In this article, learn a few highlighted strategies that are particularly useful when inspecting digital circuits!
In a recent article, I discussed a few tips and tricks you can apply when troubleshooting your electronics projects. However, it focused on a general summary of the topic that can be applied to a wide variety of electronic projects.
Inspect the PCB and solder joints
If you designed a PCB for your project, make sure to inspect the pads, traces, and solder joints on the board.
Thin traces can be quickly destroyed, and it’s also fairly easy to lift a pad, especially when you aren’t practiced in soldering. Therefore, always verify that there aren’t any broken traces or lifted pads.
Furthermore, you should inspect the solder connections on the board. Make sure that all pins are connected, and there aren’t any bridges where they aren’t supposed to be.
Solder bridges create an electrical short capable of causing the entire design to malfunction. Image credit: Sparkfun.
You can use the continuity function of a multimeter to test for broken traces and shorts. Lifted pads and solder bridges can be found by visually inspecting the PCB.
Check the Specs
Like any other circuit, it’s important to take a look at the specifications of the components in your design. Double-check the datasheets of the parts because they contain valuable information, including timing descriptions, pin diagrams, maximum ratings, and much more.
Trace the Voltages Throughout Your Circuit
Once you visually inspected the board to make sure that all the traces and pads are intact, it’s time to use a voltmeter to check whether the components in your circuit are supplied with the right voltage.
Start by measuring the input voltage and then the supply voltage of each component. Make sure that each part is supplied with the correct voltage. If it isn’t, check whether the GND pin of that component is correctly connected with a continuity tester.
You can also try to measure the current the circuit draws. If the measured value greatly differs from what you expect, something is likely wrong. This could indicate that one or more components are broken, something has shorted out, or that you have one or more broken traces on your PCB.
A multimeter is a handy tool to have when troubleshooting various types of circuits.
However, some ICs also have a low-power or power-saving mode, which needs to be disabled if you want the IC to perform correctly.
Inspect the Outputs and Inputs
Next, look at what the components do in your design, and check whether it differs from what you expect them to do.
You can use various tools to inspect the I/O pins of digital ICs. The simplest tool is an LED (preferably one with a built-in resistor). An LED allows you to quickly determine whether a pin is HIGH or LOW. The same can be accomplished with a multimeter or a simple logic probe.
However, those are only viable options when you want to troubleshoot a slow synchronous or an asynchronous digital circuit. For any other type, I recommend using an oscilloscope to look at the signals over time.
Make sure to inspect every signal of the circuit. I prefer to start at the output of the circuit itself. If it's correct, then you’re done. Otherwise, you have to take a look at the input pins of the device that produces the output.
Check whether the input signals are what you expect them to be. If they are, then the device itself might be broken (or is being used incorrectly). If the inputs are incorrect, look at the next device in the chain (the one that produced the incorrect signal). Repeat these steps until you find the point of failure.
Note that short voltage spikes can mess up the functionality of sensitive ICs. If you suspect this is the case, incorporating smoothing capacitors into your design can help.
Rethink Your Design
If all the components in your design are working correctly and the PSU can supply enough power, then it might be time to rethink your design.
It’s not uncommon that you simply forgot or misunderstood something, especially when you’re working on a prototype. Make sure that you accurately do the math when you want to implement a logic function.
A Quick Review of Troubleshooting Steps
Start by visually inspecting your project. If everything looks right, check for continuity and shorts. After that, review the datasheets of each component in your design. Usually, the parts get tested at the factory, but if they get used incorrectly, it can lead to indeterministic behavior or damaged parts.
To find the point of failure, inspect the input and output signals of each component. Start with the output of the entire circuit and work your way to the first input.
If you can’t find anything suspicious, try to get back to the sketchbook and rethink your design. Make sure that you fully understand what each component does. You should also make sure that all timing and power constraints are met.
What Are Your Tips and Tricks?
I’ve covered some common troubleshooting tips and tricks, but there are certainly more.
Feel free to share your troubleshooting tips and tricks in the comments below! Some mistakes are common and you might be able to help other makers who struggle to find an error in their design.