Booting Cortex-M3 STM32F1 microcontrollers from RAM instead of flash memory can offer performance benefits like faster boot times. However, it requires configuring the chip’s boot options and setting up a RAM-based bootloader. This guide will walk through the key steps for booting STM32F1 chips from RAM.
Overview of Booting from RAM
Most microcontrollers boot by default from internal flash memory, where the program code is stored. However, flash memory has slower read speeds compared to RAM. Booting from RAM can skip the flash access and improve boot performance.
To boot from RAM, a small bootloader must first be programmed into flash memory. On startup, this bootloader copies the main application code from flash to RAM, then jumps to execute from RAM. A reserved region of RAM holds the main code image.
Key steps to implement RAM booting on Cortex-M3 STM32F1:
- Configure system memory for RAM bootloader and application image
- Create RAM bootloader and program it into flash
- Build application code and link it to RAM address
- Set boot configuration options to boot from RAM
Configuring System Memory
The first step is planning out the memory map to accommodate both the bootloader and application image in RAM. A region of RAM needs to be reserved for storing the application code.
For example, on a STM32F103 with 64KB RAM, we could allocate the memory like this:
- Bootloader: 0x20000000-0x20000FFF (4KB)
- Application image: 0x20001000-0x2007FFFF (60KB)
The bootloader size depends on requirements, but 4KB is typical. The remaining RAM is available for the application.
This memory layout is defined through the linker script file used for building the bootloader and application code.
Creating a RAM Bootloader
With the memory map defined, we can now create a minimal bootloader program in C or assembly language. The key tasks of the bootloader are:
- Configure RAM and copy application image from flash to RAM
- Jump to application entry point in RAM
For the STM32F103 example, this could look like: // Bootloader code void bootloader() { // Enable RAM, copy application SCB->VTOR = APPLICATION_RAM_ADDRESS; copy_application(); // Jump to application jump_to_application(); }
The bootloader is compiled and linked to execute from the reserved bootloader RAM region. It is programmed into the chip’s flash memory using STM32 programming tools.
Building Application for RAM
With the bootloader set up, the main application code can be compiled to execute from the application RAM space.
In the linker script, set the code and data sections to start at the defined application RAM address. For example: MEMORY { RAM (xrw) : ORIGIN = 0x20001000, LENGTH = 60K } SECTIONS { .text : { *(.text*) } > RAM .data : { *(.data*) } > RAM }
Compile the code with these linker settings. The resulting application binary will be linked to run from the dedicated RAM region.
Configuring Boot Options
Finally, configure the Cortex-M3 system control register to boot from RAM instead of flash: SCB->VTOR = APPLICATION_RAM_ADDRESS;
This points the reset vector to the application RAM address instead of flash. Combined with the bootloader, this will cause the system to load and execute the application from RAM on startup.
The vector table offset register (VTOR) can be configured in the bootloader startup code. Alternatively, many STM32 chips have configuration bits to set the VTOR value in flash memory.
Boot Sequence
With everything set up, the boot process will be:
- On reset, CPU fetches reset vector from application RAM address
- Bootloader initialization runs from dedicated bootloader RAM
- Bootloader copies application image to RAM
- Bootloader jumps to application entry point in RAM
- Application runs fully from RAM
This sequence bypasses flash access for improved performance. The application RAM space can also be preloaded for faster booting.
Optimizing RAM Utilization
Some optimizations can further improve RAM usage:
- Place only the minimum startup and initialization code in the bootloader.
- Use a binary application image instead of full .text and .data sections.
- Compress the application image to save RAM.
- Override weak functions like printf() to minimize bootloader footprint.
Careful planning of the memory map is key to maximizing available application RAM. The bootloader should be as small as possible.
Conclusion
Booting Cortex-M3 STM32F1 chips from RAM provides faster starts up by skipping slow flash reads. A small bootloader in RAM loads the application image and jumps to execution. With careful memory configuration, most of the available RAM can be used for the application.
RAM booting requires firmware changes but can improve performance-critical boot times. Other optimization techniques like overlaying can also help minimize flash usage. With some effort, RAM booting can enable more responsive waking from sleep and faster starts.
