mercredi, mars 28, 2012

Linux on an 8-bit micro!

It is common to see newbies asking in microcontroller forums if they can run Linux on their puny little 8-bit micro. The results are usually laughter. It is also common to see, in Linux forums, asked what the minimum specs for Linux are. The common answer is that it requires a 32-bit architecture and an MMU and at least a megabyte of ram to fit the kernel. This project aims to (and succeeds in) shatter(ing) these notions. The board you see on the right is based on an ATmega1284p. I've made one with an ATmega644a as well, with equal success. This board features no other processor and boots Linux 2.6.34. In fact, it can even bring up a full Ububntu stack, including (if you have the time) X and gnome.from here


Yes, it is true that a full Linux install requires megabytes or RAM and a 32-bit CPU with an MMU. This project has all of that. First let's address the RAM. As you can see, there is an antique 30-pin SIMM memory module on the board. These were in use for 80286-based PCs. It is interfaced to the ATmega, and I wrote the code to access it as well as refresh it within spec (SDRAM requires constant refreshing to avoid losing data). How fast it is? The refresh interrupt happens every 62ms and takes up 1.5ms, thus eating under 3% of the CPU. RAM is accessed, for ease of programming, one byte at a time. This results in a maximum bandwidth of about 300 kilobytes per second.


With the RAM requirement put to rest, we have two to deal with. Storage is not too difficult a problem to solve. SD cards are quite easy to talk to using SPI, and my project does that. A 1GB SD card works fine, though 512Mb would be enough for this particular file system (Ubuntu Jaunty). The ATmega does have a hardware SPI module, but for whatever reason, it didn't quite work out, so I am bit-banging the interface. It is still plenty fast - about 200kilobytes per second. This also adds a nice touch to the project - it can be done on any microcontroller with enough pins - no hardware modules are used.


All that's left is that pesky 32-bit CPU & MMU requirement. Well the AVR has no MMU and is 8-bit. To conquer this obstacle, I wrote an ARM emulator. ARM is the architecture I am most familiar with, and it's simple enough that I could comfortably write an emulator for it. Why write one instead of porting one? Well, porting someone else's code is no fun, plus none of the emulators I saw out there were written in a way that would make them easy to port to an 8-bit device. One of the factors: AVR compiler insists on making ints 16-bit so something as simple as "(1 << 20)" will get you in trouble, producing zero. Instead you need to do "(1UL << 20)". Needless to say trawling someone else's unknown codebase looking for all places where ints are assumed and would fail would be a disaster. Plus I wanted a chance to write a nice modular ARM emulator. So I did.

Other features

The board's communication with the real world occurs over a serial port. Currently it is attached to a serial port on my PC running minicom, but it is fathomable to instead connect a keyboard and a character LCD to the board, making it entirely standalone. Two LEDs exist on the board as well. They signal SD card access. One for read, one for write. A button is onboard too. When pressed and held for a second it will spit out on the serial port the current effective speed of the emulated CPU. The AVR is clocked at 24MHz (a slight overclocking over its stock 20MHz)

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