I've implemented a simple signal generator using my AVR-Fun experimental board in order to help me in the tests. The RPM can be increased/ decreased in steps of 50 or 100 RPM within the 150..7000RPM range using the 5 buttons on the board. The third (central) button sets RPM to Idle value (RPM).

The prototype has advanced a bit with the incorporation of the frontend circuitry for sensing RPM and manifold vacuum.