Electronic – arduino – Noisy ADC readings

Here is how I would suggest going about debugging this problem:-

1. Check the connections.. if you've mixed up signal ground and chassis ground (shield) you could get funny behavior just like you describe. Pin 7 on the DB-25 is the one you want to connect to DE-9 pin 5. I'll assume you're using a commercial adapter/cable or have one wired like this.

   

   Maybe check the transmit out line to ensure it's idling at -12 ( < -3 anyway).

2. Make sure the instrument is set to RS-232, and not current loop (Jumper 7,8 and 1,2).

3. Try the most common serial port setting (9600/8-N-1), 9600 baud, NO parity, 1 stop bit switch 1, 2, 3 OFF switch 4, 5 ON switch 6 ON switch 7, 8 ON

   Make SURE you understand which is 'OFF' and which is 'ON'. Don't laugh, I've seen this happen more than once, and some types of DIP switches are kind of ambiguously marked. If in doubt, an ohmmeter across the switch.. in fact you could check all of the switches to ensure they're working.

   I like Realterm as a terminal program. It's free, and easy to change the parameters. Try it on something known working first of course, and look for an online tutorial. Really worth it.

4. If still gibberish after that, get a scope screen cap or photo and post it.

5. If you have a continuing need for a logic analyzer, this one is very cheap and relatively useful. Not as good as my Tektronix boat anchor, but this one will fit in a coat pocket. But I'd get a scope first if you don't have one. Even the worst scope is better than nothing.

So I ended up by finding myself the answer on the ATTiny85 specs.

As I was trying out the code I found out that each time I used pin 3 (PCINT3) as the interrupt pin and analog reading, the code actually worked. So I started digging the a little bit more the details of each pin and I found out the following:

Port B, Bit 3 – XTAL1/CLKI/ADC3/OC1B/PCINT3 • XTAL1: Chip Clock Oscillator pin 1. Used for all chip clock sources except internal calibrateble RC oscillator. When used as a clock pin, the pin can not be used as an I/O pin. • CLKI: Clock Input from an external clock source, see “External Clock” on page 26. • ADC3: Analog to Digital Converter, Channel 3 . • OC1B: Inverted Output Compare Match output: The PB3 pin can serve as an external output for the Timer/Counter1 Compare Match B when configured as an output (DDB3 set). The OC1B pin is also the inverted output pin for the PWM mode timer function. • PCINT3: Pin Change Interrupt source 3.

Port B, Bit 2 – SCK/ADC1/T0/USCK/SCL/INT0/PCINT2 • SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a Slave, this pin is configured as an input regardless of the setting of DDB2. When the SPI is enabled as a Master, the data direction of this pin is controlled by DDPB2. When the pin is forced by the SPI to be an input, the pull-up can still be controlled by the PORTB2 bit. • ADC1: Analog to Digital Converter, Channel 1 . • T0: Timer/Counter0 counter source. • USCK: Three-wire mode Universal Serial Interface Clock. • SCL: Two-wire mode Serial Clock for USI Two-wire mode. • INT0: External Interrupt source 0. • PCINT2: Pin Change Interrupt source 2.

The problem was that I assumed that the pin number was the same as the analogue channel, so I ended up by always reading something else.

Here's the working code (in the meanwhile I recoded it)

#include <avr/sleep.h>
#include <avr/interrupt.h>

const int switchPin = A1;
const int door = 3;
const int sleep = 4;

int outputValue = 0;

void setup() {

    //pinMode(switchPin, INPUT);
    //digitalWrite(switchPin, HIGH);

    pinMode(door, OUTPUT);
    digitalWrite(door, LOW);

    pinMode(sleep, OUTPUT);
    digitalWrite(sleep, HIGH);

    // Flash quick sequence so we know setup has started
    for (int k = 0; k < 10; k = k + 1) {
        if (k % 2 == 0) {
            digitalWrite(door, HIGH);
            }
        else {
            digitalWrite(door, LOW);
            }
        delay(250);
        } // for
    } // setup

void MCsleep() {

    GIMSK |= _BV(PCIE);                     // Enable Pin Change Interrupts
    PCMSK |= _BV(PCINT2);                   // Use PB2 as interrupt pin
    ADCSRA &= ~_BV(ADEN);                   // ADC off
    //digitalWrite(switchPin, LOW);
    set_sleep_mode(SLEEP_MODE_PWR_DOWN);    // replaces above statement

    sleep_enable();                         // Sets the Sleep Enable bit in the MCUCR Register (SE BIT)
    sei();                                  // Enable interrupts
    sleep_cpu();                            // sleep

    cli();                                  // Disable interrupts
    PCMSK &= ~_BV(PCINT2);                  // Turn off PB2 as interrupt pin
    sleep_disable();                        // Clear SE bit
    //digitalWrite(switchPin, HIGH);
    ADCSRA |= _BV(ADEN);                    // ADC on

    sei();                                  // Enable interrupts
    } // sleep

// Whatever the pin this IRS should always point to the PCINT0_vect, because is the only capable of
// providing the interrupt. The ISR must be provided otherwise, the MC just resets instead of resuming.
ISR(PCINT0_vect) {
    // This is called when the interrupt occurs, but I don't need to do anything in it
    }

void loop() {

  MCsleep();
  delay(20);

  outputValue = analogRead(switchPin);

  if(outputValue > 200){
    digitalWrite(sleep, LOW);
    delay(1000);
    digitalWrite(door, HIGH);
    delay(2000);
    digitalWrite(sleep, HIGH);
  }
  else{
    digitalWrite(sleep, LOW);
    delay(1000);
    digitalWrite(door, LOW);
    delay(2000);
    digitalWrite(sleep, HIGH);
  }
}