Electronic – Custom bootloader passing control to application code in STM32

bootloadergcckeilstm32

I am trying to understand that if for STM32F405 MCU a custom bootloader is used on the MCU then each of the two projects for the custom bootloader and the application code will have their own 'startup.s" files.

What I understand is that both of the two binaries, ie the bootloader and the application code, they both reside at different locations in the flash.

Now when the custom bootloader will exit and return the control then the control should pass-on to the startup code of the application code. But I don't know how it actually happens. Does the two binary programs resides at contiguous locations so that after the bootloader finishes the next instruction is that of the startup code?

Or they are at different locations and somehow the start address of startup of application code is saved as jump address at the end of the bootloader binary?

Will it make a difference in the way it is actually implemented if I use Keil or GCC compiler?

Best Answer

The compiled image of an STM32 application begins with a vector table, in which the first word contains the initial stack pointer value, the next one has the address of the first instruction.

The bootloader must obviously know the address the application is loaded at (how could it load it otherwise?), so it can look at the first two words of the image it had just flashed.

You must take the layout of the flash sectors into account when deciding the start address of the application. If parts of the bootloader and the application end up in the same flash sector, then the bootloader won't be able to reflash the app.

The designers of this MCU had arranged a few shorter sectors at the beginning of the flash exactly for this purpose. You might want to reserve 32k for the bootloader, and place the application at 0x08008000 for example, it's up to you (and the complexity of the bootloader).

Will it make a difference in the way it is actually implemented if I use Keil or GCC compiler?

The only difference would be in the linker configuration files, which tell the linker where to load the application. The GNU linker takes it from a linker script (*.ld), Keil calls it scatter file. They have the same purpose, but different syntax. You should adjust the linker script/scatter file of the bootloader too, limiting the amount of flash available to it.

Related Solutions

Electronic – It appears that I need two main() functions

Well, unless you are trying to do something really weird, generally the bootloader and application will be two entirely separate programs, each compiled to its own bin/hex file. The application would be compiled with .text=0, and the bootloader would be compiled with .text set to the beginning of the boot section. Note that this information must be passed to the linker, not the compiler. You can combine the hex files later for ease of flashing, but this is not required. If you need communication between the two, then you will have to implement a jump table or similar. You really don't have much space on an atmega8, though.

Electronic – Bootloader jump to main application problem using STM32 with Keil Uvision

I had the same problem: I solved it with commented this lines:

__disable_irq();
__set_MSP(*(__IO uint32_t*) APPLICATION_ADDRESS);

Some advice for anyone writing a bootloader for first time:

Turn your code into small sections:

In the first write a simple blinky code without any interrupt.
use of sector erase ability and and write your put your bootloader and application code together in flash. Now you just,must try to jump from bootloader in application code. If it works, this means your jumping is correct.
Now try receive your application code with USART (or with any other peripheral) and write in flash. It must work, if not, it means write it flash is wrong (open hex code file and compare flash data,flash address and other thing with hex file) (attention to little_endian writing)
Now write a blinky code with timer and interrupt. It must work, if not, it mean is mistake is in jumping vector table

Best Answer

Related Solutions

Electronic – It appears that I need two main() functions

Electronic – Bootloader jump to main application problem using STM32 with Keil Uvision

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