RPM Code, micros returns odd values

Projects Programming
1

Measuring RPM off of an optical tach.

Why does this code;

volatile byte rpmcount;
unsigned int rpm;
unsigned int timeold;

 void setup()
 {
   Serial.begin(9600);
   attachInterrupt(1, RPM_Func, RISING);
   digitalWrite(2, HIGH);
   rpmcount = 0;
   rpm = 0;
   timeold = 0;
 }
 void loop()
 {
   if (rpmcount >= 20) {
     unsigned int time_since = millis() - timeold;
     rpm = ((rpmcount * 1000) / time_since) * 60;
     timeold = millis();
     Serial.println(time_since, DEC);
     rpmcount = 0;
     Serial.println(rpm, DEC);
   }
 }
 void RPM_Func()
 {
   rpmcount++;
 }

...work, but this code;

volatile byte rpmcount;
unsigned long rpm;
unsigned long timeold;

 void setup()
 {
   Serial.begin(9600);
   attachInterrupt(1, RPM_Func, RISING);
   digitalWrite(2, HIGH);
   rpmcount = 0;
   rpm = 0;
   timeold = 0;
 }
 void loop()
 {
   if (rpmcount >= 20) {
     unsigned int time_since = millis() - timeold;
     rpm = ((rpmcount * 1000000) / time_since) * 60;
     timeold = millis();
     Serial.println(time_since, DEC);
     rpmcount = 0;
     Serial.println(rpm, DEC);
   }
 }
 void RPM_Func()
 {
   rpmcount++;
 }

...not?

All I can figure is there's a rollover somewhere but I used unsigned longs, figuring 4.2 billon was big enough. RPM is in the 16 to 140 Hz range.

Any help is appreciated.

-Ian

P.S. For the record, I thought I'd use micros to give me a little more resolution without slowing down the update time by increasing the number of counts it updates on.

2

you have a mix of
unsigned int
& unsigned long
for your time elements
make them all unsigned long to agree with the format that millis() returns

I don't see that you have micros() called out.

3

Yep, fixed it right up. Thanks.

-Ian