Getting started with bootloader on mbed OS

This example shows how to create a bootloader.

To read more about the bootloader, please visit bootloader tutorial.

Required hardware

• A supported target - u-blox EVK-ODIN-W2, Nucleo F429ZI or K64F.
• SD card.
• If your supported target does not have an SD card port, you need a shield or breakout board with an SD card port that uses SPI pins.

Import the example application

From the command-line, import the example:

mbed import mbed-os-example-bootloader
cd mbed-os-example-bootloader

Connecting the SD card

This example uses the onboard SD card port for the K64F and the UBLOX_EVK_ODIN_W2.

Other targets use the following Arduino form-factor SPI pins by default:

• D11 - MOSI
• D12 - MISO
• D13 - SCK
• D10 - CS

To use different SPI pins, you need to modify the mbed_app.json file. Add a key for your target to the target_overrides section with the following data (replace <TARGET_NAME> with your target's name):

{
    ...
    "target_overrides": {
        ...
        "<TARGET_NAME>": {
            "sd_card_mosi": "<MOSI_PIN>",
            "sd_card_miso": "<MISO_PIN>",
            "sd_card_sck": "<SCK_PIN>",
            "sd_card_cs": "<CS_PIN>"
        }
        ...
    }
    ...
}

Preparing the bootloader

All supported targets mentioned above are set up to build as a bootloader image. To add support for a new target, you must specify the size of the bootloader.

To do this, set the target value restrict_size to the maximum bootloader size in mbed_app.json (replace <TARGET_NAME> with your target name):

    "target_overrides": {
        ...
        "<TARGET_NAME>": {
            "target.restrict_size": "0x20000"
        },
        ...

Note: restrict_size pads the build image to the requested size.

Building the bootloader

Invoke mbed compile, and specify the name of your target and your favorite toolchain (GCC_ARM, ARM, IAR). For example, for the ARM Compiler:

mbed compile -m <TARGET_NAME> -t ARM

Your PC may take a few minutes to compile your code. At the end, you see the following result:

Merging Regions:
  Filling region application with .\BUILD\<TARGET_NAME>\ARM\mbed-os-example-bootloader_application.bin
  Padding region application with 0x11420 bytes
Space used after regions merged: 0x20000
+-----------------------------+-------+-------+-------+
| Module                      | .text | .data |  .bss |
+-----------------------------+-------+-------+-------+
| Misc                        | 18481 |    24 |  1268 |
| drivers                     |  2132 |     8 |   184 |
| features/FEATURE_LWIP       |    46 |     0 | 12560 |
| features/filesystem         | 12756 |    12 |   540 |
| hal                         |   414 |     8 |     0 |
| hal/TARGET_FLASH_CMSIS_ALGO |   520 |    28 |     0 |
| platform                    |  2759 |    32 |   380 |
| rtos                        |   144 |     8 |     0 |
| rtos/rtx                    |  6954 |   100 |  8396 |
| targets/TARGET_STM          | 15560 |   568 |   736 |
| Subtotals                   | 59766 |   788 | 24064 |
+-----------------------------+-------+-------+-------+
Allocated Heap: unknown
Allocated Stack: unknown
Total Static RAM memory (data + bss): 24852 bytes
Total RAM memory (data + bss + heap + stack): 24852 bytes
Total Flash memory (text + data + misc): 60554 bytes

Image: .\BUILD\<TARGET_NAME>\ARM\mbed-os-example-bootloader.bin

After the build completes, the build system creates two binary images in the BUILD/<TARGET_NAME>/<TOOLCHAIN_NAME> directory: <project-name>.bin and <project-name>_application.bin. <project-name>.bin is the padded bootloader (used for combining with applications). <project-name>_application.bin is the original unpadded bootloader.

In this example, main.cpp defines the update binary for the bootloader as mbed-os-example-blinky_application.bin (specified under the macro UPDATE_FILE). The bootloader looks for this file in the root of the SD card, flashes it to memory and then jumps to the application.

Building the application with the bootloader

The next step is to build an application you can combine with your bootloader to create a loadable image.

1. Instruct the build system to use the newly created padded bootloader image (named <project-name>.bin in the previous section). To do this, create a target_overrides section in your application's mbed_app.json file, and add a key for your target (see <TARGET_NAME> in the example below). If mbed_app.json does not already exist in your application, create one.

2. Set the target value bootloader_img in mbed_app.json to the file path of the padded bootloader image. For the below example, copy the padded bootloader image to the project directory.

{
    "target_overrides": {
        ...
        "<TARGET_NAME>": {
            "target.bootloader_img": "<path to padded bootloader binary>"
        },
        ...
}

2. Invoke mbed compile, and specify the name of your target and the toolchain you are using (GCC_ARM, ARM, IAR). For example, for the ARM Compiler:

   mbed compile -m <TARGET_NAME> -t ARM
   

After the build completes, the build system creates two binary images in the BUILD/<TARGET_NAME>/<TOOLCHAIN_NAME> directory: <project-name>.bin and <project-name>_application.bin. <project-name>.bin is the combined bootloader and application (suitable for flashing to the device's ROM directly). <project-name>_application.bin is the update binary (the one you place on the SD card).

Program the application with the bootloader included

Follow these steps to program the target with the combined bootloader and application image. You then can update the application from the SD card.

1. Connect your mbed device to the computer over USB.
2. Copy the combined bootloader and application binary file (named <project-name>.bin in the previous section) to the mbed device.
3. Press the reset button to start the program.

Program the application from the bootloader (load from SD card)

Note: You must first flash the combined bootloader and application image or just the bootloader image (padded or unpadded) before proceeding with the following steps.

1. Connect the SD card to your computer.
2. Copy the update application binary (named <project-name>_application.bin in the previous section) to the root of the SD card.
3. Remove the SD card from your PC, and connect it to your mbed (see the above section Connecting the SD card)
4. Press the reset button to start the firmware update.

If a terminal is open, the following prints at a baud rate of 9600:

Firmware update found
Starting application

Troubleshooting

1. Make sure mbed-cli is working correctly and its version is >1.0.0.

   mbed --version
   

   If not, you can update it:

   pip install mbed-cli --upgrade
   

2. If using Keil MDK, make sure you have a license installed. MDK-Lite has a 32 KB restriction on code size.