Digital circuit inputs turn on thresholds, turn on voltage?

[Billy Mayo]

How would a tech troubleshoot unexpected state transitions, or the failure to perform state transitions as expected? because of the input logic threshold voltage level?

Why would a troubleshooting tech need to know about each Logic IC chips, Logic Threshold Voltage Levels?

What kind of troubleshooting would a test technician be doing to have to know the logic threshold voltage levels?

When are they used for what use?


Here is a Chart of them:
http://www.interfacebus.com/voltage_threshold.html

 

[crutschow]

Those are not so much for troubleshooting as they are to determine how to interface different logic types together.

If there are problems in a digital circuit it's usually due to noise or logic or race problems, not that a logic level is incorrect. Logic circuits typically either work or they don't. Incorrect logic levels of properly designed digital circuits are rare.

 

[Billy Mayo]

a digital circuit it's usually due to noise or logic or race problems

How would you know if the logic states had noise?

What is logic race problem?

The Logic IC chips can have the same part number but they have different input logic threshold voltages, this causes unexpected state transitions or failure to perform a state transitions

But how does a troubleshooting tech find which IC chip is doing this or has this kind of problem?

 

[crutschow]

Noise can usually be seen with an oscilloscope.

A logic race problem results from a incorrect logic design which may allow a signal to arrive at a gate sooner than it should.

Different logic threshold levels are usually not a problem since normal logic output levels always comfortably exceed the threshold requirements for a logic 0 and a logic 1 input.

Finding a bad chip involves a certain amount of detective work. You need to know what the signals are supposed to be based upon the circuit design and then determine which signal(s) are different from that. It can be a difficult and time-consuming process.

 

[JimB]

How would a tech troubleshoot unexpected state transitions, or the failure to perform state transitions as expected?

The tone of your question infers that the technician is faultfinding on something which has worked satisfactorily in the past and has now failed.

With that in mind it is not necessary to have all the threshold levels in your head while chasing the signal through the circuit with an oscilloscope.

As a gross simplification, if the signal is is within 0.5volt of either power rail it is OK. Most of the time it will be.

If the signal voltage is somewhere near the middle, investigate more closely, something may be holding it up or down.

And before getting carried away looking at the signal levels, make sure that the supply is with in 5% of its nominal value and free from ripple.

JimB

 

[KeepItSimpleStupid]

With LabVIEW, you can have "Software Race Conditions" because the language allows writing parallel processes very easy.

While probably not a "Race condition", substituting a RAM with a higher access time might be an example. The set-up time isn;t long enough for the slower RAM chip to function.

 

[kubeek]

In a very high speed circuit the actual level can matter. I saw recently in a datasheet of an ethernet switch chip an example in which a clock signal with mismatched impedance. On an oscillosope they showed a screen where the singal had a small dip in the middle of the rising edge of the clock close to the threshold level, and this was said to be capable of double clocking the chip and resulting in incorrect operation.

 

[Billy Mayo]

Different logic threshold levels are usually not a problem since normal logic output levels always comfortably exceed the threshold requirements for a logic 0 and a logic 1 input.

This problem is that the outputs and the logic high state is +5 volts, so how can i get the voltage lower so i can test the IC logic states threshold voltage "in circuit" ? Because +5 volts is to high to test the inputs threshold voltage. The list shows the input voltages to be very low and if the outputs are 5 volts high i can't test if the inputs threshold is in spec and working.

Example:

Good Logic IC output is +5 volts going to a BAD input threshold voltage IC chip which will cause the output to be On all the time

To test the Bad IC chips input threshold voltage , I need to lower the output of the IC chip before the Bad IC inputs
To test at which voltage threshold it turns ON the BAD logic IC chip

 

[Billy Mayo]

I'm trying to test and check the input threshold voltage level for each IC chip "in circuit" but how can I do this please?

I need to find out and write down at what voltage level the IC chip turn on or switches the outputs state

 

[crutschow]

I'm trying to test and check the input threshold voltage level for each IC chip "in circuit" but how can I do this please?

