I need to load the Atmega328p 8kHz board definition using the board manager and can not find a way. Any help will be appreciated.
Assuming that you have a 3.3V Atmega328P and are using an external 8MHz occilator:
In the Arduino IDE click on
Scroll down until you see an option starting with the word
Select the option called
Arduino Pro or Pro Mini
Tools menu once again.
This time select the option starting with
Atmega328P (3.3V, 8MHz)
Alternatively you can edit the
boards.txt file located somewhere similar to
C:\Program Files (x86)\Arduino\hardware\arduino\avr
Fuse bits can be calculated using one of these online tools: