Adafruit seesaw is a near-universal converter framework which allows you to add add and extend hardware support to any I2C-capable microcontroller or microcomputer.
Instead of getting separate I2C GPIO expanders, ADCs, PWM drivers, etc, seesaw can be configured to give a wide range of capabilities.
For example, the ATSAMD09 breakout with seesaw gives you
3 x 12-bit ADC inputs
3 x 8-bit PWM outputs
7 x GPIO with selectable pullup or pulldown
1 x NeoPixel output (up to 340 pixels)
1 x EEPROM with 64 byte of NVM memory (handy for storing small access tokens or MAC addresses)
1 x Interrupt output that can be triggered by any of the accessories
2 x I2C address selection pins
1 x Activity LED
But you can reprogram and reconfigure the chip to have more or less of each peripheral - as long as it fits into the ATSAMD09D14's firmware! For example, there's also a UART converter but it isn't included in the default firmware.
The ATSAMD09 breakout is great for development of seesaw capabilities (Adafruit use it in-house for their design work) or you can use it as-is to give your Raspberry Pi or ESP8266 more hardware support! Each breakout comes with the assembled and tested board, as well as some header strips.
Please note: The boards do not come with a bootloader. If you want to do development on seesaw (e.g. changing the configuration) you'll need to pick up a J-Link. At this time Adafruit's project is for Atmel Studio but you could probably get it working with arm gcc and a Makefile. They don't provide any support for custom builds of seesaw - they think this is cool and useful for the Maker community!
For more details including the documentation on how to use seesaw, libraries for Arduino/CircuitPython/Raspberry Pi Python, schematics, and more check out the Adafruit seesaw guide
TECHNICAL DETAILS
Uses I2C address 0x49 - can be changed to 0x4A, 0x4B or 0x4C
Product Dimensions: 32.0mm x 12.0mm x 3.5mm / 1.3" x 0.5" x 0.1"
Product Weight: 1.6g / 0.1oz
