RPM Interrupt speed limit

Projects General Guidance
9

Hi guys,

So I've got a program where I communicate to my arduino via serial from a windows form C# app on my pc.
I'm trying to pickup rpm (hall effect) and temperature (voltage) from sensor. Code would look something like this:

int message = 0;   //  This will hold one byte of the serial message
int ledPin = 13;   //  This is the pin that the led is conected to
//int LED = 0;      //  The value or brightness of the LED, can be 0-255
volatile byte rpmcount;
unsigned int rpm;
unsigned long timeold;
int rpmPin = 12;



void setup() 
{  
  pinMode(rpmPin, OUTPUT);
  
  Serial.begin(9600);  //set serial to 9600 baud rate
  pinMode(ledPin, OUTPUT);
  attachInterrupt(0, rpm_fun, RISING);
  rpmcount = 0;
  rpm = 0;
  timeold = 0;
}

void loop()
{
  digitalWrite(rpmPin, HIGH);
  delay(100);
  digitalWrite(rpmPin, LOW);
  delay(100);
  
  if (Serial.available() > 0) 
  { 
    message = Serial.read();   

    if (message == 'A')
   {  
     digitalWrite(ledPin, HIGH);      
     Serial.println("LED on");  
   }
   if (message == 'a')
   {  
     digitalWrite(ledPin, LOW);         
     Serial.println("LED off"); 
   }
   if (message = 's')
   {
     {
       if (rpmcount >= 2) 
       { 
         //Update RPM every 20 counts, increase this for better RPM resolution,
         //decrease for faster update
         rpm = 30*1000/(millis() - timeold)*rpmcount;
         timeold = millis();
         rpmcount = 0;
         Serial.println(rpm,DEC);
       }
     }

  } 
}
}

 void rpm_fun()
 {
   rpmcount++;
   //Each rotation, this interrupt function is run twice
 }

I get stuck in the the rpm function. Is this a limitation of the arduino or should I be able to serial input commands and read values (via serial) from different pins while running an interrupt?

Thanks in advance.

To add more clarity. I would like to send a command to run function to stream rpm ('s') then change functions for temp ('t')then stream that pin. Is this possible?

Moderator edit:
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