There is no battery INPUT pin on the Gemma. You can connect a battery via the JST jack. We have found that Lipoly batteries, coin-cells, and AAA's work great. You can also make your own battery input pack using a plain JST cable. And use a JST extension cable if necessary.
You can plug anything from around 4VDC up to 16VDC, but we suggest 4-6V since higher voltages just get wasted as heat. This input is polarity protected. If the green PWR LED lights up, you're good to go. There is no off switch on the Gemma, so unplug or switch off the battery pack when done.
Half of the pads on the Gemma are related to power in and out: 3Vo , Vout and GND
- Vout - This is a voltage OUTPUT pin, it will be connected to either the USB power or the battery input, whichever has the higher voltage. This output does not connect to the regulator so you can draw as much current as your USB port / Battery can provide (in general, thats about 500mA)
- 3Vo - This is the 3.3V OUTPUT pad from the voltage regulator. It can provide up to 150mA at a steady 3.3V. Good for sensors or small LEDs or other 3V devices.
- GND is the common ground pin, used for logic and power. It is connected to the USB ground and the power regulator, etc. This is the pin you'll want to use for any and all ground connections
Next we will cover the 3 GPIO (General Purpose Input Ouput) pins! For reference you may want to also check out the datasheet-reference above for the core ATtiny85 pin
All the GPIO pins can be used as digital inputs, digital outputs, for LEDs, buttons and switches etc. They can provide up to 20mA of current. Don't connect a motor or other high-power component directly to the pins! Instead, use a transistor to power the DC motor on/off
On a Gemma, the GPIO are 3.3V output level, and should not be used with 5V inputs. In general, most 5V devices are OK with 3.3V output though.
The 3 GPIO pins are completely 'free' pins, they are not used by the USB connection so you never have to worry about the USB interface interfering with them when programming
- Pad #0 - this is connected to PB0 on the ATtiny85. This pin can be used as a PWM output, and is also used for I2C data, and SPI data input.
- Pad #1 - this is connected to PB1 on the ATtiny85. This pin can be used as a PWM output, and is also used for SPI data output. This pin is also connected to the onboard LED (like pin 13 on a regular Arduino).
- Pad #2 - this is connected to PB2 on the ATtiny85. This pin can be used as an analog input (known as Analog A1), and is also used for I2C clock and SPI clock.
On the off chance you want to reprogram your Gemma with an AVR burner, the bottom of the board has a large pad that is connected to the Reset pin. We use it for testing and you will likely never need it but it is there if you do.