Transcript PPT: Circuits 1

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
Voltage
◊ 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 Output/Input
VCC
VOH
VIH
Minimum
Minimum
Undefined
Undefined
Maximum
VOL
VIL
Maximum
Output
Input
Ground
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.
Volts
Vcc 1
Vcc 2
Elec3.8
Faulty Condition
◊ Example of incompatible voltages
Minimum=3.5V
Minimum=2.7V
CMOS
TTL
Maximum=1.5V
Maximum=0.5V
Output
Input
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
Volts
Vcc 2
Vcc 1
Too
much
Current
Shorted
Capacitor
Basic Digital Troubleshooting 1
©Paul Godin
Updated August 2013
prgodin @ 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
◊
◊
◊
Vcc
Ground
Another pin
◊
◊
◊
Logic errors
Mislabeled or wrong device
Defective device
◊
Short Circuit to:
◊
General malfunction
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
◊
◊
◊
◊
Attached circuit fault
Power supply
Environmental
Design (example: loading)
Other
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.
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http://www.eevblog.com
http://www.lovekin.net
http://www.engadget.com
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http://hackaday.io
http://caltexsci.com
http://www.element14.com
http://en.wikipedia.org
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http://www.cbldatarecovery.com
http://www.storagereview.com
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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
◊
◊
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◊
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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?
1
1
0
Place probe tip on
the pin of the IC.
Careful not to short
the pins with the
probe tip.
1
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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?
0
1
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?
1
Float
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:
0.1μF (Yellow)
0.01μF (blue)
◊ 0.01μF for each IC
◊ 0.1μF for every 5 ICs
◊ Place a similar cap in
parallel to a
suspected bad cap
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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.
Elec 1.33
Basic Principle 1
Vcc
Vcc
Float
1
0
0
1
Poor Switch Config
Good Switch Config
Elec 1.34
Animated
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.
+
X
Vcc
Elec 1.35
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
Vcc
X
X
(unless open
collector with
ext. resistor)
Examples of Wired-OR
Elec 1.36
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
Vcc
X
Examples of an error with Vcc
Elec 1.37
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.
Vcc 1
Vcc 2
Same reference
Elec 1.38
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.
Vcc
Vcc
X
X
Vcc
Examples of errors by powering a chip
Elec 1.39
https://breakrulesnotnails.wordpress.com
END
©prgodin @ gmail.com
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