I know you can make an Axiom to control crystal timer-based interrupts, because I've done it before. But how fast do you need the timer to be? Because the hardware timers on the 15MHz calculators are actually based off the crystal timers and run at extremely precise speeds. If you just need to keep accurate time over a long period rather than do something many times a second with precise intervals in between, you should be just fine with the hardware timers. The exact speeds the hardware timers run at can be found here: [wikiti]83Plus:Ports:04[/wikiti]. The most regular speed is with the hardware timer set at the fastest setting (0) on a 15MHz calculator, which will run at (almost) exactly 512Hz.
