Love Flora but want a bite-sized version? Look no further, Gemma is a tiny wearable platform board with a lot of might in a 1" diameter package. Powered by a Attiny85 and programmable with an Arduino IDE over USB, you'll be able to realize any wearables project!

We wanted to design a microcontroller board that was small enough to fit into any project, and low cost enough to use without hesitation. Perfect for when you don't want to give up your Flora and you aren't willing to take apart the project you worked so hard to design. It's our lowest-cost sewable controller!

The Attiny85 is a fun processor because despite being so small, it has 8K of flash, and 5 I/O pins, including analog inputs and PWM 'analog' outputs. We designed a USB bootloader so you can plug it into any computer and reprogram it over a USB port just like an Arduino (it uses 2 of the 5 I/O pins, leaving you with 3). In fact we even made some simple modifications to the Arduino IDE so that it works like a mini-Flora. Perfect for small & simple projects the Gemma will be your go-to wearable electronics platform.

Even though you can program Gemma using the Arduino IDE, it's not a fully 100% Arduino-compatible. There are some things you trade off for such a small and low cost microcontroller!

• Gemma does not have a Serial port connection for debugging so the serial port monitor will not be able to send/receive data
• Some computers' USB v3 ports don't recognize the Gemma's bootloader. Simply use a USB v2 port or a USB hub in between
• Gemma is not supported on Linux operating system at this time - try Mac OS or Windows!

Here are some useful specifications!

• Super small, only 1.1" / 28mm diameter and 0.28" / 7mm thick.
• Easy-to-sew or solder pads for embedding in your wearable project
• Low cost enough, you can use one for every weekend project
  
• ATtiny85 on-board, 8K of flash, 512 byte of SRAM, 512 bytes of EEPROM
• Internal oscillator runs at 8MHz
• Ultra low power, draws only 9 mA while running
  
• USB bootloader with a nice LED indicator looks just like a USBtinyISP so you can program it with the Arduino IDE (with a few simple config modifications)
• Mini-USB jack for power and/or USB uploading, you can put it in a box or tape it up and use any USB cable for when you want to reprogram.
  
• We really worked hard on the bootloader process to make it rugged and foolproof
  
• ~5.25K bytes available for use (2.75K taken for the bootloader)
• On-board 3.3V power regulator with 150mA output capability and ultra-low dropout. Up to 16V input, reverse-polarity protection, thermal and current-limit protection.
• Power with either USB or external output (such as a battery) - it'll automatically switch over
  
• On-board green power LED and red pin #1 LED
• Reset button for entering the bootloader or restarting the program.
  
• 3 GPIO - The 3 independent IO pins have 1 analog input and 2 PWM output as well.
  
• Hardware I2C capability for breakout & sensor interfacing.
  

Let me take you on a tour of your Gemma! Each Gemma is assembled here at Adafruit and comes chock-full of good design to make it a joy to use.

• Mini-B USB connector (on Version 1) - We went with the tried and true mini-B USB connector for power and/or USB bootloading on our first version of the Gemma
• Green Power LED - you'll know that the board is powered up when this bright LED is lit
• Red #1 LED - this LED does double duty. Its connected with a series resistor to the digital #1 GPIO pin. It pulses nicely when the Gemma is in bootloader mode, and its also handy for when you want an indicator LED.
• JST Battery Input - take your Gemma anywhere and power it from an external battery. This pin can take up 16V DC input, and has reverse-polarity, over-curent and thermal protections. The circuitry inside will use either the battery or USB power, safely switching from one to the other. If both are connected, it will use whichever has the higher voltage. Works great with a Lithium Polymer battery or our 3xAAA battery packs with a JST connector on the end
• Voltage Output - This pin will give you either the battery power or USB power, whichever has a higher voltage. Its great when you want to power something like NeoPixels, that might use more than the 150mA available from the onboard regulator
• 3V Regulator Out - The on-board voltage regulator can supply up to 150mA at a steady 3.3V from up to 16VDC
• Sewing friendly pads - You can easily sew to these pads, and they're gold plated so they wont corrode (oxidize). You can also use alligator clips or solder directly to them.
• GPIO! - 3 GPIO pins, at 3V logic, check the next section for a detailed pinout guide
• Reset Button - an onboard reset button will launch the bootloader when pressed and the Gemma is plugged into a computer. If it is not connected to a computer, it's smart enough to go straight to the program.

