The usual trick here is to use a current sense resistor and some analog stuff to charge the coil and hold the current and then the micro just controls the high voltage transistor to fire the coil. Nissan seems to like the full charge control of the coil done by the micro but that ties you to a particular coil (which may or may not be an issue). DYI EFI has some interesting stuff - search for 'diy efi'.