It's been done, but the the tutorials I find seems incomplete. What I'm trying to do is make the ATTiny84 talk to the 2.4 GHz NRF24L01+ radio. I understand that in order to do that I need the MCU to talk to the radio by serial. Attiny84 (and 85) does not have serial, hardware any ways.
But as I understand it you can do software serial by USI, but in order to do so the core for the board has to have this modification?
I've searched far and wide. I bump into the cores on Google Code (https://code.google.com/archive/p/arduino-tiny/downloads). I've installed them but SPI is not listed in the list of libraries. I've got another tinycore (from where I dont know), when I use this one SPI is a library I can reference, but compiling causes
error: 'SPIE' was not declared in this scope
So, could anyone point me to a known good source of cores to Attiny85 and 84 so I can enable SPI?