I need to find out and write down at what voltage level the IC chip turn on or switches the outputs state

For that you have to disconnect whatever is driving the input and use a variable power supply voltage to drive the input and determine the threshold.

 

[Billy Mayo]

you have to disconnect whatever is driving the input and use a variable power supply voltage to drive the input and determine the threshold.

Yes, that's what I have been doing, or use multiple variable power supply's or wall warts to test multiple input NOR gates, AND gates, etc multiplexers

I use "logic probes pulser mode" also to inject a signal on the inputs of the logic IC chips to make sure I get a output state to check if the IC chip is good or bad

I wasn't sure If I the only tech doing something like this

But when check the input threshold voltage level , you can't use the Logic Probe pulser mode, because the pulsers square waveform's HIGH state is 2.3 volts to +5 volts. So you don't know when the input threshold voltage level is at.

I just use a variable wall wart and use an ez hook and probe it to the input pin of a good or bad IC chip to make sure it has the correct output state and input threshold level

How do you use a logic probe? or the logic probes pulser mode?

The Logic Probes pulser is 10us pulse wdith, frequency is 500hz , at +5 volts HIGH and Low is zero volts, its a square waveform

 

[kubeek]

I somehow fail to understand why do you think that the threshold level is so important as to be tested for every IC in the circuit? Normally when you troubleshoot something you try to narrow down the part of the circuit responsible, and then change anything suspect until you find the fault or give up.

 

[Billy Mayo]

Well what causes unexpected state transitions? beside noise

What caues failure to perform state transitions as expected?

 

[kubeek]

Depends on the chip in mind, but most of the time it is ESD or plain short/overvoltage that kills transistors inside ICs. Another factor could be ageing, but I would consider that far less common.

 

[Billy Mayo]

Yes I have seen over voltage kill the inputs of op-amps, which changes the "Circuit impedance" on the resistors on the inputs of the fried op amp inputs. A lot of techs I worked with thought the resistors were bad and would change a bunch, but it was the op amps inputs that were fried inside changing the circuits impedance. Do you know what i mean? not sure why that happens tho

Over voltage with Logic Gates, mostly the pull up or pull down resistors get shorted or open

If a pull down or pull up resistor , shorts or opens , what happens?
1.) if a pull down resistor shorts it shorts the input or output to ground, which can change a high state to a low state
2.) If a pull down resistor OPENS , it causes a floating input or output, but what else does a pull down resistor that is OPEN do?
3.) If a pull up resistor is shorted, it shorts the input or output to the + supply voltage
4.) If a pull up resistor is OPEN, what happens? beside that the input or output is floating or has a high impedance state

 

[crutschow]

What pull up and pull down resistors?

 

[Billy Mayo]

in logic circuits, they are common on inputs and outputs of logic IC chips

I'm talking in general

 

[kubeek]

in logic circuits, they are common on inputs and outputs of logic IC chips

No, not really. Pull up/down resistors are common on the outer interfaces, such as connectors. However usually the bulk of I/Os is used to interconnect two logical devices and unless those are open collector buses you won´t find pullups very often.

Anyway, realize that pullups have quite large values, say 4k7. If a 0805 4k7 resistor is rated 1/8W, than you need 25V at the input to get it to rated wattage and 35V to get double that. The resistor may or may not be fried, but the IC it is connected to and probably other stuff as well will be dead for sure. So if a pullup opens due to overcurrent, you have lot more trouble in the circuit then just an open pullup.

 

[Billy Mayo]

Why would a designer choose pull down resistors instead of pull up resistors or vise versa?
Pull down sinks current, Pull up sources current?

 

[kubeek]

The main thing that makes the decision is what logic level you want to have at that input.
For example an unused SET input on a D-latch asks for a pulldown, because you want this input it to be inactive. Another example is when a microcontroller drives a high side p-mos transistor - when the micro starts the output will most likely be an input in default state until the program starts, so the output might turn on due to leakage and interference. You would use a pullup on that to keep the mosfet off until you want to turn it on. For a low side n-mos you´d use a pulldown obviously.