ Wrong IC

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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

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Question 1: Logic Probes

◊ The following circuit has an error condition. ◊ What is the error?◊ What are the possible sources?◊ Next steps?

01

1

1

Place probe tip on the pin of the IC.

Careful not to short the pins with the

probe tip.

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Question 2: Logic Probes

◊ The following circuit has an error condition. ◊ What is the error?◊ What are the possible sources?◊ Next steps?

10

1

1

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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?

Float1

1

1

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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.

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Power Supply Problems

◊ Digital circuits are sensitive to power supply problems such as:◊ Noise◊ Voltage drops and surges◊ Grounds

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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.

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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)

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Decoupling Capacitors

◊ Rule of thumb:◊ 0.01μF for each IC◊ 0.1μF for every 5 ICs

0.01μF (blue)

0.1μF (Yellow)

Other effects

◊ Many of the following design considerations will be addressed in semester 2, including:◊ Bus design considerations◊ Effective grounding◊ Interface and interconnection considerations◊ Noise◊ Electrical design requirements◊ Limitations in IC design

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END

©prgodin @ gmail.com