The Feather 328P is chock-full of microcontroller goodness. There's also a lot of pins and ports. We'll take you a tour of them now!

PDF of above pinout image available here.

Power Pins

• GND - this is the common ground for all power and logic
• BAT - this is the positive voltage to/from the JST jack for the optional Lipoly battery
• USB - this is the positive voltage to/from the micro USB jack if connected
• EN - this is the 3.3V regulator's enable pin. It's pulled up, so connect to ground to disable the 3.3V regulator
• 3V - this is the output from the 3.3V regulator, it can supply 500mA peak

Logic pins

This is the general purpose I/O pin set for the microcontroller. All logic is 3.3V

Serial Pins

These pins are the only hardware Serial pins, you shouldn't connect to them unless you absolutely have to - they're shared with the onboard USB-Serial chip!

• RX / #0 - GPIO #0, also receive (input) pin for Serial - Used for debug and bootloading, do not use these unless you're positive you mean to!
• TX / #1 - GPIO #1, also transmit (output) pin for Serial - Used for debug and bootloading, do not use these unless you're positive you mean to!
  

I2C Pins

• SDA / A4 - Analog #4, also the I2C (Wire) data pin. There's no pull up on this pin by default so when using with I2C, you may need a 2.2K-10K pullup.
• SCL / A5 - Analog #5, also the I2C (Wire) clock pin. There's no pull up on this pin by default so when using with I2C, you may need a 2.2K-10K pullup.

SPI Pins

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. Also used to reprogram the chip with an AVR programmer if you need.

• SCK / #13 - SPI Clock and GPIO #13, can do PWM output and is connected to the red LED next to the USB jack
• MOSI / #11 - SPI Microcontroller Out Serial In and GPIO #11, can also do PWM output
• MISO / #12 - SPI Microcontroller In Serial Out and GPIO #12

General Purpose I/O Pins

• #2 - GPIO #2, also Interrupt #0
• #3 - GPIO #3, can also do PWM output and act as Interrupt #1
• #4 - GPIO #4
• #5 - GPIO #5, can also do PWM output
• #6 - GPIO #6, can also do PWM output
• #7 - GPIO #7
• #9 - GPIO #9, can do PWM output
• #10 - GPIO #10, can do PWM output

Analog Inputs

• A0 thru A3 - These are each analog input as well as digital I/O pins.
• A6 - This is a special analog input only pin! You cannot use it for digital in/out! 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
• A7 - This is a special analog input only pin! You cannot use it for digital in/out!

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!