Libpinproc is C++ but easily interfaces to C.

My $0.02 worth of custom game tips:
1) a service menu/screen should be your first item to get running, especially with any custom hardware and especially with custom wiring. I used P-ROC's pinproctest module as a starting point for a simple standalone display for switch matrix activity, then later added keyboard commands to fire outputs (solenoids, LEDs, Flashers). During game hw dev you'll want to quickly verify any changes you made, or find that solder joint that broke, that noisy opto, that fuse you blew, etc...

2) assets take time. After getting your whitewood flipping and basic play sorted out, you now realize how boring modern pinball is without music, sounds, and animations (unless you're re-imagining an EM with over $1000 of custom electronics hardware.) Grinding out mode code is one thing but gathering suitable assets is a challenge. Fortunately for use there's the innerweb and DVD's!

So what happens when your auto-fire coils react to a switch hit? They pulse a coil for around 30ms, and probably ignore further switch inputs for that pulse duration (so your coil isn't on continuously.) This effectively lengthens the debounce time to the coil pulse time.

This is useful for "tuning" your jet bumper nest, where a stronger (longer) coil pulse kicks the ball harder but won't react to a quick rebound. This gives the impression of a laggy bumper. Somewhat unintuitively reducing the coil pulse allows the bumper to react to quick rebounds, giving the lively bumper action we love.

If your layout is prone to bumper resonance your software can alter the coil pulse "on the fly" to avoid such activity.