bootselBoot

A small command line application to reset Raspberry Pi Pico to BOOTSEL mode from a host via USB instead of pressing a hardware 'BOOTSEL' button.

BOOTSEL

Raspberry Pi Pico has BOOTSEL mode that a board is connect to a host via USB by device mode. Flash storage on the board can be mounted on the host as a USB flash drive.

BOOTSEL mode can be invoked by pressing 'BOOTSEL' button on the board when the board is plugged with USB or 'RUN' pin on the right card edge of the board is switched to GND. But these procedures are annoying because the board is rather delicate and there is a possibility to break a microUSB port or the board itself. It is better to handle without hardware treatments.

There is a solution to reset Pico via USB from a host with some limitations:

1. An executable code with pico_stdio_usb module is installed and executed on the board.
2. The board is connected to a host computer via USB.

Default mode of pico_stdio_usb module has three intefaces on its USB descriptor. Interface#0 and interface#1 are for CDC (Communication data class) device and interface#2 is for resetting the device. The interface#2 can receive a control transfer with bRequest field set 0x01 or 0x02. 0x01 request invokes BOOTSEL boot and 0x02 regular flash boot.

I wrote a small code using libusb-1.0 to send the control transfers to Pi Pico board.

On a Raspberry Pi as a host

Install pico-sdk with TinyUSB module. It is a standard procedure how to use Raspberry Pi for a host computer to Pi Pico board.

$ git clone -b master https://github.com/raspberrypi/pico-sdk.git
$ cd pico-sdk
$ git submodule update --init --recursive
$ cd ..
$ git clone -b master https://github.com/raspberrypi/pico-examples.git

Additionally, install a serial terminal emulator. At this time,

$ apt install screen

Any other serial emurators are OK. Plug a Pi Pico board to a USB connector of the host. Install a executable code to the board. For example, install hello_world example from pico-example.

Test example code for Pi Pico

Install any executable that includes pico_stdio_usb. For example, here is a test code named pwmled.c that is a small code to blink on-board LED continuously using PWM capability of the board and to read a character from USB port to change blink period.

To build pwmled, create a direcory named 'pwmled' neighbouring pico-sdk directory.

$ git clone https://github.com/illusiaDecafish/bootselBoot
$ cd bootselBoot/pwmled/
$ mkdir build
$ cd build/
$ cmake ..
$ make -j4
$ ls

If you can find pwmled.uf2 build was successful.

Plug Pi Pico via USB pressing BOOTSEL button and copy pwmled.uf2 to flash storage on the board.

$ sudo mount /dev/sda1 /mnt/pico
$ sudo cp pwmled.uf2 /mnt/pico/
$ sudo sync

Then the LED blinks continuously and slowly (period is may be about 1sec). Invoke the screen command

$ screen /dev/ttyACM0 9600

device file '/dev/ttyACM0' is a CDC (Communication Data Class) port of the connected Pi Pico. When you key in a character '1', LED blinks faster and '2' .. '9' LED blinks slower. To exit screen command, type C-a ('a' with control-key) -> k -> y.

How to compile and invoke bootselBoot

To build bootselBoot.c, libusb-1.0 library should be installed. Needless to say, bootselBoot is a native executable on a host, not on a Pi Pico.

$ sudo apt update
$ sudo apt install libusb-1.0

Again return bootselBoot directory and compile cootselBoot.c.

$ cc bootselBoot.c -o bootselBoot -I/usr/include/libusb-1.0 -L/usr/lib/arm-linux-gnueabihf -lusb-1.0

And invoke bootselBoot command to reset the Pi Pico

$ sudo ./bootselBoot b

The argument 'b' means BOOTSEL boot and 'f' for standard flash boot.You can see the LED on the board turning off and find a device file of the flash storage.

$  ls /dev/sd*

Then you can mount and copy other .uf2 file onto the flash storage and don't need the procedure 'unplug USB/pressing BOOTSEL/plug USB again'.

It is handy and useful.

On other unix platforms

Almost same procedures can work on other unix platforms such as debian, ubuntu or macOS. There are some notes:

• device file name of flash storage and USB CDC port of Pi Pico may be different.

• A path to libusb-1.0 library and its include file may also be different.

For example, on macOS 11.3.1,

• device file of CDC port may be /dev/tty.usbmodem0000000000001

• flash storage is mounted automatically on /Volumes/RPI-RP2 (its device file may be /dev/disk4s1 etc.)

• libusb-1.0 should be installed manually by homebrew or MacPorts etc.

• compile command is

  $ cc bootselBoot.c -o bootselBoot -I/usr/local/include/libusb-1.0 -L/usr/local/lib -lusb-1.0
  

  for the homebrew case.

For Windows

I'm sorry I don't know how.

Please encourege yourself.

References

Reset funtion description from pico_stdio_usb doxygen:

"....reset over the USB interface"

Reset interface code on Pi Pico:

C source

Constant definitions

vendorID and productID of Pi Pico on device mode:

raspberrypi/usb-pid

libusb-1.0:

libusb.info

The author's blog

decafish.blog

but sorry in Japanese.