Jump to Bootloader in STM32 through application i.e using Boot 0 and Boot 1 Pins in Boot mode from User flash

Question

I have a requirement for firmware upgrade. I am planning to use USB DFU class. But command for firmware upgrade will come from PC application in my case . so i need to switch to bootloader which is there in System Memory. As initially i am running application so it is getting booted from User flash i.e i have Boot0 and Boot 1 pins configured for User flash. As DFU bootloader is there in System flash ,now for that Boot0 and Boot1 pins settings need to be changed . is there a way like Boot 0 and Boot 1 settings remain same as User Flash memory and in the application we jump to System Memory?

Answer 1

Boot0/1 pins are sampled only when the processor starts, in order to check if it should load the user code from memory or if it should load the bootloader. The state of these pins has no effect of the bootloader afterwards.

I've been faced to a similar request, and found 2 ways to load the bootloader on-demand.

First, you can "JUMP" from user-code to the bootloader. For example, you could jump to the bootloader when a button is pressed.

But... this is far more complicated than a simple JUMP instruction : some registers and devices must be reconfigured correctly to work with the bootloader, you have to ensure that no IRQ will be triggered during the JUMP,... In fact, you have to reconfigure the processor as if it was just started after reset. You can find some information about this technic : on this video from ST.

I managed to do this kind of things on STM32F1xx project. However, on a more complex project based on STM32F4, this would become really difficult... I would have to stop all devices (timers, communication interface, ADC, DAC,...), ensure that no IRQ would be triggered, reconfigure all the clocks,...

Instead, I decided to implement this second solution: When I want to jump to the bootloader, I write a byte in one of the backup register and then issue a soft-reset. Then, when the processor will restart, at the very beginning of the program, it will read this register. This register contains the value indicating that it should reboot in bootloader mode. Then, the jump to the bootloader is much easier, as presented in the youtube video.

Answer 2

You can simulate the bootloader condition. Connect capacitor and paralel resistor from BOOT pin to ground. Connect another free pin to BOOT pin. Capacitor can be charged by external pin and is discharged by resistor. I don't remember exact values you can calculate/experiment them (important is time constant of RC circuit).

Charge this capatitor by setting external pin to 1, perform software reset by NVIC_SystemReset. After reset, it will run bootloader. Resistor connected to Capacitor will perform discharging. After firmware update, you can reset the device and it will run to your application.

We are using this in some applications and it works well. Drawback of this solution is that you need external circuitry, but it is very easy to implement and it is universal for all STM32 devices.