Hit crossing block: how to emulate them with simpler blocks

Dear all,

I have a couple of problems related to hit crossing blocks.

The first one, is that I need to implement a hit-crossing block by using only "simple" blocks, like unit-delays, memory blocks, logic operators, switch, etc, and I wonder how to do that. I have tried to use a switch block with a constant "0" and a constant "1" as inputs and to switch between such "0" and "1" according to the zero-crossing of the continuous-time signal. What I get with variable-step solvers is a zero-crossing error (too many consecutive zero-crossing). However, this does not happen with fixed-step solvers or with variable-step solvers when I explicitly use hit crossing blocks. This suggest me that there must be a way to "emulate" the hit crossing block by using simpler blocks. But... how to do that?

The second one, more related to the hit crossing block itself, is that when a hit crossing has been detected, then it generates a pulse in the output. What if I want a constant signal instead? For instance, what if I need to set the output signal to "1" when I detect a zero crossing of from "above" and a "0" when I detect it from below and hold these signals?

Can you help me?

Thanks, Ubaldo