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-hole positions, 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 at boot. That self-identification is the part that makes a HAT more than a generic daughterboard: the Pi reads the EEPROM, learns which pins the HAT drives, and applies the correct device-tree overlay without the user editing config files.
Typical HATs add displays, sensors, real-time clocks, motor drivers, power management, GPS, or radios — often talking to the Pi over I2C or SPI so they consume only a couple of the header’s pins for data. Other SBCs use similar stackable add-ons under different names (capes on the BeagleBone, for example), but the Raspberry Pi HAT is the most widely supported ecosystem.
Anatomy of the standard
| Element | What the spec fixes | Why it matters |
|---|---|---|
| Form factor | Board outline, mounting holes | HATs and cases fit any Pi |
| Connector | 40-pin GPIO header | Consistent electrical pinout |
| EEPROM | ID + pin configuration | Pi auto-sets up the board |
| Data link | Usually I2C / SPI | Few pins, many peripherals |
| Power | Feeds from the Pi’s rails | No separate supply needed |
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 when the network is down, a fan and power controller for a sealed enclosure, or a GPS HAT that feeds a pulse-per-second signal in for precise timing on a capture node. Because HATs stack, one can carry several of these at once.
Sources
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Raspberry Pi HATs — Wikipedia, on the HAT add-on board standard. ↩