user85606: I use a homemade board that hosts a PIC16F1829 and an ATmega328P on the same board.  We use the ATmega328P with the Arduino IDE and the PIC with the MPLABX IDE for about 100 EE students a semester at UTSA.  I have 100 pic chips but no ATmega328P chips, I can re-spin the board and adapt it to the ATmega8-U2, since right now I can by 250 ATmega8-U2 chips that are in stock but there are no ATmega328P chips available anywhere and I need 100 at least. I am currently using a different UART to TTL adapter, the CP2102 to connect the ATmega328P to the Arduino IDE.  I have the 2 chips talk via a shared I2C line. How difficult will it be to use the Arduino IDE with the ATmega8-U2?  (Again, not as the UART to USB interface but as the main processor.  Thanks,

user7508: I never tried, but it seems to me it should work without issues.
In the Arduino IDE, Menu Tools → Board, you can select “Arduino NG or
older”. You then have a menu Tools → Processor where you can select
either “ATmega168” or “ATmega8”. These options don't require you to
install any extra board support package: they come with the default
“classic” AVR core, which does support the ATmega8.

85606: <p>I use a homemade board that hosts a PIC16F1829 and an ATmega328P on the same board.  We use the ATmega328P with the Arduino IDE and the PIC with the MPLABX IDE for about 100 EE students a semester at UTSA.  I have 100 pic chips but no ATmega328P chips, I can re-spin the board and adapt it to the ATmega8-U2, since right now I can by 250 ATmega8-U2 chips that are in stock but there are no ATmega328P chips available anywhere and I need 100 at least. I am currently using a different UART to TTL adapter, the CP2102 to connect the ATmega328P to the Arduino IDE.  I have the 2 chips talk via a shared I2C line. How difficult will it be to use the Arduino IDE with the ATmega8-U2?  (Again, not as the UART to USB interface but as the main processor.  Thanks,</p>
 7508: <p>I never tried, but it seems to me it should work without issues.</p>
<p>In the Arduino IDE, Menu Tools → Board, you can select “Arduino NG or
older”. You then have a menu Tools → Processor where you can select
either “ATmega168” or “ATmega8”. These options don't require you to
install any extra board support package: they come with the default
“classic” AVR core, which does support the ATmega8.</p>
