Arduino has shields. BeagleBone has capes. And now Raspberry Pi has HATs. The Raspberry Pi Foundation officially announced their specification for add-on boards for their new Model B+ single board computer, which just started shipping. HAT stands for “Hardware Attached on Top” and is meant to make it easier for the end user to add hardware to their Pi.
As with BeagleBone capes, the spec includes the physical layout of the boards and on-board I2C EEPROM memory to hold information about the manufacturer, GPIO setup, and device tree fragment, which is a way for Linux to properly configure the pins to use the hardware on the HAT. According to a post by James Adams, Director of Hardware at Raspberry Pi, manufacturers aren’t required to follow the spec, but warns that manufacturers cannot call their add-on board a HAT if it doesn’t follow the spec.
“We want to ensure consistency and compatibility with future add-on boards, and to allow a much better end-user experience, especially for less technically aware users,” said James.
The full specification is available on Github and includes diagrams, a design guide, ciruits for backpowering the Pi, and the structure for the data stored on the EEPROM.