troubleshooting digital circuits

[Audioguru]

Simply look at the datasheets for the type of logic chip:
1) A TTL input floats high.
2) A Cmos input has no current so it could float high or low or in between.

 

[walters]

2) A Cmos input has no current so it could float high or low or in between

Why doesn't a CMOS input have no current?

How can you tell when the CMOS is in between STATE?

 

[Audioguru]

A Cmos logic IC is made with Mosfets that have no input current.

A Cmos logic circuit has a lot of voltage gain so if its input floats then its output will be at 0V or at the supply voltage or the circuit will oscillate.

 

[dmk]

Here is the link to Digital Desing Principles and Practicies:


Added after 6 minutes:

And here is the link to Digital Electronics Demystified:

In EDAboards you can find the books you need going to:
and make a search for the name you are looking for.

 

[soptea]

After more than 20 years of troubleshooting analog circuits (mostly CRT tv ) now I have to deal with digital circuits ,which seem to be a dificult challenge .
Troubleshooting analog circuits is a step by step pleasant journey ,you start from the effect and advance to the root cause of the problem .
But for digital circuits I can t figure out what could be wrong ,the symptoms aren t giving me any clue , anything seem to be related to everything.
Last few days I enjoy reading stories from techs repairing arcade games like this
https://wiki.arcadeotaku.com/w/Game_Board_Repair_Guide and came up to a littlle strategy when dealing with digital circuits :
-check supplies ,better with scope
-check reset ,clock address and data buses
-faults to looking for :shorted inputs/outputs (to ground/Vcc or other pins) ,Ic's seems to be from far the often faulty components
software (ROM /FLASH content )
next as often are connectors ,shorted / open traces , soldering
discrete seem to be last to be checked (caps,resistors ,tranzistors ,diodes,coils)
As for tools ,logic probe seems to be obsolete for newer IC's working at low voltage like 1.2 or 3.3 volts
Any idea / tip /link apreciated

 

[Dan Mills]

A JTAG boundary scan tool is the way to go on digital boards that have the appropriate connections.

However, 90% of the time Power supplies, Connectors and IO buffers will cover the common cases (Just shotgun the buffers, no point in wasting time on them if that looks like the problem).

My general view is that it it is not one of those things and if a can of freezer spray does not find the dud part, then a board swap will usually be more cost effective then going on a hunt for a badly soldered QFN or BGA.

Regards, Dan.