Using a timer to detect large periods of inactivity in MSP430F1611

Obviously the 32 kHz watch crystal is the best solution   Some of the MSP430s have a built in VLO that runs at ~12kHz but I don't think yours does.



That being said a simple overflow interrupt with an int32 counter should be more then sufficient for the 10-30 minute interval.  The amount of time to check/increment your counter in an interrupt even without using any prescalers should be very minimal (easily < 1% of the load, probably a lot less then that).  Using an /8 prescaler on the timer and possibly another on SMCLK would give you even less of a load on the system.



If for some reason the above isn't appropriate for your application you can also try cascading the timers, but I'm having a hard time coming up with situation where this would be appropriate for this kind of interval timer.

 It sounds like you want to wait a period before putting it into sleep mode or low power.There are many ways to achieve this but I have used this method to put the device to sleep after a period of time. It uses slices of 32mS and I use a count to determine how many I want. Your main routine sets the total time you want before going to sleep. Every 32mS on WD interrupt it either will reset the counter (detecting a user activity) but if no activity, then the count will reach the set point and the device goes to sleep. Another user input or activity from port 1 wakes up the device and the process repeats. Sample code below should help if you want. Uses the MSP430g2231 and internal clock

// Watch Dog SetupRoutine -----------------------------------------------------------------------

void WatchDog (void)
{
 WDTCTL = WDT_MDLY_32 +WDTNMI;                             // Set Watchdog Timer interval to ~32ms
 IE1 |= WDTIE;                                             // Enable WDT interrupt
P1IE |= BIT0;                                             // Enable P1.0 interrupt, - used for user detection
 __enable_interrupt();
}

Set the required time of non activity

int SleepTime = 15000;                                  // WatchDog Time to Sleep 32mS x SleepTime
                                                                            // 15000 = 32mS x 15000 = 480 seconds / 60 seconds = 8 minutes

 SleepFlag = 0;

// Port 1 Interrupt Routine ------------User activity

#pragma vector=PORT1_VECTOR                                // Port 1 interrupt service routine
__interrupt void Port_1(void)
{
if (SleepFlag == 1)                                       // Only set in the WD Interrupt Routine
   {
   wdtCounter = 0;                                         // Reset watchdog timer counter
   SleepFlag = 0;                                          // Reset SleepFlag, no longer in WD mode
                                                                        // Only ever Cleared here !!
   _BIC_SR(LPM3_EXIT);                             // Wake up from low power mode
   }

 else if (SleepFlag == 0)                                  // Not in WD so must have been a button press
  {
   wdtCounter = 0;                                         // Reset watchdog timer counter
  }

 P1IFG = 0x0;                                              // Reset P1 Interrupt Flag
}

// Watch Dog Interrupt Routine --------------------------------------------------------------

#pragma vector=WDT_VECTOR                                      // Watchdog Timer interrupt service routine
__interrupt void watchdog_timer(void)
{
  if(wdtCounter == SleepTime)
  {
  //Stuff here before putting to sleep
   SleepFlag = 1;                                          // Set Flag for WD Sleep Mode
                                                                        // Only ever set here !!
   wdtCounter = 0;                                         // Reset Counter to Zero
   _BIS_SR(LPM3_bits + GIE);                               // Enter LPM3 w/interrupt
  }
  else
  {
   wdtCounter++;                                           // Increment Counter because of user inactivity
  }
}