BCM2835-ARM-Peripherals
http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
http://www.raspberrypi.org/archives/615
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We've been leaning (gently and charmingly) on Broadcom, who make BCM2835, the SoC at the heart of the Raspberry Pi, to produce an abbreviated datasheet describing the ARM peripherals in the chip. If you're a casual user, this won't be of much interest to you, but if you're wanting to port your own operating system or just want to understand our Linux kernel sources, this is the document for you.