Wrong computation Number

Question

im trying to make a speedometer with arduino Nano Board but when i use interrupt im getting "strange" number on variables t1 or t2 going under 0 there is the code:

volatile int impulsi = 0;
int impulsiOld = 0;
volatile int appoggio = 0;
//static int inizioDiNuovo = 0;
volatile unsigned int deltaT[3];
long t1=0;
long t2=0; 
void setup() {
 Serial.begin(9600);
 pinMode(2, INPUT_PULLUP);
 attachInterrupt(0, definisciTempi, RISING);
}
void loop() {
 Serial.println(impulsi);
 if (impulsi == impulsiOld || impulsi > impulsiOld)
 {
 ;
 }
 //Serial.print("N IMPULSI: ");
 //Serial.println(impulsi);
 if (appoggio == 3) {
 appoggio = 0;
 Serial.print("DELTA T 0 : ");
 Serial.println(deltaT[0]);
 Serial.print("DELTA T 1 : ");
 Serial.println(deltaT[1]);
 Serial.print("DELTA T 2 : ");
 Serial.println(deltaT[2]);
 t1 = (deltaT[1] - deltaT[0]);
 if (t1 == 0) {
 t1 = 1;
 }
 t2 = (deltaT[2] - deltaT[1]);
 if (t2 == 0) {
 t2 = 1;
 }
 int t = t1 + t2;
 Serial.print("TEMPO T1 ");
 Serial.println(t1);
 Serial.print("TEMPO T2 ");
 Serial.println(t2);
 }
}
void definisciTempi() {
 impulsi++;
 switch(appoggio){
 case 2:
 //Serial.println("ASSEGNO DELTA T 2");
 deltaT[2] = millis();
 //Serial.println(deltaT[2]);
 appoggio = 3;
 break;
 case 1:
 //Serial.println("ASSEGNO DELTA T 1");
 deltaT[1] = millis();
 appoggio = 2;
 break;
 case 0:
 //Serial.println("ASSEGNO DELTA T 0");
 deltaT[0] = millis();
 appoggio = 1;
 break;
 }
}

how suggest i changed

unsigned int t1=0; unsigned int t2=0; into `long t1=0; long t2=0;` 

output on Serial now have negative number it's for the interrupt??:

DELTA T 0 : 8785
DELTA T 1 : 8666
DELTA T 2 : 8706
TEMPO T1 40
TEMPO T2 -119 <------how 8706-8666= -119 ?

this is my pulse generator code:

const int pin = 2;
void setup() {
 pinMode(pin,OUTPUT);
}
void loop() {
 digitalWrite(pin,HIGH);
 delay(40);
 digitalWrite(pin,LOW);
 delay(40);
}

• Gerben: appoggio Isn't defined as volatile. All variables that get changed in the interrupt, and used in the main loop need to be volatile.
• Francesco Valla: thanks i made appoggio volatile but don't change the result , every time negative number , thanks
• Gerben: Move "appoggio = 0;" to after al the serial.prints.
• Gerben: After you set appoggio = 0; and interrupt might fire, and change the variables you are currently working on.
• Andrew: What will happens if you get an interrupt between printing the deltas and calculating the t values? You'll end up with the t values being calculated with the new delta values (or half with the old ones, half with the new ones) . You need to disable interrupts, make a copy of the delta values and then re-enable them. You then base all your outputs and calculations on the copies.

Answer 1

The interrupt may be triggering between the Serial.print and the actual calculations, as stated by @Andrew.

Evidence supporting that fact is that your deltas are recorded in order 0, 1, 2 (ie: delta 0 < delta 1 < delta 2), However in the example output delta 0 is larger than delta 2, which suggests that a new round of delta recordings had started but not yet completed.

Updating this section as follows should fix the issue:

if (appoggio == 3) {
 noInterrupts(); // Disable interrupts while calculating
 appoggio = 0;
 Serial.print("DELTA T 0 : ");
 Serial.println(deltaT[0]);
 Serial.print("DELTA T 1 : ");
 Serial.println(deltaT[1]);
 Serial.print("DELTA T 2 : ");
 Serial.println(deltaT[2]);
 t1 = (deltaT[1] - deltaT[0]);
 if (t1 == 0) {
 t1 = 1;
 }
 t2 = (deltaT[2] - deltaT[1]);
 if (t2 == 0) {
 t2 = 1;
 }
 int t = t1 + t2;
 Serial.print("TEMPO T1 ");
 Serial.println(t1);
 Serial.print("TEMPO T2 ");
 Serial.println(t2);
 interrupts(); // re-enable interrupts
}

Answer 2

unsigned int t1=0;
unsigned int t2=0; 

Integers on an 8 bit system can only store 16 bits of data. For unsigned that is 0 - 2¹⁶-1, or 0 - 65535.

Make them unsigned long and they can store 0 - 2³²-1, or 0 - 4294967295.

Also, since they are unsigned, any negative value will wrap around to the maximum again. An unsigned can't store a negative value.

• Francesco Valla: I'm simulating the pulse with another arduino board, how an impulse timing be negative? i have setted the time to 40ms for each pulse(sorry for my english)
• Francesco Valla: now i make the change to` unsigned int` to` unsigned long` but now this the output: DELTA T 0 : 3269 DELTA T 1 : 3309 DELTA T 2 : 3349 TEMPO T1 4294967096 TEMPO T2 40
• Majenko: You may well be suffering from a lack of critical sections which I mentioned in your previous question.