Download Week 07 Lecture

Digital Troubleshooting Teaching Strategy
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TTL Characteristics.
TTL gates circuit.
7400, 74H00, 74L00, 74S00, 74LS00,
Comparison of TTL characteristics.
Tri state logic.
CMOS characteristics (How to handle CMOS)
Troubleshooting TTL gates circuit.
Ways an IC may fail
How to troubleshoot gates circuit.
More complex circuit decoding glitches, racing in FF etc.
System level troubleshooting (Reflection and Power Supply Glitches)
Troubleshooting Tips.
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Troubleshooting gate networks
Types of faults:
1. Open output in driving gate:Symptom:- a loss of signal to all gates, Bad floating level, Logic probe light will be “dim”.
2. Open input in a load gate:Loss of signal from the faulty gate but will not affect other gates connected to that
node. Use a pulser to pulse the node and check the pulse activity at the output of each
load gate. The faulty one will show no activity.
3. Shorted output in driving gate:This causes the node to be stuck in low state. With the power ON, use current Tracer to
observe current activity at the node. Since the short is internal to the chip, no current
activity will be seen at the node to anywhere. Now turn Off the power and pulse the
node with a pulser and follow the current path with a current tracer. The current tracer
will show a shorted path into the output of the faulty driving gate. No current activity
will be seen into the input of the load gate.
4. Shorted input in the load gate:If one of the inputs is shorted, the current tracer will show a current activity into the
shorted path of the faulty input.
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Prob 1:
For the gate network shown, the waveforms on the right are not correct for the gate input
(Observed on a logic analyser). Assuming that one of the gates is faulty (output stuck either at
high or low), determine the faulty gate and the failure node.
TROUBLESHOOTING A “STUCK NODE”
Consider the figure shown:
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Gate G1 is Driving the node and other gates are load gates. Possible failures are:
1. OPEN OUTPUT IN DRIVING GATE:- Loss of signal to all load gates.
2. OPEN INPUT IN A LOAD GATE: - Results in a loss of signal output from the faulty gate only.
3. SHORTED OUTPUT IN DRIVING GATE:-This failure can cause the node to be “stuck“in the low
state.
4. SHORTED INPUT IN A LOAD GATE: - This failure will also cause the node to be “stuck” in the
“Low State”.
1. OPEN OUTPUT IN DRIVING GATE:-
With circuit power “ON”, there will be no pulse activity on the node. DIM probe light indicates a “bad”
logic level.
2. OPEN INPUT IN A LOAD GATE:-
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Apply the logic pulser tip to the node. Monitor the output of each gate with the logic probe. Faulty gates
output will show no activity.
3. SHORTED OUTPUT IN A DRIVING GATE:This fault can cause the node to be stuck “Low”. You will need a current Tracer to locate the faulty gate.
First pulse one of the input of gate 1, with a logic pulser (with the other input tied High), and monitor its
output with a probe. The probe output lamp will be off, indicating a short.
There are three possibilities:a) Output of G1 shorted internally to ground.
b) Input of one of the gates tied to the node is internally shorted to ground.
c) There is an external short to ground somewhere along the line.
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Step 1: With Power off, Pulse the node with a pulser and monitor various inputs and the output of G1
with a current tracer. If no current activity is seen at point B but a large current activity is seen at the
output of G1, then G1 output is shorted internally to ground.
Step 2: If no current activity is observed at the output of G1, then apply the tracer tip at the input of G2,
G3 and G4 in that order. If G2 input is shorted, current tracer’s lamp will turn ON at that input but will
be Off at all other inputs. Current tracer’s lamp will turn ON only at that input which is shorted to
ground.
Step 3: If the short is external, follow the line with the current tracer. The lamp will be ON if the current
is flowing through the line. As shown in figure (d), there is a solder bridge between X and Y. As soon as
you pass point X, no current activity will be observed. Follow the current path and it will lead you to the
faulty gate or short.
WIRED AND PROBLEMS
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One of the most difficult problems in troubleshooting ICs of any types is a stuck WIRED AND
CIRCUIT. One of the open collector gates may still continue sinking current after the gate has
been turned Off. Refer to figure (f). Place the current tracer on the gate side of the pull up
resistor. Adjust the tracer’s sensitivity control until the indicator is just fully lighted. If the
indicator doesn’t light, use a logic pulser to excite the line. Place the tracer tip on the output of
each gate, only the faulty gate or pin will cause the indicator to light.
Testing for an external open signal path
Use a probe to locate a bad floating input. New follow the line back until the probe indicates a
good logic level.
VCC to ground short:
If there is a direct short between the VCC and ground, there will be a drop in power supply
voltage or a complete failure of the power supply. However locating the exact point of the short
can be quite difficult. To use the current tracer, disconnect the power supply from the VCC line,
and pulse the power supply terminal using the logic pulser with the supply return connected to
the ground lead of the pulser. Then follow the path of greatest current using the current tracer.
