Hit-and-Hold Valve Control

Numerous valves cannot be energized and left on at their nominal voltage, or they risk burning up. The solution is a *hit-and-hold* strategy where the valve is initially energized at it's nominal voltage and then left energized at a reduced voltage that is just high enough to keep the valve energized, but low enough so as not to burn out the solenoid if left on continuously.

We can achieve a simple hit-and-hold strategy using PWM to modulate the output voltage and current sent to the valve and simple state machine to control hit-to-hold transition logic.

We will need hit-and-hold pwm control for all 16 valve outputs.

### References
* [Hit and Hold Driver Strategy from Lee Co](https://www.theleeco.com/support-resources/engineering-tools/electrical-engineering/drive-circuits/)