Can I run a program from a SD memory instead of flash on an evaluation board (embedded programming)?

Question

I have an evaluation board (Olimex STM32-P103) which supports a SD-card connector. I want to put my program in to a SD memory instead of internal flash of the micro-controler; and run it from there. I don't know if it is possible to do that according to boot-loader issue!

P.S my goal is running linux on this board and then port my application over it.

Answer 1

AS other says you cannot directly execute your code directly from the SD CARD. But like those "linux board", you can load the stored kernel/programm into an external SDRAM that can be mapped and execute it from there. You'll still need to write that "bootloader" and store it in the internal flash.

That'is a lot work to my opinion, for limited application. If you want to write your application in a linux environnement then port it suck small target, I would rather design my application using dependency injection, or even use an emulator.

Answer 2

The data on the SD card is not memory mapped, so cannot be executed directly.

It is possible to dynamically load the data from the card into RAM for execution. WindRiver's VxWorks RTOS supports loading and linking object modules dynamically, I know of no other OS that would scale to a Cortex-M that directly supports that but it would be possible to write your own.

However, I would suggest that in the case of the microcontroller you are using the idea is ill-advised; optimal performance on Cortex-M is achieved when the code is in on-chip flash and data in RAM allowing the data and instruction to be fetch to occur simultaneously on the separate buses (Harvard architecture). If you execute the code from RAM the performance will be severely hit since then data and instructions must be fetched sequentially over the same bus.

The board is entirely unsuited to running Linux, with only 128K Bytes Program Flash, and 20K Bytes RAM is is not at all feasible. Even the smallest Linux distribution requires 600Kb RAM plus whatever is needed by application code. uClinux can just about run on higher-end STM32 with external RAM and Flash, but that would suffer from the same bus contention performance hit and Linux without an MMU is rather missing the one major benefit of using Linux at all. The part on your board lacks an external memory interface, so cannot be expanded to support Linux.

If you need an OS consider a RTOS such as uC/OS-II, FreeRTOS, or emBOS for example.

Answer 3

To run programs from SD-Card in general you should know that you can't run them "right away". This means, you have to load it in a executable memory somewhere in your address space which is done by a (more or less) simple bootloader. In the simplest instance, the bootloader is capable to read from a SD-Card a specific binary and copy it into the memory.

That being said you should think about this considering you only got 20k of RAM and 128k of Flash on your board. So where should your program go? Or better: Why not flashing the program in the 128k of Flash from the very beginning? Especially you should know that Linux is a bit "hungry" in terms of memory.

If your goal is to run a "normal" Linux on this board, I'm afraid you're screwed. This because from what I know Linux needs a MMU to run and the chip on this board does not provide one (as far as researchable without access to datasheets from ST).

If you're lucky you can go with uCLinux. I'm not sure if a finished port exists for the STM32 but it seems there are some resources based on a short google search for "STM32 uCLinux". But even if you manage to run uCLinux I'm afraid there's not much left in your system for your application, so the result might be a bit disappointing.

Depending on why you are looking for Linux running on this MCU, there are maybe other solutions like a FreeRTOS in combination with a lwIP-stack (if networking is needed) or a FAT library like FullFAT if you are looking for reading SD-Cards and stuff.

Edit: One thing i'd like to add is that booting from the SD-Card is typically something you do with "bigger" (not much but slightly) systems where you have enough RAM to keep the whole image you'd like to run in it and still have some space left for the data you want to process.

Answer 4

You're going to have to have some code in the STM's onboard flash (typically called a "boot loader") that implements this since the "bare metal" very likely can't boot from SD card.

You're going to have to build that code, which figures out how to use the STM's onboard peripherals to talk to the SD card, finds the file you want to run in the file system (which you also have to implement), and loads it.

I wanted to include a link to the STM standard peripheral library, but it seems to be down (being moved). :/