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I'm working on a custom Cortex-M3-based device and I need to implement in-application programming (IAP) mechanism so that it will be possible to update the device firmware without JTAG (we'll use TFTP or HTTP instead). While the IAP-related code examples available from ST Microelectronics are clear enough to me, I don't really understand how the re-flashing works.
As far as I understand, the instructions are fetched by the CPU from the Flash through the ICode bus (and the prefetch block, of course). So, here's my pretty silly question: why doesn't the running program get corrupted while it re-flashes itself (i.e. changes the Flash memory from which it is being run)?
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The Cortex-M3 processor is specifically developed for high-performance, low-cost platforms for a broad range of devices including microcontrollers, automotive body systems, industrial control systems and wireless networking and sensors.
The processor supports two modes of operation, Thread mode and Handler mode: The processor enters Thread mode on Reset, or as a result of an exception return. Privileged and Unprivileged code can run in Thread mode. The processor enters Handler mode as a result of an exception.
For beginners in embedded programming, using C language for software development on the Cortex-M3processor is the best choice. It is to be noted that the use of the C language can often speed up application development.
Arm® Cortex®-M3 in a nutshell. The 32-bit Arm® Cortex®-M3 core processor is designed for high-performance, real-time processing in cost-constrained applications and can handle complex tasks. Any Arm® Cortex®-M3 microcontroller offers high scalability combined with an optimal trade-off between performance and cost.
A common solution is to have a small reserved area in the flash, where the actual flashing program is stored. When new firmware has been downloaded just make a jump to the code in this area.
Of course, this small area is not overwritten when flashing firmware, it can only be done by other means (like JTAG). So make sure this flashing-program works good to start with. :)
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I'm not familiar with STM implementation, but in NXP chips the IAP routines are stored in a separate, reserved ROM area which can't be erased by user code.
If you're implementing the flash writing code yourself by using HW registers directly, you need to either make sure that it doesn't touch the sectors it's running from, or runs from the RAM.
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