New in Version 2

• Micro-B USB connector (on Version 2) - In version 2 we upgraded to a micro USB connector, which is now the standard for all cell phones and other devices. It is used for power and/or USB bootloading on our first version of the Gemma
• On/Off switch (in version 2 only) - With the space we saved by moving to a smaller chip and micro USB connector we had enough space to add an on/off slide switch!

This diagram shows the physical size of the Gemma (diameter in mm) and the distance from pad to pad. The pads are at exact 90 & 45 degree angles except for 0 and 180 which is where the USB/JST connectors go

PDF For Gemma PrettyPins Diagram

JST Battery Input

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.

Power Pads

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

Input/Output Pads

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.

Secret Reset Pad

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.

One of the challenges with the Gemma is that we wanted to have a built-in USB bootloader, but the ATtiny85 doesn't have built-in USB hardware! There are existing USB bootloaders that can work on the 't85 but they use other companies' USB VID/PIDs. Since it not permitted by the USB developer's group to use others' VID/PIDs we had to adapt one of these existing bootloaders to use our USB ID, but we also wanted to not have to re-compile avrdude or the Arduino IDE since that's such a pain.

So instead, Frank (our awesome engineer with mad USB chops) created a USB bootloader that combines the elegance of V-USB with the well-supported and tested nature of the USBtinyISP. This bootloader looks just like a USBtinyISP - and since it uses the unique Adafruit VID/PID we own and that we added to avrdude so long ago, it works with only very minimal configuraton tweaks. No need to recompile anything, whew!

Don't forget to plug in the Gemma via a known-good USB cable to start the process. You should see the green power LED lit and the red bootloading LED pulse indicating that the Gemma is ready to start programming. If you've programmed the Gemma since getting it, you can always get it back to the bootloader state by pressing the small onboard reset button.

Special Notes on using Gemma with Linux

Linux is fairly picky about who can poke and prod at the USB port. You can always run avrdude or Arduino IDE as root, which will make sure you have the proper permissions. If you want to be super-cool you can add a udev rule which will let any user (who is not root) connect to the USBtiny driver. That way you don't have to be root all the time!

Check http://learn.adafruit.com/usbtinyisp/avrdude#for-linux for what to add to your udev file.

How to start the bootloader

Before you try to upload code to the Gemma it must be in the Bootloader Mode. That means its listening for a sketch or program to be sent to it

The Gemma must be connected to a computer via a USB cable to enter bootloader mode. You can enter the bootloader mode by pressing the little button on the board with your fingernail. The bootloader will 'time out' after 10 seconds, so to re-enter the bootloader mode just re-press the button!

Don't press-and-hold the reset button, be sure to press-and-release!

See the video below for what it looks like to plug it in, have the LED pulse in bootloader mode, time out and then press reset to restart the bootloader. The board shown is a Trinket, which uses the same upload system as Gemma.

Chances are, you picked up a Gemma because it is programmable with the Arduino IDE. Note that the Gemma is not a full Flora or Arduino-compatible, it uses a different (smaller) chip than the Flora, Uno, Mega, Leonardo or Due. However, there are many small sketches and libraries that will work just fine. Some may not even need anything other than pin number changes.

Arduino IDE Setup

Just follow the steps in the steps in the Adafruit Arduino IDE setup guide to easily install a pre-configured Arduino IDE to program Gemma!

If you are running Arduino IDE 1.6.4 or greater, you can also use this quickstart guide to add in the Gemma plugin

When you're finished installing the IDE come back to this page to continue the Gemma guide.

Blink!

After installing the Arduino IDE with support for Adafruit's boards you can load a simple blinking LED example to test uploading to Gemma works as expected.  Open the Arduino IDE and replace the sketch code with the following blink code:

/*
  Blink
  Turns on an LED on for one second, then off for one second, repeatedly.
 
  This example code is in the public domain.

  To upload to your  or Trinket:
  1) Select the proper board from the Tools->Board Menu (Arduino Gemma if
     teal, Adafruit Gemma if black)
  2) Select the uploader from the Tools->Programmer ("Arduino Gemma" if teal, 
	 "USBtinyISP" if black Gemma)
  3) Plug in the Gemma into USB, make sure you see the green LED lit
  4) For windows, make sure you install the right Gemma drivers
  5) Press the button on the Gemma/Trinket - verify you see
     the red LED pulse. This means it is ready to receive data
  6) Click the upload button above within 10 seconds
*/
 
int led = 1; // blink 'digital' pin 1 - AKA the built in red LED

// the setup routine runs once when you press reset:
void setup() {
  // initialize the digital pin as an output.
  pinMode(led, OUTPUT);

}

// the loop routine runs over and over again forever:
void loop() {
    digitalWrite(led, HIGH); 
    delay(1000);
    digitalWrite(led, LOW);
    delay(1000);
}

/*
  Blink
  Turns on an LED on for one second, then off for one second, repeatedly.
 
