Electrical Characteristics of ICs Technician Series.

Electrical Characteristics of ICs

Technician Series

Electrical Performance

◊ Every technician facing a faulty device hopes that the bad component has completely failed, making it easier to find and identify.

◊ Sometimes the component is functional but the circuit to which it is attached has failed or, the component has only partially failed. These types of faults are less obvious and more difficult to isolate. Electrical and signal parameters must be analyzed to determine these faults.

◊ This presentation will describe the basic electrical and signal parameters of logic ICs.

IC Logic Families

◊ We have seen in a previous presentation that logic ICs are either TTL (transistor) or CMOS (MOSFET) based.

◊ These are expanded further into numerous families of devices that each have their own electrical and performance characteristics.

Specification Sheets

◊ IC specification sheets from all manufacturers are generally similar in layout and appearance.

◊ Additionally, for many devices, the different manufacturers share:◊ base model numbers◊ pin layouts◊ similar electrical characteristics for similar families

Important Specifications

◊ Electrical:◊ Output voltages (VO) and current (IO)

◊ Input voltages (VI) and current (VO)

◊ Power (wattage)◊ Timing

◊ Physical:◊ package type and size◊ pin layout

◊ Other:◊ Temperature ratings

◊ Devices may function on different voltage values:◊ Most new logic designs use 3.3 Volt◊ More advanced designs are looking at lower voltages◊ Communication systems may use higher voltages, and

may even use negative voltages

◊ Voltage values define the logic input or output state of the device.

Voltage

Voltage

Voltage Output/Input

VOH

VOL

VIH

VIL

Undefined

VCC

Ground

Undefined

Output Input

MinimumMinimum

MaximumMaximum

Gate inputs that receive voltage levels within the undefined zone are unable to reliably determine the logic level.

Gate inputs or outputs that do not have a proper voltage interface indicates a defective device input, output or circuit.

Elec3.7

Voltage Measurement

◊ The logic voltage output of a (driving) gate must be interpreted properly by the receiving (loading) gate.

◊ Use a good logic probe or a voltmeter to locate this problem.

Elec3.8

Vcc 1 Vcc 2

Volts

Faulty Condition

◊ Example of incompatible voltages

CMOSTTL

Output Input

Minimum=3.5V

Minimum=2.7V

Maximum=1.5VMaximum=0.5V

Voltage Problem: External to the IC

◊ If the voltage measures lower than it should on a logic high output there may be a problem outside of the IC

Vcc 1Vcc 2

Volts

Too much

CurrentShorted

Capacitor

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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.

Solder Bridge

http://www.npiengineer.com/

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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 can be frustrating at times, but some simple approaches can improve success.

◊ A circuit or logic diagram is vital for troubleshooting.

NASA

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Troubleshooting Process part 1

◊ Check for the obvious, common, basic problem. Example: check the fuse, power switch, plug, connectors, 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 trace.

◊ 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. Think what may be the cause of the poor values and where the fault may originate.

◊ Visually inspect the components and the wiring. Look for clues such as discoloration. Use your nose.

◊ Substitute a suspected faulty device.

TS 1.18http://www.eevblog.com http://www.engadget.com

http://www.lovekin.net

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http://en.wikipedia.orghttp://hackaday.iohttp://www.element14.comhttp://caltexsci.com

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http://www.storagereview.com

http://www.cbldatarecovery.com

Troubleshooting “Tricks”

◊ Many manufacturers produce a service manual with test conditions, and several “TP” (Test Points) on the board.

◊ Use cold spray on the circuit to create a little frost. The melting frost indicates where the current is flowing.

◊ Use a working unit to compare voltages and signals at various points.

◊ The power supply is the most likely to fail. Check it first.

◊ Use the internet to search for model numbers and look for trends.

◊ Check solder joints for breaks, cold-solder joint. Retouch solder points with an iron to reflow the solder.

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Cautions When Troubleshooting

◊ Be extremely cautious when attaching an external power supply. Reverse voltage or too high a voltage will instantly kill components.

◊ Never use the “Ohms” setting on a meter on a digital board. The voltages produced by the meter may damage components. Never probe for Amps as this requires a series connection. Only use Volts settings.

◊ Be aware of grounding. Ground yourself due to static. Careful when probing components.

◊ Do not drop anything on the circuit board.

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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

Bad Design

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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.

◊ Capacitors are often to blame for timing or noise problems.

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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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Decoupling Capacitors

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

◊ Place a similar cap in parallel to a suspected bad cap

0.01μF (blue)0.1μF (Yellow)

Design and Build Considerations

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Basic Principle 1

◊ Never allow a gate input to float.◊ A floating input creates a situation where a logic gate can:

◊ Not perform as expected◊ Be damaged by static discharge◊ Operate erratically (often difficult to troubleshoot)◊ Oscillate (likely to cause problems to other parts of the

circuit)

◊ Even unused gates in an IC should have their inputs tied to logic high or low.

◊ DO NOT tie outputs to Vcc or Ground! It’s the inputs that need to be addressed.

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Float0

Basic Principle 1

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Vcc

1

Animated

01

Poor Switch Config

Vcc

Good Switch Config

Basic Principle 2

◊ Over-voltage, wrong polarity and negative voltage damages ICs.◊ Ensure voltages are set and checked before connecting to the

ICs.◊ Mixing Vcc and GND is usually fatal for the device

◊ In some cases the ICs may continue to function but their

performance and lifespan will be unsure.

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Vcc

+

-X

Basic Principle 3

◊ Never tie active outputs together (called a “wired-OR”), or to Vcc, or to GND.◊ This will likely damage certain families and types of devices.

◊ Leave unused outputs disconnected◊ Use the appropriate logic gate to connect outputs together

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Vcc

Examples of Wired-OR

X

(unless open collector with ext.

resistor)

X

Basic Principle 4

◊ Do not use resistors for Vcc input.◊ The resistor will drop voltage and the device will not receive

adequate voltage.◊ The connections for Vcc and Ground must have low resistance

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Vcc

Examples of an error with Vcc

X

Basic Principle 5

◊ When electrically connecting separate circuits, ensure there is a common reference (ground in most cases)◊ Without a common reference the communication link

between the two separate circuits will be unreliable.

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Vcc 1 Vcc 2

Same reference

Basic Principle 6

◊ Do not power an IC with the output of another IC.◊ The IC’s gate cannot supply enough current or voltage to the load.

◊ Do not use the power input of an IC as an enable/disable.◊ Use steering gates to enable or disable a device’s output.

Elec 1.39Examples of errors by powering a chip

X

Vcc Vcc

VccX

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END©prgodin @ gmail.com

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