Thanks to Drew for sharing. Via Ken Shirriff’s blog
The BeagleBone is a inexpensive, credit-card sized computer with many I/O pins. These pins can be easily controlled from software, but it can be very mysterious what is really happening. To control a general purpose input/output (GPIO) pin, you simply write a character to a special file and the pin turns on or off. But how do these files control the hardware pins? In this article, I dig into the device drivers and hardware and explain exactly what happens behind the scenes. (Various web pages describe the GPIO pins, but if you just want a practical guide of how to use the GPIO pins, I recommend the detailed and informative book Exploring BeagleBone.)
This article focuses on the BeagleBone Black, the popular new member of the BeagleBoard family. If you’re familiar with the Arduino, the BeagleBone is much more complex; while the Arduino is a microcontroller, the BeagleBone is a full computer running Linux. If you need more than an Arduino can easily provide (more processing, Ethernet, WiFi), the BeagleBone may be a good choice.
The BeagleBone uses the Sitara AM3358 processor chip running at 1 GHz – this is the thumbnail-sized chip in the center of the board above. This chip is surprisingly complicated; it appears they threw every feature someone might want into the chip. The diagram below shows what is inside the chip: it includes a 32-bit ARM processor, 64KB of memory, a real-time clock, USB, Ethernet, an LCD controller, a 3D graphics engine, digital audio, SD card support, various networks, I/O pins, an analog-digital converter, security hardware, a touch screen controller, and much more. To support real-time applications, the Sitara chip also includes two separate 32-bit microcontrollers (on the chip itself – processors within processors!).
The main document that describes the Sitara chip is the Technical Reference Manual, which I will call the TRM for short. This is a 5041 page document describing all the feature of the Sitara architecture and how to control them. But for specifics on the AM3358 chip used in the BeagleBone, you also need to check the 253 page datasheet. I’ve gone through these documents, so you won’t need to, but I reference the relevant sections in case you want more details.