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I see "HAT-Compatible" in marketing but a description is not easy to find. What is "HAT"? Is it an acronym for something? Is it simply the 40-pin pin out for connecting supported add-on modules?

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Hat compatible refers to the standard developed by the Pi Foundation for add on boards, and yes HAT it is an acronym for Hardware Attached on Top. You can read more about it here. The release coincided with the move to 40 pin boards B+. Quoting from the page linked above:

In a nutshell a HAT is a rectangular board (65x56mm) that has four mounting holes in the (nicely rounded) corners that align with the mounting holes on the B+, has a 40W GPIO header and supports the special autoconfiguration system that allows automatic GPIO setup and driver setup. The automatic configuration is achieved using 2 dedicated pins (ID_SD and ID_SC) on the 40W B+ GPIO header that are reserved for an I2C EEPROM. The EEPROM holds the board manufacturer information, GPIO setup and a thing called a ‘device tree‘ fragment – basically a description of the attached hardware that allows Linux to automatically load the required drivers.

So yes it is more than the 40 pin connector it includes the size and location of the mounting holes, how the board and its capabilities are identified and software is configured.

It remains to be seen if and how this changes given the recent release of the Pi zero which is much smaller than the size of current HAT standard.

  • I think it is true to say that HATs are inferior to Arduino shields. You can generally stack Arduino shields but I don't believe that to be the case for HATs. – joan Nov 30 '15 at 8:57
  • @joan your correct stacking is not supported by the HAT standard, but comparing it to an Arduino shield is a bit of an apples and oranges comparison. Besides the GPIO the Pi has USB and Ethernet (except on the model A and Zero boards) built in meaning that there is less of a need to stack addons. Also, stacking Arduino shields physically is usually not a problem, it can often present several issues with pin assignments etc. – Steve Robillard Nov 30 '15 at 13:17
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    I mentioned it as I think those from an Arduino background may assume HATs could be stacked. – joan Nov 30 '15 at 13:22

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