RP2040 Serial bootloader

Field	Description
Author	Brian Starkey (github.com/usedbytes)
Function	Bootloader for the custom embedded apps in Mr Beam.
Info	Customized for use with Mr Beam 3 Pico boards.
This version	v0.1.1
Date	2022-09-11

The serial bootloader is used to jump to the custom embedded app developed for the Pico boards. The usage is in the Mr Beam 3 devices where two RPi RP2040 Pico boards are used.

Using a serial bootloader enables updates for the embedded software to be applied during a software release.

The .uf2 file is copied and pasted on the RPi Pico during initial configuration.

Opening a serial terminal with PuTTy or minicom or any similar app with the corresponding Serial/UART module address will show an output of the bootloader identifier string like the one below.

BL##v0.1.1##(8000)##

Flashing is done using the serial-flash tool. https://github.com/mrbeam/RP2040-serial-flash-tool

POINTS TO NOTE:

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1.bootloader.ld

• FLASH(rx) : ORIGIN = 0x10000000, LENGTH = 32k //maintain the length as multiple of 4k.
• Make similar changes in the other .ld files and CMakeLists.txt file
  • combined.ld
  • standalone.ld

Refer the branch - https://github.com/usedbytes/rp2040-serial-bootloader/tree/bl32k and the changes.

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2. main.c

IMAGE_HEADER_OFFSET should be the same as the FLASH LENGTH variable in the bootloader.ld file.

#define IMAGE_HEADER_OFFSET (32 * 1024) 

When using different baud rates make sure to recompile the GOlang serial flash tool available in the following repo. - https://github.com/mrbeam/RP2040-serial-flash-tool.

 #define UART_BAUD   (115200)

Best to keep the baudrate same across the tools to avoid updating the flash tool itself.

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3.CMakeLists.txt

Changing the size of the FLASH LENGTH becomes necessary new libraries are added.When the preprocessor #define DEBUG is declared in the cmake compile options the stdio_usb library is added. Make the following change in CMakeLists.txt file.

target_link_libraries(bootloader
                      pico_stdlib
                      hardware_dma
                      hardware_flash
                      hardware_structs
                      hardware_resets
                      pico_stdio_usb //Add this line.
                      cmsis_core)

Compile with the following options on the terminal for DEBUG option.

mkdir Debug
cd Debug
cmake -DUSE_DEBUG=ON ..  
make -j8

For Release builds

mkdir build
cd build
cmake ..  
make -j8

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