I've been wondering for a long time what these pins, labeled ICSP and blank, do.


I can't seem to find a diagram that explains them, so I've been wondering:

  • What are they for?
  • Are they GPIO, or do they do something special?
  • How can you use them?

Thanks for the help.

1 Answer 1


They're for programming, as well as carrying the non-select signals for SPI. The right one is for programming the main MCU, the left for the USB communication MCU. You should almost never need to use them unless you need to write a bootloader to a new chip or change the fuse or lock settings on them.

  • 2
    I'd like to add that the pins on the right header are VCC, GND, RESET, and Digital pins 11, 12, 13. So it's not like you can use them if you run out of digital pins.
    – Gerben
    Jan 19, 2015 at 14:36
  • There is a case where a "normal" user use the once at the back. That is when you use a shield that connects to those pins. An example of such a shield is the ethernetshield. The tag (that is what these 6 pins are called) at the end is actually added to fix a compatibility issue between the mega and uno.
    – jantje
    Feb 28, 2016 at 13:45

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