Just received the first two pre-production prototypes of the Wireless Debug Probe (WDBP) from MacroFab. After initial inspection and power-up I am so far very pleased with the quality of the work done. There are a few firmware issues to be addressed and I will be publishing the results of the full testing and firmware updates in a future blog post.

Sid.

For several decades I have designed, built, and tested my own PCBs for several products/projects. During all of that time I made use of traditional “through-hole” mounting components. Beyond the schematic capture and PCB design software no special tools were required for this process other than a good soldering iron and some good quality wire snips to trim the component leads after soldering.

In recent years it has become increasingly difficult to source through-hole mounted components, particularly for the modern microcontrollers that offer the kinds of features new designs require. The time was right to transition to using Surface Mount Technology(SMT) components.

A while ago I began a new project, the design of a Wireless Debug Probe (WDBP) to connect a computer based debugger to and embedded system using either JTAG or SWD. This design, that will be open-sourced upon release, is based up the Blacksphere “Black Magic Probe” (BMP), an excellent open source debugging probe. The BMP itself is very, very small, using some of the smallest available SMT components.

The BMP is about 15mm x 35mm … REALLY small.

Since the WDBP is my first project using SMT components I decided not to use the smallest components so the PCB size is about 35mm x 90mm. While using larger components contributes to the size of WDBP, an additional factor was the Wi-Fi module with integrated antenna. Plus, I chose to add a footprint for the Tag-Connect TC-2050-IDC “bed-of-nails” connector for attaching my debug probe to the WDBP for debugging the firmware:

Here is a prototype of WDBP:

The WDBP prototypes have been built by a contact of mine who has been working with SMT components for some long time so I avoided the need to purchase any specialized assembly tools. However, during the testing there were a few issues that needed addressing, it was at this time that decided to investigate the minimum set of tools that would enable me to rework the prototypes.

The first issue when working with such small components is being able to visually inspect soldered joints. I tried a head-mounted magnifier, however the magnification was just not good enough to efficiently find any soldering issues. This led me to research and purchase of a microscope. The one I chose is from AmScope, model number SW-3T24Z, their Trinocular Stereo Microscope. I chose to get a trinocular ‘scope so that at some point in the future I could add a camera to the setup. Also, after reading the reviews of this microscope being used for SMT I purchased an additional lens to ensure the distance between the lens and the item under inspection was as great as possible to allow tools to be used on the PCB.

Apart from some small tools like anti-static tweezers the remaining tool investment was a hot-air rework station. This station permits a PCB to be held securely while hot air flows over it, allowing components to be removed, or simply reflowed to fix poor solder joints. Again after some extensive Internet searches I chose the Aoyue 866 rework station. This station comes with not only the base rework unit, but also a selection of nozzles for the hot air gun and a temperature controlled soldering iron. It is a good starter kit for anyone setting out on the SMT journey.

So far, the above tools have worked out really well in testing my prototype PCBS. The next phase of the WDBP project is the pre-production prototypes arriving in about 10 days. Having invested in this tool-set I feel confident that any minor issues with the PCBS can be addressed.

Sid Price

In my previous post I wrote about interrupts and atomic operations in embedded systems. After getting bitten myself with just such an issue I thought I would write about the details of what I just experienced on my current project.

First, the background of the issue; the device I am working on connects to a wireless access point to provide an interface to some PC-based software. The issue was that when the PC software started up sometimes the device would appear to drop messages or stop responding to messages. The issue was hard to track down because if the device was run under debugger control the messages appeared to be received just fine. This led me to suspect the code’s logic was being corrupted by interrupts disrupting the management of messages out of the receive buffers.

When a message is received from the network it is placed into a circular buffer, the index of the next free location is updated, and the total number of characters in the buffer is updated. This all happens in the response to an interrupt from the wireless module. Since there was no related interrupt that could be disrupting this I turned my attention to the background task that unpacks the data in the circular buffer into packets for processing. However, what I saw there was my buffer manipulation being protected by the wireless module interrupt being turned off, just as I wrote about previously.

This deepened the mystery since it appeared I was following good practice and keeping the buffer management code atomic, at least to the extent of protecting it to further interrupts from the wireless module, the only other code that manipulated the buffer variables.

The next step was to instrument the two methods that enabled and disable the wireless module interrupt. I did this to make sure that each disable call was matched by an enable call. The results of this test revealed that there were more enable operations than disable operations. After some thinking I realized that the enable and disable methods themselves were not atomic, therefore it was possible for an interrupt to arrive during the execution of the disable call, but before the interrupt had been disabled. What was needed was to track the enable and disable calls and only re-enable them when all callers had requested the re-enable.

To do this, a counter was incremented when interrupt enable was requested and decremented when disable was requested. However, the disable action would only be taken if the counter decremented to zero.

This update appears to have resolved my lost message and non-received message issues.

So, if your system has what appear to be random failures, closely examine the data flow through it and the effects that interrupts may have on that flow. Plus, double check that the code being used to manage the interrupts themselves is clean and does not make assumptions like mine did. Just because a caller requests an interrupt be enabled the code needs to be ensure the action is only taken when each caller that requested a disable has also requested an enable.

I have been trying to track down a particularly tricky problem on my Wireless Debug Probe over the last few days. When embedded systems begin to perform in strange, apparently illogical, ways the culprit is usually related to interrupts disrupting the logic of the non-interrupt, or background tasks.

While this turned out not to be the issue I was facing with the project, the first things checked were the potential impacts of interrupts. The first area of interest was to check if there were any variables or code lines that the compiler may have been optimizing out. In general ‘C/C++’ compilers these days have really good optimizers that remove unnecessary code or reduce it to a minimal number of instructions. The compiler does this by analyzing code in the context of the surrounding code. This can be problematic when for example a variable is being examined in a background task and modified in an interrupt task. A typical use of this may be a boolean flag that gets asserted by an interrupt function and is tested in a background function. As the compiler examines the code around the background testing it may find no other references to the variable and decide it can optimize the test out, or change it in some other way. The result of this is that the background task may never detect the boolean being asserted. Fortunately modern ‘C/C++’ compilers have the “volatile” keyword that allows us to inform the compiler that the code should not be optimized when the variable is accessed.

For a good discussion of the volatile keyword see this article.

A second potential issue is reading and writing of variables being interrupted mid-way through the process by a function that also modifies the variable. When a microcontroller needs to modify a memory-based variable it needs to perform a read, modify, write sequence. This sequence may take several instructions and an interrupt may occur at any point during the sequence. Should the interrupt function modify the variable the value partially read or written by the background function may be corrupted. To avoid such non-atomic operations the background function should temporarily disable any interrupt that may affect the variable to be manipulated, restoring the interrupt state once the operation is completed.

Now back to finding my real problem.

Sid