I want to use the avr interface to sample analog inputs with nothing connected to the usb data. how is this done?
You don’t need to use the usb interface part of the avr_interface module. You can simply set new_tx_data to 0, tx_data to 8hXX to never send anything.
You could also copy/paste the avr_interface component into your own module and remove all the serial/USB stuff leaving only the ADC/SPI stuff.