Author Topic: suggestion: digital trouble shooting  (Read 1965 times)

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Offline PA4TIM

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suggestion: digital trouble shooting
« on: December 15, 2016, 11:32:08 pm »
Not how to debug your software or testing a design, but trouble shooting hardware to repair mixed digital electronics/circuits.

Modern electronics like for instance a scoop or DMM have a mix of analog and digital circuits working together. 

To learn analog electronics you can find tons of info, made since the early 30's upto now. Most about designing the stuff. But also on trouble shooting.

For digital stuff there is tons of info about writing the firmware upto how to use a FPGA to blink a led. How to use logic IC's and how to connect digital to the analog world and vise versa.

But after the design is bug free and running smooth it goes in production and people start to use it. And service manuals and schematics become rare.

There comes a time it stops working. And then you have to trouble shoot a circuit that is often analog and digital. The difference with preproduction trouble shooting is the fact that this circuit can and did work.

But there is not a lot of info about trouble shooting the digital part.

For instance is a logic analysers of any use if you do not know the software ?
How to use your  scope, for instance single shot, roll mode, triggering ?

For instance in a Fluke scopemeter you have a mix of ASICs, a processor, display, keyboard, Vref, analog circuits, ADC's, analog switches, some logic IC's etc.  All working together and smd (that makes it harder to swap parts as a test and even probing can be a challenge.   

Where to start (after you checked the power rails)

Are there "tricks" or rule of tumbs? Like in analog you have a diode drop between base and emitter, or measure voltage over a resistor to see if there flows current in analog circuits. Some people use a component tester/octopus to test gates (looking for a chair shape)

Also things like voltage levels, runts, clock's, enable pins. Digital is in theory only 0 or 5V (as an example) but I often see a mix of amplitudes. 2V on a hex inverter input  and 5V out. Is de gate dead or is the 2V to high or is it perfectly fine because 2V is seen as low for the IC you are testing.

One thing I find often a challenge is to find out if a voltage on an input or output pin is maybe to low or to high or distorted or OK and for most, is it wrong because the pin (and so the IC) or part of an IC you measure is faulty or is the circuit connected to that pin the problem.

An extra problem is that an IC can work for several functions or just for one or even sometimes for one function. So it is possible to see no activity because the IC is dead, or because it is not active for the state/function the DUT is in.
Ghost  signals due to high impedance stated.
Or Is that peak/spike real of just a measurement fault, and if real what is causing it.
The temp of the IC
How to use test circuits from the datasheet.
A perfect working uP, at least it looks that way but with a dead in or outputs or 1 function missing/failing.

I do a lot of this stuff and I use some Technics I learned over the years, most by trial and error. Looking at youtube most trouble shooting is about changing caps or boards in TV's. And almost always about a repair an sich.

It would be nice as a series of fundamental trouble shooting Technics.




 
www.pa4tim.nl my collection measurement gear and experiments Also lots of info about network analyse
www.schneiderelectronicsrepair.nl  repair of test and calibration equipment
http://www.youtube.com/user/pa4tim my youtube channel
 


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