Using a software USB boot-loader has some disadvantages:
- It has to occupy some part of the user Flash, which leads to less available Flash memory (usually a whole page)
- it is difficult (or impossible) to protect it from accidental erase or data corruption.
- the user firmware has to be build with the boot-loader in mind
- it has to have own USB PID/VID. Having a VID is a little bit expensive, especially for small hobby projects.
The solution I have is to use the STM32 internal loader. This allows me to use the whole Flash memory for my user code. On top of that I have some more flexibility with this solution - I have a control over the hardware reset pin from the software.
However, all it comes at a price - an external components are needed. The schematic is very simple. A popular FTDI FT232 USB-to-UART converter is used.
Two GPIO from FT232 are used to control STM32_RESET and STM32_BMODE pins, and two pins for UART communication.
Operation is simple - when a firmware has to be loaded, the boot mode pin is set to enable the internal boot-loader and a hardware reset is issued. The STM32 goes to boot-loader mode. Via UART the new firmware is loaded, the boot pin is set to a state to enable user code execution and the STM32 is reset again. At this point the new firmware gets executed.
The source code can be found on https://code.google.com/p/stm32-loader/. The GUI is based on Qt.
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