Why would you use an external real time clock for this? Those are meant to keep real time as in years, days, hours, minutes - they're not meant for timing applications where you need 0.1 second resolution or better.

Instead of using interrupts to count the pulses from the driveshaft why don't you use one of the timers to count the pulses for you and use another timer as the timebase? Mike gave you an exceptional explaination of how to use a timer to generate an accurate timebase with just about any resolution. I honestly believe it's going to be a lot easier AND give you better results then trying to use an external RTC chip.

Also, timing 600 seconds with 0.1 second resolution is only 6000 "counts" so it not require a LONG. If 600 seconds is the longest time you can get 10ms resolution and still stay within the bounds of a WORD variable.

What is the longest time you'll likely want to measure? Are you getting 1 pulse per revolution of the driveshaft? What's the distance traveled per pulse?

/Henrik.