I want to use a ATtiny85 to check the voltage of two 12 V batteries (later to control a relay). Therefore I'm using the analogRead() function. For programming I'm using an Arduino Mega 2560 board.

// +-\/-+
// Ain0 (D 5) PB5 1| |8 VCC
// Ain3 (D 3) PB3 2| |7 PB2 (D 2) INT0 Ain1
// Ain2 (D 4) PB4 3| |6 PB1 (D 1) pwm1
// GND 4| |5 PB0 (D 0) pwm0
// +----+

For A1 this works perfectly fine. But for A2 or A3 I'm getting always a value of 15 V (thus the analog input gives 1023 respectively it thinks there is 5 V on the pin, which is not the case). I tried A0 as well, but it's the same.

For seeing the current value I put a serial - USB converter on RX, TX on PB3, PB4. So I guess they cannot work properly as analog inputs then. But still A0 does not work and without serial my LED is always on without having a voltage over 12 V.

At the moment I don't know what to do about it. No matter which of the analog pins I choose, I'm getting always a wrong value (except A1).

I'm not a hundred percent sure about A0. On some diagrams there is an A0 pin, on some not. But I tried to remove all the serial communication code and still having the LED 0 on high.

Thanks in advance!

void setup() {
 pinMode(0, OUTPUT);
 pinMode(1, OUTPUT);
 digitalWrite(0, LOW); // led for displaying the voltage
 digitalWrite(1, LOW); // blinking status led
 int sensorValue1 = 0;
 int sensorValue2 = 0;
 Serial.begin(9600);
}
void loop() {
 int sensorValue1 = analogRead(A1);
 int sensorValue2 = analogRead(A0);
 // <- the problem, A2, A3 do not work as well
 float voltage1 = sensorValue1 * (5 / 1023.00) * 3;
 float voltage2 = sensorValue2 * (5 / 1023.00) * 3;
 Serial.print(voltage1);
 Serial.print("--");
 Serial.print(voltage2);
 Serial.println();
 if ((voltage1 > 12) || (voltage2 > 12)) {
 digitalWrite(0, HIGH);
 } else {
 digitalWrite(0, LOW);
 }
 digitalWrite(1, HIGH);
 delay(1000); 
 digitalWrite(1, LOW);
 delay(1000);
}

SOLUTION: As suggested by Andrei Dragan the problem was the serial communication, which uses just the same pins as the analog inputs A2 and A3. When turned off, the problem disappeared.

As the serial communication is quite useful for me, right now I'm still using the serial communication but only with the pin for sending data. The pin for receiving data is turned off. It looks like this:

#include <SoftwareSerial.h>
#define RX -1 // *** turned off
#define TX 4 // *** D4, Pin 3
SoftwareSerial Serial1 (RX, TX);

(I don't know if this is the best way, but it works for me.)

Once again, thanks for the help!