Being able to track down a bug in a mountain of source code is a skill in its own right, and it’s a hard skill to learn from a book or online tutorial. Besides the trial-by-fire of learning while debugging your own project, the next best thing is to observe someone else’s process. [Uri Shaked] has given us a great opportunity to brush up on our debugging skills, as he demonstrates how to track down and squish a bug in the Wokwi Arduino simulator.

A user was kind enough to report the bug and include the offending Arduino sketch. [Uri]’s first step was to reduce the sketch to the smallest possible program that would still produce the bug.

Once a minimal program had been produced, it was time to check whether the problem was in one of the Arduino libraries or in the Wokwi simulator. [Uri] compiled the sketch, loaded it onto a ATtiny85, and compared the behavior of the simulator and the real thing. It turns out the code ran just fine on a physical ATtiny, so the problem must have been in the Arduino simulator itself.

To track down the bug in the simulator, [Uri] decided to break out the big gun—GDB. What follows is an excellent demonstration of how to use GDB to isolate a problem by examining the source code and using breakpoints and print statements. In the end, [Uri] managed to isolate the problem to a mis-placed bit in the simulation of the timer/counter interrupt flag register.

If you’d like to see more of [Uri]’s debugging prowess, check out his dive into an ATtiny’s write protection and configuration fuses. If you’ve been wowed by the power of GDB and want to learn more, check out this quick tutorial!

[Lee] wrote in to tell us about a Set Top Box he hacked. Before the cable industry lawyers get out their flaming swords… he’s not stealing cable, or really doing much of anything. This is a hack just for the adventure and thrill of making someone else’s hardware design do your bidding without any kind of instructions.

He posted about the adventure in two parts. The first is finding the JTAG header and identifying the pins. Arduino to the rescue! No really, and this is the type of Arduino use we love. Using a package called JTAGenum the board becomes a quick tool for probing and identifying JTAG connections.

The image above shows a different piece of hardware. From looking at it we’re pretty sure this is a Bus Blaster which is specifically designed for JTAG debugging with ARM processors. This is the beginning of the second part of his documentation which involves code dumping and stepping through lines code (or instructions) using OpenOCD and GDB. It’s a chore to follow all that [Lee] discovered just to write his name to the display of the box. But we certainly found it interesting. The display has a convoluted addressing scheme. We assume that there are cascading shift registers driving the segments and that’s why it behaves the way it does. Take a look for yourself and let us know what you think in the comments.