In the above code, macros and bits definitions are used (i.e ADMUX, REFSO) instead of pointers, etc. Add a comment for each line of code that describes what each line does. Describe the function of ADCW.
This code tells us that When ADCL is read the ADC register is not updated consequently,if the result is left adjusted and no more than 8 bit precision is required it is sufficient to read ADCH.
