Programming flash memory is a crucial task for managing firmware and performing updates in embedded systems, especially when dealing with STM32 microcontrollers. One of the most effective ways to manage firmware updates in STM32 is through the use of a bootloader, a small piece of software that allows the microcontroller to load new application firmware, even without a debugger or external programmer. In this tutorial, we’ll explore how to program flash memory and create a bootloader for STM32, enabling firmware updates through various communication interfaces such as USB, SPI, or UART.
Programming flash memory is essential for firmware management and updates in embedded systems, Particularly STM32 microcontrollers.
Implementing a bootloader—a tiny piece of software that allows the STM32 microcontroller to load application firmware even in the absence of a debugger or external programmer—is a potent way to manage firmware updates. This tutorial will demonstrate how to program flash memory and create and implement a bootloader for the STM32 that allows firmware updates through various communication interfaces, such as USB, SPI, or UART.
Firmware is frequently stored in flash memory, a kind of non-volatile memory found in embedded systems. Application code can be stored in the flash memory that is integrated into STM32 microcontrollers. Bootloaders are useful because they provide a way to load new code when updating the firmware on a deployed device. A bootloader is a tiny program that launches at startup and is kept in a designated area of flash memory. If a fresh firmware image is available, it loads it into the main memory of the microcontroller. This enables USB firmware updates, over-the-air (OTA) updates, and communication via additional interfaces like SPI or UART.
Flash memory, usually segmented into sectors, is integrated into STM32 microcontrollers. It is possible to erase and rewrite each sector, but this is a slow process with a finite number of write cycles. Therefore, when implementing bootloaders and firmware updates, it is crucial to carefully manage the write and erase cycles.
Important Features:
The size of flash memory varies depending on the STM32 model, usually falling between 64KB and several megabytes.
Sector Size: Usually falls between 1 and 16 KB for each sector. Write/Erase Cycles: Each sector of flash memory can normally handle 10,000–100,000 write/erase cycles.
Flash Memory Layout: Usually, the STM32 bootloader is located at the start of the flash (from 0x08000000 or another specified location) or in the system memory.
The user application code typically starts at 0x08008000 and is stored after the bootloader.
3.1 Overview of the Bootloader
The function of the bootloader is to: Set up the fundamental hardware elements (clocks, USB, UART, etc.). Verify whether a firmware update request has been made (for example, when a particular button is pressed or when data is received via USB or UART). Remove the relevant flash segments if there is a legitimate firmware update. Update the flash memory with the new firmware. Check the firmware’s integrity (checksum or CRC are optional). Navigate to the flash memory-stored application code.
3.2 Example of Memory Layout
This is a typical STM32 flash memory layout: Bootloader: Usually kept at the beginning of flash memory (0x08000000, for example). Software (User Firmware): usually begins at 0x08008000 or a higher address.
A few kilobytes are used by the bootloader code, which leaves room for the application.
Features
Writing, erasing, and verifying flash memory are among the functions that the STM32 offers to manage flash memory programming.
4.1 Flash Sector Erasing
The target flash sector needs to be cleared before writing new data to it. The STM32 has hardware-specific features that allow it to delete entire flash memory sectors.
Writing data to the flash memory comes after it has been erased. Writing 32-bit data (words) to flash is possible with STM32.
5.1 Interfaces for Communication
The bootloader can communicate through a variety of interfaces for firmware updates, including: A popular interface for slow communication is UART.
USB: Another widely used technique for firmware updates is USB DFU (Device Firmware Upgrade) mode. SPI: Helpful for peripheral or external memory communication.
5.2 Flow of the Bootloader
Typical bootloader operations could include: Launching Initialization: Set up USB or UART to facilitate communication.
Verify whether the firmware update has been requested by checking for data or button presses.
Firmware Update: Clear the relevant flash sectors where the updated firmware is to be stored. Write the updated firmware that was obtained through USB, SPI, or UART.
Check for Integrity (optional): Use a checksum, CRC, or signature to confirm the received firmware’s integrity.
Go to the Application: Go straight to the user application code if the firmware update was successful.
Receiving firmware updates via UART is a popular technique. To start the update process, the bootloader can look for a UART command (such as a particular key or byte sequence).
The STM32 provides a flexible and effective method of managing firmware updates through the implementation of a bootloader and flash memory programming. Embedded systems can be easily updated and maintained by utilizing the STM32’s integrated flash memory programming capabilities and communication interfaces like USB and UART to create a reliable firmware update mechanism that operates in the field.