The iCufflinks use an Atmel ATtiny4 microcontroller (MCU) as the brains to controlling the LED lighting pattern. The MCU is an 8-bit processor with 32 bytes of SRAM, only a handful of registers, and 512 bytes of flash for program storage. The stack is stored in the SRAM so you don’t really get to use it for anything.
The original hardware design and software are all open source and can be found on the Adafruit GitHub. One of the things about the design is that it runs on CR1220 batteries and it is recommended that they be changed after 24 hours of use. That is what got me thinking that I could improve this product to increase the amount of time between battery changes.
I have also never read nor written assembly code for an AVR processor and the last time I probably looked at assembly was 386 stuff about 20 years ago. So excuse any minor assembly style issues. I was temped to rewrite the code in C but with the limited flash space I had to rule this out. Had this been a ATtiny9 with 1k bytes I would have gone this route. The small overhead that AVR Studio introduces was just a tiny bit too much for this limited memory space.
This is a great example of how well open source hardware can work, Scott was able to recreate and improve our wearable electronics product(s) – it’s being modded and improved and it’s only been out a few weeks.