Datasheet

#5 - GPIO #5

#6 - GPIO #6

#9 - GPIO #9, also analog input A7. This analog input is connected to a voltage divider for the lipoly battery so

be aware that this pin naturally 'sits' at around 2VDC due to the resistor divider

#10 - GPIO #10

#11 - GPIO #11

#12 - GPIO #12

#13 - GPIO #13 and is connected to the red LED next to the USB jack

A0 - This pin is analog

input

A0 but is also an analog

output

due to having a DAC (digital-to-analog converter).

You can set the raw voltage to anything from 0 to 3.3V, unlike PWM outputs this is a true analog output

A1 thru A5 - These are each analog input as well as digital I/O pins.

SCK/MOSI/MISO (GPIO 24/23/22)- These are the hardware SPI pins, you can use them as everyday GPIO pins

(but recommend keeping them free as they are best used for hardware SPI connections for high speed.

Other Pins!

RST - this is the Reset pin, tie to ground to manually reset the AVR, as well as launch the bootloader manually

ARef - the analog reference pin. Normally the reference voltage is the same as the chip logic voltage (3.3V) but if

you need an alternative analog reference, connect it to this pin and select the external AREF in your firmware.

Can't go higher than 3.3V!

SWCLK & SWDIO - These pads on the bottom are used to program the chip. They can also be connected to an

SWD debugger.