GPIO access over USB

oroymd

I want to send commands from Matlab to the BelaMini over USB to write certain digital pins, as would be done with an Arduino for example. In my IDE script, I have digitalRead calls that poll the values of the digital pins of interest in order to produce sounds according to precise conditions. Currently, I have to use a workaround which consists of writing pins of an Arduino which I connect to the pins I want to write to on the BelaMini. It works but it is not elegant and introduces a small delay. On my computer, the BelaMini appears as "/dev/cu.usbmodembela6". I tried using the Serial library as in the Serial example but get the message "Error opening /dev/ttyUSB0: No such file or directory". I use the BelaMini as the hardware for producing sounds according to an experimental Matlab script which I cannot embed entirely in the BelaMini code since it requires other functions such as response logging, on-screen visuals, etc.

I have searched for a while on the forum but did not find an answer. This is probably trivial, I know...

Thank you!

giuliomoro

/dev/ttyUSB0 in the example code is just an example. That would be the name of a USB-UART adapter plugged into Bela. The available ttys can be seen by running ls /dev/tty* in the console at the bottom of the Bela IDE. The one that represents the USB connection to the host is called /dev/ttyGS0 and it operates at a speed of 115200bps .

Currently it is configured as a login shell, so if you use that, only some of the characters will be sent to the Bela program, while others will be grabbed by the shell. You should therefore also run in the console at the bottom of the IDE:

 systemctl stop serial-getty@ttyGS0.service

This will stop the shell on ttyGS0 till the next reboot. If you want to disable it permanently, run

 systemctl disable serial-getty@ttyGS0.service

(you can revert either with start or enable respectively).

oroymd

giuliomoro Thanks, I can now communicate through serial over USB! Would you know how to manipulate GPIO pins from there (as, for example, can be done with Arduino)?

giuliomoro

is any Bela program running on Bela while you want to control the GPIO via serial?

oroymd

giuliomoro Yes, ideally I’d like to use the pin states changes to trigger the sounds.

giuliomoro

What language are you using for the Bela program?

oroymd

giuliomoro I'm using C++. I'm sorry for not having included this information at the beginning. I assumed this was kind of the default.

giuliomoro

Right so you only need to add to your existing C++ project the few lines from the Serial example that deal with starting the auxiliarytask and getting the serial data. Then you can either set some global variable, read it from render() and use digitalWrite{once}() or use the Gpio class to handle the gpio from the auxiliarytask itself.

oroymd

giuliomoro Totally makes sense, thank you I will try that!

giuliomoro

Here for the Gpio class http://docs.bela.io/classGpio.html and some more details about non-Bela-Digital pins :https://learn.bela.io/using-bela/technical-explainers/other-uses-of-gpio-pins/

oroymd

giuliomoro Yes thank you. I was able to make it work. Now I have a good idea of the latency involved and the difference between the GPIO class and the usage within Bela.h.

Would you have an idea in mind (except for cutting the USB cable and probing the wire directly) to measure the latency for serial communication? The idea is to see if I can achieve similar latency with USB serial communication as when using the digital signal accessed through a given pin. Thanks a lot for your help!

giuliomoro

This is a virtual serial port over the USB, so there is no actual 115200bps signal to scope. Even if you had access to the data lines, I don't think ou can really just scope the USB bus and hope to extract meaningful timing information without a dedicated USB protocol analyzer. The easiest way would be to have an external USB-to UART device going into one of the UARTs on Bela (e.g.: /dev/ttyS4) and at that point you can scope the UART pins.

oroymd

giuliomoro Thanks, I'll try that!

oroymd

@giuliomoro Is there a way to use gSerial.write() in a real-time safe fashion to send feedback to the host?

giuliomoro

oroymd Is there a way to use gSerial.write() in a real-time safe fashion to send feedback to the host?

no, that's writing directly to the file descriptor, so it needs to be done from a separate thread. You can use a Pipe to send POD between the two. There already is a Pipe object in the example for Serial. You can use it to also send data back from the audio thread to a new thread. So you can send data from the Rt side with Pipe::writeRt(). You probably want to set the non-rt side to be blocking and have a dedicated thread calling readNonRt() repeatedly, blocking on each call, to minimise the jitter that is typical of polling and non-blocking reads. Whenever valid data is received, pass it to gSerial.write() immediately.

Class documentation: http://docs.bela.io/classPipe.html