Debugging a Custom Flash Loader

Technical Note 201118






12/10/2020 10:14 AM


Creating a custom flash loader can be a tricky process, because it is difficult to debug flash loader activity the usual way. (See Technical Note 200214 on how to log flash loader activity). This Technical Note shows an alternative way of debugging a flash loader under development.

For more information about developing custom flash loaders, see also the Flash Loader Development Guide in $TOOLKIT_DIR$\doc\FlashLoaderGuide.ENU.pdf.


During development of a custom flash loader, it is possible to use semihosting printf to debug flash loader activity. The general idea is to:

  1. Build a flash loader project with semihosting printf
  2. Configure the application project to produce a Terminal I/O log file


In this example, an STM32F429II-ACA board is used with an I-jet debug probe.

Build a flash loader project with semihosting printf

  1. Open the flash loader workspace from $TOOLKIT_DIR$\src\flashloader (in this example from ST\FlashSTM32F4xx)
  2. Select the correct configuration (in this example FlashSTM32F4xxRAM192K_Dual)
  3. Confirm that Project > Options > General Options > Library Configuration > stdout/stderr: Via semihosting is selected.

  4. Add a call to __iar_data_init3 and a printf statement at the beginning of the FlashInit function, like this:
    __iar_data_init3(); // Required to initialize .bss
    parallelism = 1;
    blankcheck = 0;
    printf("Hello! World\n\n");
    printf("image_size:%d\n\n", image_size);
  5. If you get a linker error [Lp005]: placement includes a mix of sections with content and sections without content, you need to modify the linker configuration file:
      * Comment out the lines do not initialize { zi } or
        do not initialize { section .bss }.
      * Add a line initialize by copy { readwrite } if not already there.
  6. Build and replace the flash loader .out file in $TOOLKIT_DIR$\config\flashloader
    (In this example in the ST folder)

Configure the application project to produce a Terminal I/O log file

The flash loader printf output will not be visible in the Terminal I/O window. However, it will be logged in the Terminal I/O log file.

  1. Open the STM32F429II-ACA LightEffects project from Help > Information Center > Example Projects.

  2. Temporarily change the debugger driver to Simulator just in case the debug session cannot be launched with I-jet or other hardware debug probes (because of problems with the custom flash loader): Project > Options > Debugger > Setup>Driver: Simulator

  3. Start a debug session by choosing Project > Download and Debug

  4. Enable the Terminal I/O log file by choosing Debug > Logging > Set Terminal I/O Log File

  5. Exit the debug session, and change the debugger driver back to I-jet: Project > Options > Debugger > Setup>Driver: I-jet

  6. Start a new debug session by choosing Project > Download and Debug

  7. Flash loader printf output is now recorded in the Terminal I/O log file.


For debugging and logging details of custom flash loader development, semihosting printf and the Terminal I/O log file are useful.

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