pihat

Pihat, often written as Pi HAT or HAT, refers to a class of add-on circuit boards designed to plug into the GPIO header of a Raspberry Pi to extend its capabilities. HATs are standardized by the Raspberry Pi Foundation and are used to provide a wide range of functions, including sensors, displays, motor drivers, audio interfaces, and more. They are commonly stacked on top of the Raspberry Pi using the 40‑pin GPIO header and may be accompanied by mounting hardware or enclosures.

How they work and the standard aspects: most HATs include an I2C-based ID EEPROM that stores metadata such as the board name, manufacturer, and pinout. This enables the Raspberry Pi’s operating system to auto-detect the device and apply appropriate device-tree overlays, simplifying setup. HATs typically operate at 3.3V logic and are designed to minimize conflicts with the Pi’s existing interfaces. The official HAT specification also addresses power requirements, mounting, and a standardized pin mapping to promote compatibility across models.

Variants and usage: the term pihat is often used to refer to Pi HATs in general, with pHATs being a related, smaller form factor designed for compact boards such as the Raspberry Pi Zero and certain 26‑pin configurations. Notable examples of HATs include the Sense HAT, which adds sensors and an LED matrix, and a variety of sensor, motor, and display HATs from third-party manufacturers. HATs are widely used in education, hobbyist projects, robotics, and embedded systems to expand Raspberry Pi functionality without custom wiring.