Categories

Makahiya – PCB, integration & capacitive sensor

Hi everybody,

On Monday, I started to review my code and clean it. I found that the touch detection algorithm was not adapted for 2 sensors, so I modified it (double the size of all the buffers, add one parameter to each function,…) And this day, we finally received our PCB. With Alexis, we made the first tests and discovered 2 mistakes: the power jack is connected in the wrong sense and the pins of the SWD aren’t mapped properly on the connector. For the first problem, Alexis inversed the wires coming from the power block, and for the second he made a patch.

After that, we flashed our program and began to test the components. The Bluetooth worked immediately, and the communication with the capacitive sensor was also immediately functional. Tanguy had to make some corrections for the LEDs and I spent some time trying to understand why I wasn’t able to communicate with the WiFi module. The problem was desperately stupid: I set the pins in the wrong alternate function (the alternate functions table was organized in a different way than the others I had seen but I hadn’t noticed it before yesterday). Now, the Wi-Fi works well (I’ve forgotten to test the reception of messages with websockets so I hope I won’t have any problems with that).

The next component I tested is the capacitive sensor. The communication worked well but I wasn’t able to detect any touch. I investigated a bit, reviewed the configuration values and decided not only to copy the one that worked for the demo board but to evaluate the appropriate ones for our board. First problem with that: we use the internal oscillator of these sensors and its frequency is used in the formulas. But the datasheet only says that this frequency is between 35 and 55 MHz, which is a rather large interval. The second problem I faced is : how to know the capacitance of a banana leaf? With Tanguy, we went to see a professor who could have helped us, but he didn’t find any relevant way to perform this measure. So I decided to redo some tests with the demo board and my own configuration to see if it worked. Then, I did some new tests with our board and this time, the values read were much more coherent. So, it was a good news. The problem now is that the algorithm I wrote some months ago to detect touches doesn’t work with these values so I will have to work a little on this to make it work.

During the end of the week, I will work on the touch detection algorithm, on the “mechanic” (container for the boards, the servomotors and the LEDs,…) and on the code that will run on the board during the presentation.

Leave a Reply