Also known as: HAT, Hardware Attached on Top
A HAT (Hardware Attached on Top) is an add-on board that stacks onto a Raspberry Pi’s GPIO header to add hardware.1
Overview
The HAT specification fixes the board size, mounting holes, and 40-pin connector so add-ons fit predictably, and a small EEPROM on the HAT can identify itself so the Pi auto-configures the right pins. Typical HATs add displays, sensors, real-time clocks, motor drivers, power management, or radios — often talking to the Pi over I2C or SPI. Other SBCs use similar stackable add-ons under different names.
Where it fits
A HAT is the quickest way to extend a board without designing your own electronics — a middle ground before committing to a custom carrier for a Compute Module. For GopherTrunk, a HAT can supply the parts a bare Pi lacks at the antenna: a real-time clock for accurate timestamps, a fan and power controller, or a GPS HAT for precise timing on a capture node.
Sources
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Raspberry Pi HATs — Wikipedia, on the HAT add-on board standard. ↩