The Tarte-Py boards have arrived, they've been assembled, and they've tested. In fact, one of them is running a series of tests right now as I type this post. How did it go? In summary, it was one of the worse board bring-ups I've experienced in recent memory. The troubleshooting process was both frustrating and enjoyable at the same time. It was also quite time consuming, which accounts for the tardiness of this post. Fortunately, the problems were eventually solved and the boards are now working.
Today we will go over the hardware design of the MCU board, which I've named Tarte-Py: Tester for Automatic Resistive Trace Experiments in Python. The board follows closely in concept to the
Pyboard as noted in Part 4, but with some slight circuit changes and big mechanical changes to better fit our application.
In this article, I'll discuss the testing approach for this project. Since I'm basically lazy, the goal is to keep things as simple as possible and try not to reinvent the wheel.
The gist of these tests is to take various parts of the circuit of interest, say a serial data link, and first observe it while its operating in the normal way. In this project, normal means with highly conductive copper traces. In the serial data example, this would mean checking the that data is not corrupted and perhaps watching the waveform on the oscilloscope.
In this article, I am going to review the variable trace resistance simulator that I've designed for this project. I'll go over some design options and how I made my decision, and wrap up with the completed design, whose PCB is being produced even as I type. In case you've just missed the blogs leading up to this point, you can find them here.
In the previous articles, we've taken a look at conductive ink PCB traces using a few back-of-the-envelope calculations. Now that we have a rough idea what to expect, it is time to get on with the fun part of this series -- building a real printed circuit board and testing how it behaves as we tweak the trace resistances.
Last article we looked into the ramifications of using conductive ink PCB traces from a static, DC perspective. Today I'm going to consider the implications from a dynamic point of view. Most of the signal interfaces we use in microcontroller designs today drive very high impedance loads. The impact of increasing the trace resistance connecting to the input gate is an increase in the rise time. Let's take a look at that in more detail.
During this series, I plan to learn about printed ink conductors primarily on my own, through analysis and experiments. Therefore I have intentionally avoided digging too deep into the details of how they are commonly used in the industry (an approach I wouldn’t recommend for someone doing this for a professional product). But I do know people are indeed using printed inks for a variety of circuits, so I don’t expect to find any big showstoppers in the analyses and experiments that follow.
While repairing an old Ham radio transmitter back in high school, I found a bad capacitor. It was a large metal-can electrolytic type, bolted to the steel chassis of the radio. Because it was part of the negative 250 Vdc bias supply, the can was isolated from the chassis with an insulating fiber washer. Unfortunately, the replacement capacitor didn’t come with such a washer, and I couldn’t use the old one because it didn’t fit. Not to be deterred, I decided to make my own custom insulator using a piece of rubber cut from an old bicycle inner tube. To this day, I still remember the small explosion that resulted when I flipped on the power switch, not to mention the odor that was released – I dubbed it “Essence of Midnight in Pittsburgh”. I learned the hard way that inner tubes are not just rubber, but contain a substantial amount of carbon black and therefore don’t make very good insulators.