Here is a demonstration of ADC that can be run with a potentiometer connected across Vtg and GND with its wiper connected to PB4. In this case the upper 2 bits of the 10-bit conversion result are displayed on the diodes. It was chosen to work with the 8-pin ATtiny13 processor. Its a simple project that shows how to set up ADMUX, ADCSRB and SDCSRA registers to read ADCH. Code included.