When uploading with the Arduino IDE, I get a lot of "(expected 4, got -5)" warnings and then "avrdude: verification error; content mismatch"

Hmm I'm still having problems with Arduino/Avrdude - and I definitely did the required updates

One fix that works for some people is to edit avrdude.conf and set the
chip_erase_delay = 900000;
under the ATtiny85 heading to
chip_erase_delay = 400000;
That is, a shorter delay.

Can Gemma drive Neopixels (strips, squares, etc)? How many?

Yes! Gemma was designed to drive short segments of NeoPixels. There is enough RAM on the attiny85 to drive 100 pixels, but depending on program RAM usage you may have to scale back to 60 or 40.

To use with neopixels:
  1. Connect the + power line of the strip to Vout on the Gemma, or to a separate 4-7VDC power source such as a 3 or 4 pack of AA batteries.
  2. Connect the - common ground to the battery pack (if being used) and also to the Gemma GND pin
  3. Connect the data in line to Gemma #1 - this will let you also see when data is being sent because the #1 red LED will flicker. You can use other pins but start with #1 since its easiest to debug and use
  4. Install the NeoPixel library as detailed in our Uber Guide, and change the PIN to 1 (its 6 by default)
  5. Upload and enjoy!

Can Gemma use the Flora Lux/Compass/Accelerometer/Color/GPS Sensors?

Maybe! We think we can get Gemma working with some of the basic sensors, but at this moment we don't have tutorials or examples, Flora code will not compile directly for the Gemma since the processors are different. Right now we think Gemma is best suited for basic buttons/LEDs/Neopixels type stuff

Can Gemma drive your Adafruit I2C LED Backpacks for 7-segment/matrix displays?

Short answer: yes! Check out http://learn.adafruit.com/tap-tempo-trinket for a tutorial on driving the 7-segment displays. Long answer: we think there's not enough space for all of the fonts for the 8x8 so you might be able to drive the 8x8 matrix in 'raw' mode (see the HT16K33 example sketch in the LEDBackpack Library) but unfortunately not with built-in font support.

That tutorial also shows how to use the TinyM I2C driver, which works great on the ATtiny85, and adapt other existing libraries for the Gemma/Trinket

Can Gemma drive a Servo?

Yup! In fact you can use 3 servos as long as they are powered by a good 5V supply, check out this guide for more details

Gemma runs at 8MHz, but I really need it to run at 16 MHz, is this possible?

It is possible to run the Gemma at 16MHz, but the processor is not specified for 16MHz at 3.3V logic so it is considered overclocking!
However, the AVR series is pretty forgiving for overclocking, so you may be able to run the 3V Gemma at 16 MHz. Note that this is still overclocking, your code may run flakey or not at all! Overclocking should not damage the AVR, but we still recommend sticking with 8 MHz only if you can get away with it!

To run at 16Mhz, use the Trinket 16Mhz board definition and modify your sketch as described here.
Last updated on 2015-05-18 at 03.35.07 PM Published on 2013-09-13 at 10.39.52 AM