The firmware runs on an ATMega48/88/168/328/P/PB AVR microcontroller (the firmware is around 700 bytes when compiled with -Os, so a smaller MCU may be warranted). The trigger is a snap-action limit switch that is attached directly to the rifle's trigger housing via a PCB. The trigger is debounced entirely in hardware, which enables for direct interrupt control on the MCU and simpler code overall. The hardware debouncing also enables the MCU to enter full power-down mode, which saves a significant amount of current draw. The trigger debouncing consists of an RC-filter passed through a Schmitt trigger to produce an excellent digital signal for the MCU's interrupt. The programming switches are not debounced and do not have to be because they are polled only after a trigger pull. It would be quite difficult to enter a state where the toggle switches are bouncing directly after a trigger pull. Not having any debounce on the switches reduces the size and cost of the PCB (if debounced in hardware) or the complexity of the firmware (if debounced in software).

The system maximizes its use of the AVR's hardware features to squeeze the best performance out of the chip. These features include a true real-time system that is entirely interrupt driven. In addition, a hardware timer is used to create an accurate and precise pulse for the solenoid valve. The solenoid is activated by the trigger interrupt and deactivated in hardware when the timer matches the specified compare value.

The rising edge from a trigger pull fires a pin change interrupt on PORTB0. Falling edges are ignored. In the corresponding interrupt service routine (ISR), the system drives the solenoid and starts the 16-bit TIMER1. The timer is set up for CTC mode, non-PWM functionality during power-on. The computation for the timer compare value is outlined in the "Programming Switches and Delay" section. After the timer is started, the trigger pull ISR exits and the system runs idle. When the timer reaches the compare value, the solenoid is deactivated in hardware on the OC1B pin and a timer match interrupt is fired. This interrupt turns stops and resets the timer. The timer interrupt also presents the OC1B pin to set when the next compare happens, which will be when the trigger is pulled. The system subsequently powers down and waits for the next interrupt to occur. During the period between the handling of the trigger ISR and the timer ISR, a flag is set to indicate that the firing sequence is being handled. This flag prevents the system from handling any additional trigger pulls while the firing sequence is happening.

The programming switches control the desired delay time. The state of the switches is polled after every trigger release. Therefore, any new settings will not take effect until the trigger is pulled. A delay constant is added to the time represented by the switches to account for the lag of the solenoid valve.

This hardware revision of the FCU improves upon a few aspects of v1.0 hardware. The battery measurement circuit and supporting LEDs have been removed to reduce BOM cost, soldering time, and overall complexity. The programming switches are now half pitch and placed on the top side of the PCB, such that the entire circuit can be soldered with hot air. Only the flying JST power leads require hand soldering. Additionally, the programming pins now use Pogo pins instead of a soldered header.

This unit is the first revision of the fire control unit. It includes the features described above and additional circuitry for detecting low battery. The low battery detection does not function (I used an N-channel MOSFET as a high side switch...). The worm v1.0 hardware draws 3.4uA when powered with 5v.