  This example code is in the public domain.

  To upload to your  or Trinket:
  1) Select the proper board from the Tools->Board Menu (Arduino Gemma if
     teal, Adafruit Gemma if black)
  2) Select the uploader from the Tools->Programmer ("Arduino Gemma" if teal, 
	 "USBtinyISP" if black Gemma)
  3) Plug in the Gemma into USB, make sure you see the green LED lit
  4) For windows, make sure you install the right Gemma drivers
  5) Press the button on the Gemma/Trinket - verify you see
     the red LED pulse. This means it is ready to receive data
  6) Click the upload button above within 10 seconds
*/
 
int led = 1; // blink 'digital' pin 1 - AKA the built in red LED

// the setup routine runs once when you press reset:
void setup() {
  // initialize the digital pin as an output.
  pinMode(led, OUTPUT);

}

// the loop routine runs over and over again forever:
void loop() {
    digitalWrite(led, HIGH); 
    delay(1000);
    digitalWrite(led, LOW);
    delay(1000);
}

Adafruit Gemma (Black Gemma)

If you're using the Adafruit Gemma (with a black PCB board) select the Adafruit Gemma 8MHz board from the Tools->Board menu.

Then, select USBtinyISP from the Tools->Programmer sub-menu

Arduino Gemma (Teal Gemma)

However if you're using the newer Arduino Gemma (with a teal PCB board) select the Arduino Gemma board from the Tools->Boards menu.

Then select Arduino Gemma as the Programmer type

Get Into Bootloader Mode

Plug in the Gemma, make sure you see the green LED lit (power good) and the red LED pulsing. Press the button if the red LED is not pulsing, to get into bootloader mode.

Click the Upload button (or select File->Upload)

If everything goes smoothly you should see the following (no red error messages) and of course, the red LED on the Gemma will blink on/off once a second

Something Went Wrong!

If you get the error message avrdude: Error: Could not find USBtiny device (0x1781/0xc9f)

That means the bootloader wasn't active. Make sure to press the button on the Gemma to activate the bootloader before clicking the Upload button.

If you get a lot of red text, errors and also a warning about Verification Failed

Check that you updated the avrdude.conf file above - if you don't update the description of the Attiny85 in the configure file by replacing it, the IDE wont know to be patient with the Gemma bootloader and will have many upload errors

On Linux if you get the error message "usbtiny_receive: error sending control message: Protocol error (expected 4, got -71)"

These can generally be ignored and should not interfere with the program upload.  Unfortunately Linux's USB core is a little flakey communicating with the ATtiny85 processor on the Trinket/Gemma and can cause these errors.  If an upload does fail, try it again as it is likely an intermittent issue.

Once you've gotten the basic Blink example to work, you can try some of the other Arduino functions and libraries. We'll be filling out this section with more example code and links to tutorials - this is just to get you started!

pinMode() & digitalWrite() & digitalRead()

You can use pinMode() to make inputs and outputs on any of digital pins #0 thru #2
digitalWrite also works well, and you can also use it with pinMode(INPUT) to activate the internal pull-up resistor on a pin

For example, to set up digital #0 as an input, with an internal pullup, and then check if it is being pulled to ground via a button or switch and turn on the red LED when it is pressed:

/*
  Button
  Turns on an LED when a switch connected from #0 to ground is pressed
 
  This example code is in the public domain.

  To upload to your Gemma or Trinket:
  1) Select the proper board from the Tools->Board Menu
  2) Select USBtinyISP from the Tools->Programmer
  3) Plug in the Gemma/Trinket, make sure you see the green LED lit
  4) For windows, install the USBtiny drivers
  5) Press the button on the Gemma/Trinket - verify you see
     the red LED pulse. This means it is ready to receive data
  6) Click the upload button above within 10 seconds
*/

#define SWITCH 0
#define LED 1

// the setup routine runs once when you press reset:
void setup() {
  // initialize the LED pin as an output.
  pinMode(LED, OUTPUT);
  // initialize the SWITCH pin as an input.
  pinMode(SWITCH, INPUT);
  // ...with a pullup
  digitalWrite(SWITCH, HIGH);
}

// the loop routine runs over and over again forever:
void loop() {
  if (! digitalRead(SWITCH)) {  // if the button is pressed
    digitalWrite(LED, HIGH);    // light up the LED
  } else {
    digitalWrite(LED, LOW);     // otherwise, turn it off
  }
}

/*
  Button
  Turns on an LED when a switch connected from #0 to ground is pressed
 
  This example code is in the public domain.

