I have switched from plain Arduino to bare ATtiny and ATmega chips about two months ago. It was a difficult extremely difficult uphill battle - instructions on internet are fragmented, different schematics are available for the same method... I lost at least two weeks because of all the inconsistencies.
All the time I was making notes and I came to a point that I feel comfortable to publish them online so I might save some novice a week of wondering around. But, before I publish them, I have to clear up several things:
Hardware (SPI) upload method:
1. What is the difference between ArduinoISP and Arduino as ISP in Arduino IDE?
2. Does uploading sketch with this method overwrites the bootloader?
3. Why is it necessary to burn fuses (burn bootloader procedure) for a new ATtiny chip and not for a new ATmega chip, even if bootloader is not used?
Bootloader (UART,USB) upload method:
4. At the beginning of the UART transmission, DTR changes its state and the 100 nF capacitor converts this change into a pulse at RESET pin. Is that correct?
5. Does any of programmers had to be selected in Arduino IDE before the transmission or is this selection ignored?
6. Does "burn bootloader" procedure always include burning fuses?
7. It seems that Micronucleus bootloader does not use RESET pin. So it wouldn't be reasonable to turn RESET pin in ATtiny85 to standard pin if only Micronucleus is going to be used?
8. It seems that different pins are used by Micronucleus for ATtiny85 and ATtiny84 chips. Is that right?
If someone can spare five minutes, you are very welcome to read what I have written so far and check if there are mistakes - quite possible they are. The URL is https://www.pinteric.com/avr.html
Thanks for help and best regards.