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Looking at my Arduino Uno R3 board and its reference design schematic I noticed that there's a second ICSP 6-pin header. In the linked schematics, the connector I'm referring to is named ICSP1.

What's that for? What useful things can an average Arduino user like me do with it?

The relevant part of the schematic is below (marked in red).

Arduino Uno R3

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I've been wondering a similar thing, not so much what it's for, but what else we could do with it. It has to be asked: arduino.stackexchange.com/questions/991/… –  Madivad Mar 29 at 21:58
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up vote 6 down vote accepted

It's for the other MCU on the board.

The main MCU on the Uno is labelled "ZIC1". This is the one that is programmed when you press "Upload" in the IDE. But unlike other Arduinos that use a FTDI chip to connect to the serial port on the main MCU, the Uno uses another MCU instead, labelled "U3" on the left. This MCU runs a small bit of code that acts as a serial port via USB and passes through the bytes to its USART1. This then connects to the USART on the main MCU, where the bootloader on that chip accepts the bytes coming through and writes to flash and EEPROM on the main MCU.

You can use this additional MCU as you would any other, but the only I/O it has available are the USB connection, the SPI connection via its ICSP header, the UART1 connection to both the main MCU and pins 0 and 1, and two LEDs on PD4 and PD5.

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that's really interesting! Thanks for the info. +1 –  Ricardo Mar 29 at 19:56
    
I noticed this only yesterday after trying to utilise the TX and RX LEDs in my project. I pulled out the schematic to realise they weren't connected where I expected. –  Madivad Mar 29 at 21:32
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