Thread: Passing a global variable from Main.c to an ISR file

I am writing my first C program. It is going very good but I have run into one problem... This is being written in Codewarrior for a Freescale MCU.

I have declared a variable in Main.c

Code:

volatile int seconds=10;    //a seconds counter

Then in Main.c loop, I check the value of counter and turn on an LED if it is zero:

Code:

 if (!seconds)
{
  led_blue_on();
}

I have an ISR that is running and clocking at 1 seconds that decrements the variable 'seconds'. Here is the ISR code:

Code:

*/
__interrupt void isrVrtc(void)
{
  /* Write your interrupt code here ... */
#include "MyEquates.h"
volatile int seconds;       //have to add this to compile this file

RTCSC_RTIF=1;             //reset RTC flag
 
 if (seconds!=0){           //update seconds counter
  seconds--;                    //reduce count
 }
}
/* end of isrVrtc */

So my problem is that the variable 'seconds' is not the same in Main.c as it is in my ISR file. So 'seconds' in main never gets decremented. But I can see that 'seconds' in my ISR does decrement in another part of memory.

How can I declare my variable 'seconds' so that its value is the same in Main.c and my ISR file? I tried adding 'volatile', which I thought would solve the problem by making it global. But it did not.

Thank you!

Originally Posted by Salem

Assuming the variable is actually global in main.c, then in isr.c you need this at the global scope also.
extern volatile int seconds; //have to add this to compile this file

That is perfect! I tried it and it solved the problem. Thank you!

But once the seconds variable reaches 0 you need to reload the value else it will execute only once.

Originally Posted by Satya

But once the seconds variable reaches 0 you need to reload the value else it will execute only once.

Yes...fully understood. This code was just to get it working.