The Feather 32u4 FONA is chock-full of microcontroller goodness. There's also a lot of pins and ports. We'll take you a tour of them now!

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. Note that the cellular module is powed by VBAT so this will only disable the microcontroller
• 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

• #0 / RX - GPIO #0, also receive (input) pin for Serial1 and Interrupt #2
• #1 / TX - GPIO #1, also transmit (output) pin for Serial1 and Interrupt #3
  
• #2 / SDA - GPIO #2, 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. Also Interrupt #1
• #3 / SCL - GPIO #3, 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. Can also do PWM output and act as Interrupt #0.
• #5 - GPIO #5, can also do PWM output. Also connected to the FONA's DTR pin if you want to use it for powersaving functionality, which is not enabled by default
• #6 - GPIO #6, can also do PWM output and analog input A7. Also connected to FONA RTS in case you want to use flow control, which is not enabled by default
• #9 - GPIO #9, connected to FONA RXD.
• #10 - GPIO #10, also analog input A10 and can do PWM output.
• #11 - GPIO #11, can do PWM output.
• #12 - GPIO #12, also analog input A11 and can do PWM output.
• #13 - GPIO #13, can do PWM output and is connected to the red LED next to the USB jack
• A0 thru A5 - These are each analog input as well as digital I/O pins.
• SCK/MOSI/MISO - These are the hardware SPI pins. Also used to reprogram the chip with an AVR programmer if you need.

Cellular Module

Now to the fun part, the cellular module. THere's a few pins that are used to control the module. It uses SoftwareSerial to communicate with the microcoller

• #8 - used as the FONA TXD (data out from module to AVR). This pin is not exposed on the Feather
• #9 - used as the FONA RXD (data out from AVR to module)
• #7 - used as the FONA RI (ring interrupt) pin, you can use this to alert you when an SMS or phone call comes in. Note that this is interrupt #4 (but on pin #7). This pin is not exposed on the Feather
• #4 - used as the FONA Reset pin. You can pulse this pin low to reset the FONA, handy when starting up. This pin is not exposed on the Feather

Optional pins:

• #5 - This pin is available on the breakout and is also connected to the FONA's DTR pin if you want to use it for powersaving functionality, which is not enabled by default
• #6 - This pin is available on the breakout and is connected to FONA RTS in case you want to use flow control, which is not enabled by default

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!
• Key - this is by default tied to ground, cut the trace on the bottom and wire to a microcontroller pin to manually turn the module on and off. (Pulse low for a few seconds to change from on to off) This is the only way to truly disable the cellular module.
• Mic+ and Mic- connections for attaching an electret microphone for audio applications (external audio interface)
• Spk+ and Spk- connections for attaching a 8 ohm 1W speaker for audio applications (external audio interface)

FONA connections & LEDs

All the way to the right we have the cellular-only connection parts. Up top is a standard uFL connector, you attach your GSM antenna here

Below that is the bluetooth antenna (small white rectangle with red marking)

At the bottom are the two cell-status LEDs.

The middle PWR LED will light up green whenever the cellular module is active and has good power.

The NET LED will blink in blue let you know the status of the cellular connection You can use this for checking the current state without sending an AT command:

• 64ms on, 800ms off - the module is running but hasn't made connection to the cellular network yet

• 64ms on, 3 seconds off - the module has made contact with the cellular network and can send/receive voice and SMS

• 64ms on, 300ms off - the GPRS data connection you requested is active

By watching the blinks you can get a visual feedback on whats going on

On the bottom is a microSIM push-push holder. Slot your microSIM in here and press in until it clicks

There's also a spot you can solder an antenna if you want to use the FM receiver capability.

Above that is nicely labeled Mic/Speaker pads

At the very bottom is the jumper for the KEY pad - cut this to let you control the module's on/off key manually. By default KEY is tied to ground so the module is always powered and on.