I forgot to mention that, the 12v pulses from an electronic ignition need to be dropped to less than 5v before applying them to a microcontroller ckt. On my motorcycle, I fed the ignition pulses to a 220K res. in series with 110K res to ground, then took the voltage pulses from across the 110k to ground, which resulted in pulses of less than 5v. I tried using a 5v Zener diode across the 110K, but I found that Zener's require a certain level of current to operate properly and that the 220k res. was restricting the current to a level that was too low for the Zener to operate properly. So took the zener out, since it wasn't working anyway.
So I use a clipper circuit to clip the pulses to whatever voltage level I want. Here's where you can find info on clipper ckts*.
http://www.ee.lamar.edu/EELABS/ELEN3108/Lab2.pdf)*
In place of the clipper ckt. battery I used a 3.3 voltage regulator, and it worked.
Patrick