ADC port manipulation

Question

I was modifying this ADC Touch implementation, namely switching analogRead and pinMode to what I thought would be port manipulation equivalents.

I get very different readings (better and more stable in my case).

So, what I think is equivalent is actually quite different. Can someone point the differences to me?

Original:

 pinMode(ADCChannel, INPUT_PULLUP);
 ADMUX |= 0b11111;
 ADCSRA |= (1<<ADSC); //start conversion
 while(!(ADCSRA & (1<<ADIF))); //wait for conversion to finish
 ADCSRA |= (1<<ADIF); //reset the flag
 pinMode(ADCChannel, INPUT);
 _value += analogRead(ADCChannel);

Modified:

 DDRC &= ~(1 << ADCChannel);
 PORTC |= (1 << ADCChannel);
 ADMUX |= 0b11111;
 ADCSRA |= (1<<ADSC); //start conversion
 while(!(ADCSRA & (1<<ADIF))); //wait for conversion to finish
 ADCSRA |= (1<<ADIF); //reset the flag
 DDRC &= ~(1 << ADCChannel);
 _value += analogRead(ADCChannel);

Answer 1

Your modified code doesn't disable the pullup on the pin.

PORTC &= ~_BV(ADCChannel);