Deprecation Warning: The Pro Trinket bit-bang USB technique it uses doesn't work as well as it did in 2014, many modern computers won't work well. So while we still carry the Pro Trinket so that people can maintain some older projects, we no longer recommend it. Please check out the Metro Mini (ATmega328 @ 5V 16 MHz), ItsyBitsy 32u4 5V 16MHz, ItsyBitsy 32u4 @ 3.3V 8MHz or ItsyBitsy M0 @ 3V 48MHz. All have built-in USB and are comparable in price! The ItsyBitsy's especially are about the same size and have native USB and tons of pins, so they're a very close compatible.
The Pro Trinket 3V is a small microcontroller with the familiarity of the core Arduino chip, the ATmega328. It's like an Arduino Pro Mini with more pins and USB tossed in. Adafruit designed Pro Trinket, with 18 GPIO, 2 extra analog inputs, 28K of flash, and 2K of RAM.
Like the Trinket, it has onboard USB bootloading support - we opted for a MicroUSB jack this time. We also added Optiboot support, so you can either program your Pro Trinket over USB or with a FTDI cable just like the Pro Mini and friends.
The Pro Trinket PCB measures only 1.5" x 0.7" x 0.2" (without headers) but packs the same capability as an Arduino UNO. So it's great once you've finished up a prototype on an official Arduino UNO and want to make the project smaller.
The Pro Trinket 3V uses the Atmega328P chip, which is the same core chip in the Arduino UNO/Duemilanove/Mini/etc. However, it uses 3V logic and 12MHz clock which means it's good when you want a lower-power and lower-voltage setup. Pro Trinket is programmable using the Arduino IDE as you already set up, and 95% of Arduino projects will work out of the box!
Here's some things you may have to consider when adapting Arduino sketches:
Here's some handy specifications: