Disable interrupts when accessing ISR variables

[utmba95]

You need to disable interrupts when accessing any of the variables modified in the ISR. This will prevent incorrect speedM calculations.

noInterrupts(); // make sure ISR doesn't modify lastTime
if(micros() - lastTime >= minSpeedTime){
speedM = 0;
}
else{
// speed = 1min in microseconds/time between subsequent interrupt times
speedM = 60000000/(timePassed*n);
}
interrupts();