  To upload to your Gemma or Trinket:
  1) Select the proper board from the Tools->Board Menu
  2) Select USBtinyISP from the Tools->Programmer
  3) Plug in the Gemma/Trinket, make sure you see the green LED lit
  4) For windows, install the USBtiny drivers
  5) Press the button on the Gemma/Trinket - verify you see
     the red LED pulse. This means it is ready to receive data
  6) Click the upload button above within 10 seconds
*/

#define SWITCH 0
#define LED 1

// the setup routine runs once when you press reset:
void setup() {
  // initialize the LED pin as an output.
  pinMode(LED, OUTPUT);
  // initialize the SWITCH pin as an input.
  pinMode(SWITCH, INPUT);
  // ...with a pullup
  digitalWrite(SWITCH, HIGH);
}

// the loop routine runs over and over again forever:
void loop() {
  if (! digitalRead(SWITCH)) {  // if the button is pressed
    digitalWrite(LED, HIGH);    // light up the LED
  } else {
    digitalWrite(LED, LOW);     // otherwise, turn it off
  }
}

analogRead()

You can read an analog voltage from digital #2 (called A1)

For example, to read an analog voltage on pin #2, you would call analogRead(A1)

analogWrite()

There are a few PWM outputs on the Trinket, you can call analogWrite() on digital #0 and #1

For example, to pulse the built-in LED slowly, upload this code:

/*
  Pulse
  Pulses the internal LED to demonstrate the analogWrite function
 
  This example code is in the public domain.

  To upload to your Gemma or Trinket:
  1) Select the proper board from the Tools->Board Menu
  2) Select USBtinyISP from the Tools->Programmer
  3) Plug in the Gemma/Trinket, make sure you see the green LED lit
  4) For windows, install the USBtiny drivers
  5) Press the button on the Gemma/Trinket - verify you see
     the red LED pulse. This means it is ready to receive data
  6) Click the upload button above within 10 seconds
*/
 
int led = 1; // pulse 'digital' pin 1 - AKA the built in red LED

// the setup routine runs once when you press reset:
void setup() {
  // initialize the digital pin as an output.
  pinMode(led, OUTPUT);
}

// the loop routine runs over and over again forever:
void loop() {
  for (int i=0; i<256; i++) {
    analogWrite(led, i);  // PWM the LED from 0 to 255 (max)
    delay(5);
  }
  for (int i=255; i>=0; i--) {
    analogWrite(led, i);  // PWM the LED from 255 (max) to 0
    delay(5);
  }
}

/*
  Pulse
  Pulses the internal LED to demonstrate the analogWrite function
 
  This example code is in the public domain.

  To upload to your Gemma or Trinket:
  1) Select the proper board from the Tools->Board Menu
  2) Select USBtinyISP from the Tools->Programmer
  3) Plug in the Gemma/Trinket, make sure you see the green LED lit
  4) For windows, install the USBtiny drivers
  5) Press the button on the Gemma/Trinket - verify you see
     the red LED pulse. This means it is ready to receive data
  6) Click the upload button above within 10 seconds
*/
 
int led = 1; // pulse 'digital' pin 1 - AKA the built in red LED

// the setup routine runs once when you press reset:
void setup() {
  // initialize the digital pin as an output.
  pinMode(led, OUTPUT);
}

// the loop routine runs over and over again forever:
void loop() {
  for (int i=0; i<256; i++) {
    analogWrite(led, i);  // PWM the LED from 0 to 255 (max)
    delay(5);
  }
  for (int i=255; i>=0; i--) {
    analogWrite(led, i);  // PWM the LED from 255 (max) to 0
    delay(5);
  }
}

More...

We also know the following libraries work:

• Adafruit NeoPixel - control up to ~150 Neopixels via a Trinket!
• SoftwareSerial - the built in SoftSerial library can (at least) transmit data on any digital pin.
• More as we do more testing and verification!
  

Datasheets

• Datasheet for the onboard regulator used (MIC5225 3.3V)
• Webpage for the ATtiny85, the microcontroller used in the Gemma
• EagleCAD PCB files on GitHub
• Fritzing object in Adafruit Fritzing Library

Source code

Original code for the Trinket/Gemma bootloader on github
We do not offer any support for this code, it is released as-is!

Files

• PDF For Gemma PrettyPins Diagram

Schematic