readme update and standalone build

Author: benedekkupper

.github/workflows/build.yml

@@ -0,0 +1,47 @@
+name: Build bootloader
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install Arm GNU Toolchain (arm-none-eabi-gcc)
+        uses: carlosperate/arm-none-eabi-gcc-action@v1
+        with:
+          release: latest
+          path-env-var: GNUARMEMB_TOOLCHAIN_PATH
+
+      - name: Normalize GNUARMEMB_TOOLCHAIN_PATH
+        run: |
+          # Ensure the path ends with a trailing slash so CMake preset prefix works
+          echo "GNUARMEMB_TOOLCHAIN_PATH=${GNUARMEMB_TOOLCHAIN_PATH%/}/" >> $GITHUB_ENV
+
+      - name: Install build dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y cmake ninja-build
+
+      - name: Configure
+        run: cmake --preset bluepill
+
+      - name: Build
+        run: cmake --build build/bluepill
+
+      - name: Upload firmware artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: firmware
+          path: |
+            build/bluepill/bluepill-can-bl.elf
+            build/bluepill/bluepill-can-bl.hex

README.md

@@ -1,13 +1,22 @@
 # Bluepill CAN Bootloader
 
-A minimal CAN bootloader for Blue Pill STM32F103C8T6 boards. It occupies only the first 2 kB of flash, leaving ~62 kB free for the user application. The bootloader speaks a compact CAN protocol with segmentation identical to the Bluetooth ACS scheme and integrates cleanly with Zephyr via sysbuild; the `app/` folder shows a minimal Zephyr client that can request an update.
+> *The cheapest of bootloaders for the cheapest of STM32 boards!*
+
+If you are shopping for the cheapest STM32 that can do CAN, you might just end up with the STM32F103C8T6 chip,
+which is also prominently featured on the famous [*Blue Pill*](https://stm32-base.org/boards/STM32F103C8T6-Blue-Pill.html) boards.
+Once you start developing the firmware, you might quickly find that 64kB of flash code space
+is not that much. Then you realize that you still need a bootloader for firmware update,
+so you search around / try to put something with an LLM, and end up with a quarter of the flash
+reserved for the bootloader.
+
+**How about this instead: a CAN bootloader that only takes up 2kB of flash, and is effortlessly integrating into your zephyr RTOS application?**
 
 ## Highlights
 - 2 kB flash footprint; rest is available for the application.
-- Simple CAN protocol with 1 kB page transfers and ACS-style segmentation.
+- Simple CAN protocol using Bluetooth ACS packet segmentation scheme.
 - Default CAN IDs: prefix `0x7E5` (updater: `prefix+0`, loader: `prefix+1`); standard frames by default (IDE=0).
-- App validity ensured by flashing MSP and reset vector **after** all payload pages are written.
-- Zephyr module with sysbuild application example; shell command `image update` triggers bootloader mode.
+- Application integrity ensured by flashing MSP and reset vector **after** all payload pages are written.
+- Zephyr module with full sysbuild integration, see [app](./app) example (try `image update` shell command).
 
 ## Flash layout
 - Bootloader: 0x0800_0000 – 0x0800_07FF (2 kB)
@@ -53,6 +62,13 @@ Notes:
 - The sample enables shell and adds `image update` ([app/src/main.cpp](app/src/main.cpp)) which sets the update request flag and reboots.
 - Partitioning comes from [zephyr/bluepill_partitions.overlay](zephyr/bluepill_partitions.overlay).
 
+## Non-zephyr application
+
+When building a non-zephyr application, two things have to be adapted for bootloader support
+(this is true in general):
+1. Modify the linker script to ensure the application's flash area starts where the bootloader's ends.
+2. Write the flash start address to the VTOR register (update vector table address).
+
 ## Update flow
 1. Application sets the update request flag (see `can_bl::set_update_request()` or run `image update` in the sample shell), then resets.
 2. Bootloader stays if no valid app or an update was requested; otherwise it jumps to the app.