ADC autostart does not work. For example, we do "ADCSRA = 0xe2" in the program, which means, among other things, that the ADC autorun bit is enabled, but autorun does not work, the value of the ADCH register is not updated.
#define ADC_input A0
// the setup routine runs once when you press reset
// initialize serial communication at 9600 bits per second
// ADC setup
ADMUX = 0x60 - A0 + ADC_input; // ADC input, 0x60 is A0, 0x61 is A1, ...
ADCSRA = 0xe2;
bitWrite(ADCSRA, 5, 1); // ADATE bit for auto trigger
// the loop routine runs over and over again forever
int v = ADCH;