Electronics, Embedded Systems, and Software are my breakfast, lunch, and dinner.
In my most recent project I selected an ARM Cortex-M0 microcontroller (the STM32F042). I soon realized that there is a key architectural piece missing from the Cortex-M0 which the M0+ does not have: The vector table offset register (VTOR).
I want to talk about how I overcame the lack of a VTOR to write a USB bootloader which supports a semi-safe fallback mode. The source for this post can be found here (look in the "bootloader" folder):
As my final installment for the posts about my LED Wristwatch project I wanted to write about the self-programming bootloader I made for an STM32L052 and describe how it works. So far it has shown itself to be fairly robust and I haven't had to get out my STLink to reprogram the watch for quite some time.
The main object of this bootloader is to facilitate reprogramming of the device without requiring a external programmer. There are two ways that a microcontroller can accomplish this generally:
Each of these ways has their pros and cons. Option 1 allows for the user program to use all available flash (aside from the blob size and bootstrapping code). It also might not require a relocatable interrupt vector table (something that some ARM Cortex microcontrollers lack). However, it also means that there is no recovery without using JTAG or SWD to reflash the microcontroller if you somehow mess up the switchover into the bootloader. Option 2 allows for a fairly fail-safe bootloader. The bootloader is always there, even if the user program is not working right. So long as the device provides a hardware method for entering bootloader mode, the device can always be recovered. However, Option 2 is difficult to update (you have to flash it with a special program that overwrites the bootloader), wastes unused space in the bootloader-reserved section, and also requires some features that not all microcontrollers have.
Because the STM32L052 has a large amount of flash (64K) and implements the vector-table-offset register (allowing the interrupt vector table to be relocated), I decided to go with Option 2. Example code for this post can be found here:
**https://github.com/kcuzner/led-watch**