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.A1 thru A5 - These are each analog input as well as digital I/O pins.You can set the raw voltage to anything from 0 to 3.3V, unlike PWM outputs this is a true analog output A0 - This pin is analog input A0 but is also an analog output due to having a DAC (digital-to-analog converter).#13 - GPIO #13 and is connected to the red LED next to the USB jack.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 There's no pull up on this pin by default so when using with I2C, you may need a 2.2K-10K pullup. #21 / SCL - GPIO #21, also the I2C (Wire) clock pin.

#20 / SDA - GPIO #20, also the I2C (Wire) data pin.#1 / TX - GPIO #1, also transmit (output) pin for Serial1, also can be analog input.#0 / RX - GPIO #0, also receive (input) pin for Serial1 (hardware UART), also can be analog input.This is the general purpose I/O pin set for the microcontroller.

Underclock adafruit trinket 5v