Will this work in an ATtiny85?

[Al-eXs]

Hi, I was just wondering if this would work on an ATtiny85, something like the Digistump's Digispark or the Adafruit's Trinket.

I didn't find anything like this for that kind of AVR boards.

It's interesting to use them as they have the right amount of I/O pins for the job, a compact size and just the right amount of flash and SRAM.

I just want to know if this could work with this tines, what changes should be made to the code.

[raphnet]

It looks possible with some work. I had a look at the project current code size and indeed it should fit if you remove the bootloader. Otherwise if you were to strip stuff you don't need (mouse support and raw I2C access maybe?) you could probably save enough space to keep the loader.

Some code adaptation would be required of course. Based on a quick compilation test without detailed analysis, I spotted the following:

• This chip does not have a TWI controller. But it has a USI instead which seems to support I2C. Either rewrite the I2C part to use the USI or do it with bit-banging.
• The IO ports are different (not PORTD nor PORTC on this chip)
• You need to make sure the clock frequency is supported by V-USB. It's not clear from the digispark page what clock source is used by default. I assume the high speed PLL clock source (16MHz) would be usable.
• The ATTint85 timer registers seem different.

[Al-eXs]

I've been trying to figure it out, and this is what I see after researching some things from another similar projects that use V-USB.

• The IO port seem to be straight forward, convert/merge the usage of PORTD and PORTC on the Mega to only PORTB on the Tiny.
• The clock frequency can be set to 12, 16 or 16.5 MHz without problem.
• The timers seem different but work in a similar way, need to test some things.
• The I2C code is the only part that I'm missing on most, I just have the basic knowledge of C and have hard time to figure out how to convert that part. At the moment I need to modify i2c.c and i2c_gamepad.c to test if the changes I made to the IO and timer parts are OK.

I find that there is a library to use I2C on the Tiny AVR's but I can't understand how to use here.