Arduino due temperature reading shows random temperatures

Question

I am using an Arduino Due board to read three temperatures from three separate smt160-30 digital sensors but the program is behaving very randomly. the code I am using is one I got from arduino official site and changed the structure to get three temp readings and display them on a 16x2 lcd. The problem is when I use one sensor the output is reasonable but when I use all three the temperature showing are all off by 4 degrees. also I tried removing the line lcd.print(temperature1, 1) and the output temperatures changed. Is this odd behavior because of SRAM issues ? if so how can I find out about it and fix it ?

#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

unsigned long now;
unsigned long lastSample;
unsigned int sampleSize;

unsigned int highCount1;
unsigned int highCount2;
unsigned int highCount3;

unsigned long lastOutput;

float temperature1;
float temperature2;
float temperature3;
float hcf1, hcf2, hcf3;
float ssf;

int sensorPin1 = 23;
int sensorPin2 = 24;
int sensorPin3 = 26;

void setup(){

    pinMode(sensorPin1, INPUT);
    pinMode(sensorPin2, INPUT);
    pinMode(sensorPin3, INPUT);

    digitalWrite(sensorPin1, LOW);
    digitalWrite(sensorPin2, LOW);
    digitalWrite(sensorPin3, LOW);

    lastSample = 0;
    sampleSize = 0;

    highCount1 = 0;
    highCount2 = 0;
    highCount3 = 0;

    temperature1 = 0;
    temperature2 = 0;
    temperature3 = 0;

    lastOutput = 0;

    delay(2000);
    lcd.setCursor(0, 0);

}

void loop() {
    now = micros();
    if (now - lastOutput > 3000000) {
        lcd.clear();

        hcf1 = highCount1;
        hcf2 = highCount2;
        hcf3 = highCount3;

        ssf = sampleSize;

        temperature1 = ((hcf1 / ssf) -0.32)/0.0047;
        temperature2 = ((hcf2 / ssf) -0.32)/0.0047;
        temperature3 = ((hcf3 / ssf) -0.32)/0.0047;

        lastOutput = now;
        sampleSize = 0;

        highCount1 = 0;
        highCount2 = 0;
        highCount3 = 0;


        lcd.print(temperature1, 1);
        lcd.print(" ");

        lcd.print(temperature2, 1);
        lcd.print(" ");

        lcd.print(temperature3, 1);

    }
    if (now - lastSample > 50) {
        sampleSize++;
        highCount1 = highCount1 + digitalRead(sensorPin1);
        highCount2 = highCount2 + digitalRead(sensorPin2);
        highCount3 = highCount3 + digitalRead(sensorPin3);
        lastSample = now;
    } else {
        delayMicroseconds(10);
    }
}

Thank you!

Answer 1

Did you read about this smt160 sensor ? It relies on the calculation of duty cycle of the square wave output from the sensor, to calculate the temparature.

temperature = (dutyCycle) -0.32)/0.0047

The square wave output has to be sampled at 8KHz or at 125us for no loss of information. This block samples that data.

if (now - lastSample > 50) {

sampleSize++;

highCount1 = highCount1 + digitalRead(sensorPin1);

highCount2 = highCount2 + digitalRead(sensorPin2);

highCount3 = highCount3 + digitalRead(sensorPin3);

lastSample = now; }

else { delayMicroseconds(10); }

when you increased the number of code lines , information loss also increased, cz more number of instructions have to be executed now , till reaching the corresponding digital reads.