'How Does STM32 Factory Bootloader Allow for Reflashing at any Time?
I'm aware that on some STM32 boards, they come with a built in bootloader in their ROM. Quoting from STM32 Application Note
The bootloader is stored in the internal boot ROM (system memory) of STM32 devices, and is programmed by ST during production. Its main task is to download the application program to the internal Flash memory through one of the available serial peripherals (such as USART, CAN, USB, I2C, SPI).
However the ability to reflash at any time, i.e. trigger a reflash when your board is a running a program confuses me. My understanding is that a bootloader is the first piece of code to execute and then launches your user program. However, to facilitate this flashing at any time, won't the bootloader have to constantly poll for a request to reflash?
Solution 1:[1]
The bootloader only polls for an update on start-up/reset. So not "anytime" in the sense of "spontaneously" - you do have to trigger it. If the programmer has access to the reset pin or control of power, it can of course invoke a restart.
Moreover the ROM bootloader only runs at all if the BOOT pins are set appropriatlely. The boot modes are Flash, SRAM or ROM.
Solution 2:[2]
Your understanding is right. Almost.
In case of STM32, you need to set up the chip to run the bootloader, typically using BOOT pins. When the bootloader runs, it checks if the programming is necessary by monitoring the specific peripherals. If the programming is not triggered, it jumps to the user program.
Therefore, you cannot reprogram at any time, only at boot time.
Here is an example bootloader flowchart for programming over UART or I2C.
See this application note on STM32 bootloader for more details.
Sources
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Source: Stack Overflow
| Solution | Source |
|---|---|
| Solution 1 | Clifford |
| Solution 2 |