Getting Started with Raspberry Pi Pico using MicroPython and C

Raspberry Pi Pico Getting Started Guide

Raspberry Pi Pico board was just launched last Thursday, but thanks to Cytron I received a sample a few hours after the announcement, and I’ve now had time to play with the board using MicroPython and C programming language.

I went to the official documentation to get started, but I had to look around to achieve what I wanted to do, namely blinking some LEDs, so I’ll document my experience with my own getting started guide for Raspberry Pi Pico using a computer running Ubuntu 20.04 operating system. The instructions will be similar for Windows and Mac OS.

Preparing the hardware

In theory, we could just get started with the board alone, but since I got some headers with my board, I also took the opportunity to try out Pine64 Pinecil soldering iron powered by MINIX NEO P2 USB-C power supply.

Pinecil Soldering Iron RPi Pico

The soldering iron worked great for about one minute, and then I started to have problems with soldering… Looking a the screen I could see Zzzz and the temperature dropped. I did not move the soldering enough so it failed to detect any activity and entered into sleep. Changing the motion sensitivity or sleep timeout can easily fix this issue, and I could complete the task at hand.

Raspberry Pi Pico Headers Soldered

It would be a shame not to use those headers so I inserted Raspberry Pi Pico into a breadboard and added an LED with the accompanying circuitry.

Raspberry Pi Pico Breadboard

5V is connected to VBUS (pin 40), GND to pin 38, and I decide to use the GPIO the closest to the LED namely GP15 (pin 20). The GPIO markings on Raspberry Pi Pico are only shown on the bottom of the board, so when the board is connected to a breadboard a pinout diagram helps.

Raspberry Pi Pico Pinout DiagramThe hardware setup is now complete, and all I need is a Micro USB to USB-A cable to connect the board to my laptop.

MicroPython on Raspberry Pi Pico

We should first copy MicroPython firmware to the board. To do so we can download the latest firmware from the getting started guide (pico_micropython_20210121.uf2 at the time of the review), then press the BOOTSEL key on the board while connecting to a computer with a USB port, and release the key after connection. I did so, but nothing happened. That’s because I used my bicycle headlight’s USB cable that lacks the data line… So I went to select a proper micro USB to USB-Type-A cable, and Raspberry Pi Pico was properly recognized on my laptop:

.. and mounted as RPI-RP2 mass storage device.

Raspberry Pi Pico RPI-RP2 mass storageAfter copying pico_micropython_20210121.uf2 file to the drive, it automatically unmounts and the Pico board reboots as a serial device:

At this point, the getting started guide on Raspberry Pi website is not very useful, and we have to switch the Python SDK documentation (PDF).

The documentation uses minicom for the serial console, but now I prefer Bootterm since it’s easier to use. In any case, if you program the board in Linux, make sure your current user is added to the dialout group, or you’ll need to run all programs as root:

Bootterm properly detected ttyACM0 port, so I just ran “bt” to access MicroPython REPL interface, and type some MicroPython commands.

I could turn on the onboard LED (GP25), but when I did the same for the LED on the breadboard (GP15), it did not work. I rechecked my circuit, and used a multimeter to check the voltage levels, and found out GP25 was still pulled low. A web search showed GP15 was disabled in CircuitPython because it would interfere with the USB interface.

This is on purpose, GP15 should not be used, it is used by the internal USB peripheral

Ah… I suppose it’s the same for MicroPython, so I switch to the neighboring pin (GP14, pin 19):

Raspberry Pi Pico Blink LED

and sure enough, it worked! If you want to learn more about the MicroPyton API, press Ctrl+B and type help():

We can exit bootterm with “Ctrl+]” followed by “q”. But what if we want to save our Python program on the board and run it automatically? Unless, I missed there’s nothing about that in the Python SDK documentation, so I had to jump to a third guide to find out the best way is to use Thonny.

Ubuntu 20.04 does have Thonny 3.2.7 in its repository, which we can install with sudo apt install thonny, but it does not support Raspberry Pi Pico, so instead I installed the latest version of the program (v3.3.3) with pip3:

I then went to Run->Select interpreter… to select “MicroPython (Raspberry Pi Pico)“.

Thonny Raspberry Pi Pico

From the user interface, I could type some code to turn off the onboard LED:

Thonny Raspberry Pi Pico Sample

In order to alternatively blink the onboard LED and breadboard LED with a one-second interval, I copied and modified some code from the Python SDK documentation:

I save the file as on my PC, and it ran fine. But if you’d like to run the code without a PC, it’s possible to save it to Raspberry Pi Pico. Click on File->Save copy, then on “Raspberry Pi Pico” button,

MicroPython Save to Computer or Raspberry Pi Pico

and save the program as You can now run the program automatically by connecting your board to any USB power source.

C/C++ on Raspberry Pi Pico

Let’s try the “C/C++ SDK” that is basically all C language, except some tools written in C++. We can go back to the official Getting Started documentation, where we are asked to copy blink.uf2 to Raspberry Pi Pico while in boot mode, and it does blink the onboard LED. It works, and it’s super easy, as the binary is pre-built, but what we really want to do is modify the source code, and build our own binary to blink both the internal and external LED.

So we’ll have to install the C/C++ SDK, dependencies, and samples as follows:

We can have a look at the blink example in pico-examples/blink/blink.c:

Good. Before modifying it, we should try to build it by first export the SDK path, and configuring the build:

We can now enter the blink directory (a new one, generated by cmake), and run make:

We now have a bunch of files:

The most important ones are blink.uf2 that we can copy to Raspberry Pi Pico to run the program, and blink.elf that can be used by the debugger (OpenOCD + GDB), but that is out of the scope of this getting started guide.

Now that we know how to compile a C program for the Pico board, let’s modify the blink.c example to alternatively turn on and off the onboard LED and external LED connected to pin 14:

and build the program again:

Then enter boot mode and copy blink.uf2 to the board, and success!

Raspberry Pi Pico LED Blink

We can achieve the same results with either a C or Python program. We’ll have a look at RP2040’s PIO (Programmable IO) interface in an upcoming post, as AFAICT it’s what differentiates Raspberry Pi RP2040 most from other microcontrollers.

I’d like to thank Cytron for sending Raspberry Pi Pico for review. If you are based in ASEAN, you can purchase the board on their store for $4.98, or if you have some time there are offering a $5 carrier board for Raspberry Pi Pico for the same price with the board already soldered that’s expected to ship on February 10th, and available worldwide, not only in ASEAN.

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