STUCK line caused by DEAD DRIVER
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Stuck 3 – state Data or Add bus
Use a current tracer.
a) If the current tracer indicates a high current at several outputs of a RAM or ROM, it is
most likely only one element is bad. Put CS of each RAM or ROM high one by one and
check with the current tracer which one conducts heavy current even with CS High.
b) If two RAM or ROM indicates high current activity, problem is the bus fight, i.e. both of
them are trying to drive the bus at the same time. This is caused by improper timing of
control signals to RAM (one RAM buffer is opened before the other one is closed).
c) If the C.T. indicates the absence of abnormally high current activity on all elements, yet
the problem persists. The problem is an element stuck in the high impedance state. This
can be found by placing a short to ground and using the C.T. to check for the element
which fails to show high activity.
TROUBLESHOOTING A “STUCK NODE”
Consider the figure shown:
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Gate G1 is Driving the node and other gates are load gates. Possible failures are:
5. OPEN OUTPUT IN DRIVING GATE:- Loss of signal to all load gates.
6. OPEN INPUT IN A LOAD GATE: - Results in a loss of signal output from the faulty gate only.
7. SHORTED OUTPUT IN DRIVING GATE:-This failure can cause the node to be “stuck“in the low
state.
8. SHORTED INPUT IN A LOAD GATE: - This failure will also cause the node to be “stuck” in the
“Low State”.
4. OPEN OUTPUT IN DRIVING GATE:-
With circuit power “ON”, there will be no pulse activity on the node. DIM probe light indicates a “bad”
logic level.
5. OPEN INPUT IN A LOAD GATE:-
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Apply the logic pulser tip to the node. Monitor the output of each gate with the logic probe. Faulty gates
output will show no activity.
6. SHORTED OUTPUT IN A DRIVING GATE:This fault can cause the node to be stuck “Low”. You will need a current Tracer to locate the faulty gate.
First pulse one of the input of gate 1, with a logic pulser (with the other input tied High), and monitor its
output with a probe. The probe output lamp will be off, indicating a short.
There are three possibilities:d) Output of G1 shorted internally to ground.
e) Input of one of the gates tied to the node is internally shorted to ground.
f)
There is an external short to ground somewhere along the line.
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Step 1: With Power off, Pulse the node with a pulser and monitor various inputs and the output of G1
with a current tracer. If no current activity is seen at point B but a large current activity is seen at the
output of G1, then G1 output is shorted internally to ground.
Step 2: If no current activity is observed at the output of G1, then apply the tracer tip at the input of G2,
G3 and G4 in that order. If G2 input is shorted, current tracer’s lamp will turn ON at that input but will
be Off at all other inputs. Current tracer’s lamp will turn ON only at that input which is shorted to
ground.
Step 3: If the short is external, follow the line with the current tracer. The lamp will be ON if the current
is flowing through the line. As shown in figure (d), there is a solder bridge between X and Y. As soon as
you pass point X, no current activity will be observed. Follow the current path and it will lead you to the
faulty gate or short.
WIRED AND PROBLEMS
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One of the most difficult problems in troubleshooting ICs of any types is a stuck WIRED AND
CIRCUIT. One of the open collector gates may still continue sinking current after the gate has
been turned Off. Refer to figure (f). Place the current tracer on the gate side of the pull up
resistor. Adjust the tracer’s sensitivity control until the indicator is just fully lighted. If the
indicator doesn’t light, use a logic pulser to excite the line. Place the tracer tip on the output of
each gate, only the faulty gate or pin will cause the indicator to light.
Testing for an external open signal path
Use a probe to locate a bad floating input. New follow the line back until the probe indicates a
good logic level.
VCC to ground short:
If there is a direct short between the VCC and ground, there will be a drop in power supply
voltage or a complete failure of the power supply. However locating the exact point of the short
can be quite difficult. To use the current tracer, disconnect the power supply from the VCC line,
and pulse the power supply terminal using the logic pulser with the supply return connected to
the ground lead of the pulser. Then follow the path of greatest current using the current tracer.
STUCK line caused by DEAD DRIVER
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Stuck 3 – state Data or Add bus
Use a current tracer.
d) If the current tracer indicates a high current at several outputs of a RAM or ROM, it is
most likely only one element is bad. Put CS of each RAM or ROM high one by one and
check with the current tracer which one conducts heavy current even with CS High.
e) If two RAM or ROM indicates high current activity, problem is the bus fight, i.e. both of
them are trying to drive the bus at the same time. This is caused by improper timing of
control signals to RAM (one RAM buffer is opened before the other one is closed).
f)
If the C.T. indicates the absence of abnormally high current activity on all elements, yet
the problem persists. The problem is an element stuck in the high impedance state. This
can be found by placing a short to ground and using the C.T. to check for the element
which fails to show high activity.
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