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NOTE All references to GPIO numbers within this document are referring to the BCM283x GPIOs (NOT pin numbers on the Pi GPIO header).
The Raspberry Pi boards with 40W GPIO headers (Model B+ and onwards) and have been designed specifically with add-on boards in mind. These boards support 'HATs' (Hardware Attached on Top). A HAT is an add-on board that conforms to the HAT specifications. HATs are not backward compatible with original Raspberry Pi 1 models A and B.
In October 2018 Raspberry Pi introduced the Micro-HAT (uHAT) specification. uHATs must follow all of the standard electrical HAT rules as laid our for normal HATs, but they have a smaller mechanical form factor as specified here
There are obviously a lot of add-on boards designed for the original model A and B boards (which interface to the original 26 way GPIO header). The first 26 pins of the 40W GPIO header are identical to those of the original models, so most existing boards will still work.
The biggest change with HAT add-on boards versus older boards designed for models A and B is that the 40W header has 2 special pins (IDSC and IDSD) that are reserved exclusively for attaching an 'ID EEPROM'. The ID EEPROM contains data that identifies the board, tells the Pi how the GPIOs need to be set up and what hardware is on the board. This allows the add-on board to be automatically identified and set up by the Pi software at boot time including loading all the necessary drivers.
While we cannot force anyone to follow our minimum requirements or HAT specification, doing so will make users lives easier, safer, and will make us more likely to recommend a product. Likewise if one of the minimum requirements is ignored we are unlikely to look on a product very favourably.
So why are we bothering with all this? Basically we want to ensure consistency and compatibility with future add-on boards, and to allow a much better end-user experience, especially for the less technically aware users.
Finally if you have any questions please head over to the forums to ask them.
If you are designing a new add-on board that takes advantage of the pins on the 40W GPIO header other than the original 26 then you must follow the basic requirements:
Note that for new designs that only use the original 26 way GPIO header pins it is still recommended to follow requirement 2. if the board supports back-powering a Pi.
A board can only be called a HAT if:
Of course users are free to put an ID EEPROM on boards that don't otherwise conform to the remainder of the specifications - in fact we strongly encourage this; we just want things called HATs to be a known and well-specified entity to make life easier for customers, particularly the less technical ones.
NOTE that the Pi3B+ introduced a new 4-pin PoE header near the top-right corner mounting hole. Newly designed HATs that do not provide a connector for this header must avoid fouling it.
Before designing any new add-on board (HAT compliant or not) please read the design guide carefully.
For what to flash into the ID EEPROM see the ID EEPROM data format spec.
There are tools and documentation on how to flash ID EEPROMs here.
Q: I want to keep shipping an existing board / ship a new board that only connects to the original 26W GPIO pins.
This is OK. You can't call it a HAT. If the board will back-power the Pi we recommend adding the safety diode as per requirement 2. of the basic add-on board requirements.
Q: I want to ship a board that attaches to the 40W GPIO header and covers IDSD and IDSC but does not include an EEPROM.
This is OK as long as it meets the basic requirements. You can't call it a HAT.
Q: I want to ship a board that has an ID EEPROM but does not conform to the remaining HAT specs.
This is OK as long as it also meets the basic requirements. You can't call it a HAT but you can say it supports GPIO autoconfiguration if the EEPROM contains valid vendor, GPIO map and DT blob information.
Q: I want to ship a HAT but the software for creating the EEPROM and/or DT blob isn't ready yet.
We expect all HATs to have a correctly programmed EEPROM, but bugs can happen, therefore make sure the EEPROM is user flashable. You will need to add some ability for a user to un-write-protect the EEPROM to (re-)flash it themselves as suggested in the design guide. Please provide instructions on your website / product packaging for how to reflash the board when any new image becomes available.
Q: I'm using the HAT mechanical spec but don't want to / can't add the cutout / slot for the display / camera flex.
This is OK and the board still conforms to the HAT specification. Some HATs will not be able to support the slot/cutout based on where the connectors and components must be placed (but it is recommended to support them if at all possible).
Q: I want to create a board that connects to the 'RUN' or 'PEN' header pin(s).
No problem but you can't call it a HAT. HATs are designed to be easy to use. Using the RUN pin requires a user to solder a header onto the Pi hence this is not something we wish to include in the HAT spec.