zero crossing detection iwth a matlab function

I have a question about zero crossing detection.

I'm modeling something in simulink that I need the exact time when a Boolean changes value. I'm running my block at continuous time but I was hopping by adding the crossing block I can assure that my block takes a step time and records the exact time of the change in Boolean. However, this doesn't seem to work. 1. Where should I have the crossing detection block? can it be after my function? 2. does it even work on a matlab function? if so, any ideas why it's not working?!

Thanks

[dpb]

I've never even seen a Simulink installation but you might try the following to see if it helps...

http://www.mathworks.com/help/simulink/examples/accurate-zero-crossing-detection.html