Russ,
Have a look at Darrel Taylor's Instant Interrupts. http://darreltaylor.com/DT_INTS-14/intro.html


From this (and Darrel's elapsed timer) you can get an interrupt driven timer to count out your .1sec intervals.

Then you can put the conditional tests and assosiated code either:
a) In the interrupt handler itself (OK if the code is short, but not very good for lots of code)
b) In your main loop, and poll the timer value for changes.

This should get you started in the right direction. There are otherways to do this, but Darrel Talyor has generously done all the hard work for you getting the interrupts working.

HTH,
Steve