Understanding the benefit of cape EEPROMs

At a glance, the CAT24C256 Electrically Erasable Programmable Read Only Memory (EEPROM) doesn't appear to add much value to the board. After all, the BeagleBone has a 2GB eMMC on the early revisions and a 4GB eMMC on revision C. An extra 256 kB of memory is hardly food scraps for the beagle. However, it serves a greater purpose; it's what enables automatic cape detection by the BBB.

The BBB has two 46 pin female expansion ports offering much more I/O capabilities than any other hobbyist board on the market. Certain pins can actually support eight different modes, mode 0 through mode 7. The mapping of pin features to a mode is known as pin muxing, short for pin multiplexing. To use a pin in a certain mode, the software must enable and configure this pin through the kernel's interface. This can be manually performed or scripted, but the easiest method is to use a BeagleBone cape.

During the kernel startup, the software will probe the...