When You Need a Scope, You Need a Scope

Sometimes there’s just no substitute for the right diagnostic tool. [Ankit] was trying to port some I2C code from an Arduino platform to an ARM chip. When the latter code wasn’t working, he got clever and wrote a small sketch for the Arduino which would echo each byte that came across I2C out to the serial line. The bytes all looked right, yet the OLED still wasn’t working.

Time to bring out the right tool for the job: a logic analyzer or oscilloscope. Once he did that, the problem was obvious (see banner image — Arduino on top, ARM on bottom): he misunderstood what the ARM code was doing and was accidentally sending an I2C stop/start signal between two bytes. With that figured, he was on the right track in no time.

We just ran an epic post on troubleshooting I2C, and we’ll absolutely attest to the utility of having a scope or logic analyzer on hand when debugging communications. If you suspect that the bits aren’t going where they’re supposed to, there’s one way to find out. It’s conceivable that [Ankit] could have dug his way through the AVR’s hardware I2C peripheral documentation and managed to find the status codes that would have also given him the same insight, but it’s often the case that putting a scope on it is the quick and easy way out.

What Could Go Wrong? I2C Edition

I should really like I2C more than I do. In principle, it’s a brilliant protocol, and in comparison to asynchronous serial and SPI, it’s very well defined and clearly standardized. On paper, up to 127 devices can be connected together using just two wires (and ground). There’s an allowance for multiple clock-masters on the same bus, and a way for slaves to signal that the master to wait. It sounds perfect.

In reality, the tradeoff for using only two wires is a significantly complicated signalling and addressing system that brings both pitfalls and opportunities for debugging. Although I2C does reduce the number of signal wires you need, it gets dangerous when you have more than a handful of devices on the same pair of wires, and you’re lucky when they all conform to the same standard. I’ve never seen twenty devices on a bus, much less 127.

But still, I2C has its place. I2C was designed to connect up a bunch of slower, cheaper devices without using a lot of copper real estate compared to its closest rival protocol: SPI. If you need to connect a few cheap temperature sensors to a microcontroller (and their bus addresses don’t clash) I2C is a great choice. So here’s a guide to making it work when it’s not working.

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What Could Go Wrong: SPI

Serial Peripheral Interface (SPI) is not really a protocol, but more of a general idea. It’s the bare-minimum way to transfer a lot of data between two chips as quickly as possible, and for that reason alone, it’s one of my favorites. But that doesn’t mean that everything is hugs and daffodils. Even despite SPI’s simplicity, there are still a few ways that things can go wrong.

In the previous article in this series, inspired by actual reader questions, I looked into troubleshooting asynchronous serial connections. Now that you’ve got that working, it’s time to step up to debugging your SPI bus! After a brief overview of the system, we’ll get into how to diagnose SPI, and how to fix it.

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Don’t Tempt the Demo Gods

Including a live technical demonstration as part of a presentation is a lot like walking a tightrope without a net. Which isn’t to say that we don’t do it — we just keep our fingers crossed and bring our lucky horseshoe. The demo gods have smote [Quinn] a mighty blow, in front of a class at Stanford, no less.


[Quinn]’s scratch-built computer, Veronica, failed to boot in front of a hall of eager students. When the pressure was off, in the comfort of her own lab, [Quinn] got to debugging. You should read her blog post if you’re at all interested in retrocomputing or troubleshooting of low-level hardware bugs. But if you just can’t spare the five minutes for a pleasant read, here’s a spoiler: watch out for flaky card-edge connectors. All’s well that ends well, with a game of pong.

We’ve been following Veronica from her very first clock cycles, so we’re happy to see her back on her feet again. Good job, [Quinn]!

Squash Your ESP-8266 Bugs with ESP-gdbstub

We hope we’re not insulting you by suggesting this, but it’s possible that even the best among us may be faced with bugs in our embedded code from time to time. And while we’re great fans of printf debugging over the serial port, and its high-speed equivalent — flipping a GPIO pin — there’s a time when your bug is so deep that having a real debugger is the best way to dig it out.

[slaff] has been doing some great work documenting C/C++ programming on the ESP-8266, mostly using Eclipse and some of the Arduino libraries. In the fourth part of his series of posts, he walks through using a couple debugger options for the ESP. What makes this all work is the ESP-gdbstub code from Espressif themselves. gdbstub looks great — it works both with the standard SDK as well as with FreeRTOS, so you can debug your ESP-8266 code whether it’s in an OS or on the bare metal. And all this just using the standard serial connection that’s used for programming.

Now, this still may not help with timing-related bugs. ESP-gdbstub uses the serial port, after all. But having the ability to set breakpoints and interactively inspect what’s going on in the chip’s memory is priceless, and doing so with no extra hardware connections is brilliant.

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uController Code Profiler Debugs Your Microcontroller

When working on digital circuits that operate at high frequencies it helps to have some special tools on hand. Things like oscilloscopes and logic analyzers are priceless when something isn’t working right. Another great tool would be this hardware-based profiler that [Mike] came up with while he was working on another project.

The profiler connects to USB and shows up as a serial port. Normally [Mike] used a set of LEDs to get information about how his microcontrollers work, but for this project that wasn’t enough. The uController Code Profiler can provide the main loop running time, time functions and sections of code, keep track of variables, and a few other tasks as well, all with nanosecond resolution.

The source code isn’t provided but a hex file is available, along with a schematic and an include file, if you want to try this one out on your next project. Like this homemade logic analyzer, this could be a powerful tool in your microcontroller arsenal. Simply include the file with various pieces of your code to get it up and running!

Execution Tracing on Cortex-M3 Microcontrollers

The higher-power ARM micros have a bunch of debugging tools for program and data tracing, as you would expect. This feature – CoreSight Trace Macrocells – is also found in the lowly ARM Cortex M3 microcontroller. The Cortex M3 is finding its way into a lot of projects, and [Petteri] wondered why these debugging tools weren’t seen often enough. Was it a question of a lack of tools, or a lack of documentation? It doesn’t really matter now, as he figured out how to do it with a cheap logic analyzer and some decoders for the trace signals.

There are two trace blocks in most of the Cortex M3 chips: the ITM and ETM. The Instrumentation Trace Macrocell is the higher level of the two, tracing watchpoints, and interrupts. The Embedded Trace Macrocell shows every single instruction executed in the CPU.  Both of these can be read with a cheap FX2-based logic analyzer that can be found through the usual outlets for about $10. The problem then becomes software, for which [Petteri] wrote a few decoders.

To demonstrate the debugging capability, [Petteri] tracked down a bug in his CNC controller of choice, the Smoothieboard. Every once in a great while, the machine would miss a step. With the help of the trace tool and by underclocking the micro, [Petteri] found the bug in the form of a rounding error of the extruder. Now that he knows what the bug is, he can figure out a way to fix it. He hasn’t figured that out yet. Still, knowing what to fix is invaluable and something that couldn’t be found with the normal set of tools.