STM8 Bootloader Template Project

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Overview

This project provides a flexible bootloader implementation for STM8 microcontrollers using the SDCC compiler. The bootloader resides in the option bytes reserved area and enables in-application programming (IAP) capabilities via UART communication.

Note: The bootloader implementation is based on and inspired by the STM8uLoader project, with significant modifications and enhancements for integration into this template structure.

Features

• Three-stage Bootloader:

  • Boot0: Reset/TRAP isr for load boot1
  • Boot1: Minimal bootloader stored in reserved option bytes
  • Boot2: Full-featured bootloader loaded via serial communication

• Flexible Configuration:

  • Enable/disable bootloader via ENABLE_OPTION_BOOTLOADER Makefile macro

• Complete Toolchain:

  • PC-side programming utility for firmware updates
  • Support for read, write, go and exec operations

• Safe Operation:

  • Bootloader integrity protected in option bytes
  • Fallback to application on timeout or communication failure

Bootloader Reference

This project's bootloader implementation is derived from the excellent STM8uLoader project by ovsp. Key adaptations include:

• Integration into a modular template project structure
• Three-stage bootloader approach (Boot1 in option bytes, Boot2 loaded dynamically)
• Enhanced Makefile system with configuration macros
• Extended command set and error handling

Bootloader Operation Flow

1. Power-on/Reset:

   • MCU starts execution at reset vector(0x8000)
   • Control transfers to bootloader_enter()

2. Stage 1 (Boot1):

   • Copies Boot1 from option bytes (0x480E-0x483F) to RAM and Run
   • Sends synchronization sequence 0x00 0x0D via UART
   • Waits for PC to send Boot2 code

3. Stage 2 (Boot2):

   • Receives and validates Boot2 code from PC
   • Executes Boot2 which provides full command interface
   • Processes PC commands for programming, reading, and device control

4. Application Start:

   • On successful programming or timeout, jumps to main application(0x8004)
   • Option to stay in bootloader mode for debugging

Bootloader Integration

1. The main application includes bootloader.h, which redirects the TRAP interrupt vector to bootloader_enter() in bsp/boot0.c.

2. During the build process, the Makefile swaps the reset vector (0x8000) and trap vector (0x8004) positions. This ensures that upon startup, the bootloader entry routine executes first.

3. It sends a handshake signal (0x00 0x0D) via UART1 and waits approximately 200ms for a response. Within the timeout period, execution proceeds to the main application.

Building the Project

Prerequisites

• SDCC (Small Device C Compiler) installed
• stm8flash or similar programming tool
• Python 3.x (for PC tools)

Compilation Options

Enable bootloader support:

make ENABLE_OPTION_BOOTLOADER=1

Disable bootloader (direct application execution):

make ENABLE_OPTION_BOOTLOADER=0

Build Targets

# Build both application and bootloader option
make all

# Program device (requires stm8flash)
make flash

Option Bytes Configuration

The bootloader uses the reserved option byte area for storage:

Address Range	Content	Size
0x4800-0x480A	Device option bytes	11 bytes
0x480D-0x483F	Boot1 code	51 bytes

Important: These addresses are specific to STM8S103/003. Adjust for other STM8 variants.

PC Communication Protocol

Connection Parameters

• Baud Rate: 128000 bps
• Data Bits: 8
• Parity: None
• Stop Bits: 1

Command Set

Command	Opcode	Description
CMD_READ	0xF1	Read memory from device
CMD_WRITE	0xF2	Write memory to device
CMD_GO	0xF3	Jump address for execution
CMD_EXEC	0xF4	Execute Machine-Code

Communication Sequence

1. Boot1 sends sync bytes: 0x00 0x0D
2. PC responds with Boot2 reversed bytes
3. Boot1 receves Boot2 and checksum
4. Boot2 executes and prepare to receive cmd
5. PC sends commands with appropriate parameters

Usage Example

Programming New Firmware

# 1. Build the application
make flash

# 2. Enter interactive mode
python scripts/stm8loader.py /dev/ttyUSB0

# 3. Use command line:
python scripts/stm8loader.py /dev/ttyUSB0 --write 0x8000 firmware.bin

Supported Devices

Currently tested with:

• STM8S103/003 (default configuration)

Important: To port to other STM8 variants need verify peripheral register definitions

Troubleshooting

Common Issues

1. No response from device:

   • Verify baud rate settings
   • Check UART pin connections (TX/RX swapped?)
   • Ensure option bytes are correctly programmed

2. Bootloader not starting:

   • Verify ENABLE_OPTION_BOOTLOADER is set during compilation
   • Check main.c include bootloader.h
   • Check bsp/boot0.c
   • Confirm option bytes

Safety Considerations

1. Power Stability: Ensure stable power supply during programming
2. Watchdog Timer: Disable or properly handle watchdog in bootloader
3. Interrupts: Save/restore interrupt context during bootloader operations
4. Memory Protection: Never overwrite bootloader area in option bytes

References

• STM8uLoader
• STM8S Reference Manual
• STM8 CPU programming manual
• SDCC User Guide
• STM8 Bootloader AN2659

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Note: This implementation is for educational and development purposes. Always verify bootloader behavior in your specific application context before deployment.