Mitch Nelke designed the circuit, which contains two essential integrated circuits. IC1 acts as the flip flop IC, responsible for latching its EN-bar output to either HIGH or LOW states following a HIGH pulse on its Clock Pulse input or a LOW pulse on its C-bar input, respectively. IC2 functions as the 12v-5v switching regulator, reducing the battery’s 12v supply to 5v for powering the microcontroller and other elements.

The Flip Flop

While the OPEnS Power Control Board boasts impressive low-power capabilities, understanding its magic requires a peek under the hood. This section dives into the details of how the board itself is powered and controlled.

Fueling the Flip-Flop:

The brain of the board, the flip-flop circuit, gets its power directly from the battery’s raw 12V supply. However, to ensure safe operation, a zener diode (D1) acts as a voltage regulator, dropping it down to a more manageable 4.3V.

Calculating the Right Current:

A resistor, R1, plays a crucial role in ensuring the flip-flop gets the current it needs to function. This value is carefully chosen based on two factors:

1. Minimum Current Requirement: The switching regulator’s enable pin and the flip-flop itself have a minimal current draw. R1 is selected to provide at least 60uA to meet this combined need.
2. Buffer: A small additional current allowance is factored in as a buffer for potential variations or future modifications.

Setting the Stage for Latched Power Control:

This 4.3V source also serves another purpose. It keeps the flip-flop’s D input permanently pulled HIGH. This acts as a default setting, influencing the flip-flop’s output based on other control signals.

Clock Pulse: The Power-On Signal:

The Clock Pulse (CP) input is typically pulled LOW by a resistor (R10k in this case). When a rising edge (voltage increase) is detected on this pin, it acts as a trigger for the flip-flop. Importantly, at this point, the flip-flop also reads the state of its D input (which is HIGH, as mentioned earlier). This combination sets the flip-flop’s output (Q) to HIGH as well.

In simpler terms, a brief voltage spike on the CP input acts like a “turn on” switch, and the HIGH D input ensures this always results in powering up the connected devices.

Overriding the On-State: The C-bar Input

The C-bar input has a different role. It has a weak pull-up resistor (R2) that keeps it at 4.3V most of the time. However, when a LOW signal is applied to this input, it overrides the CP input entirely. This LOW signal can come from two sources:

1. RTC_INT: This input is typically connected to the interrupt pin of a Real-Time Clock (RTC) module. When the RTC triggers an interrupt, it pulls C-bar LOW, effectively shutting down the device.
2. High Input from INT_2: This alternative allows a microcontroller’s output pin (INT_2) to control the power state. A HIGH signal on INT_2 sends a LOW signal to C-bar, triggering a power down.

C-bar’s Dominance and the Role of GPIO_READ:

Once C-bar is pulled LOW, it essentially ignores any subsequent clock pulses. The flip-flop’s output (Q) is forced LOW, turning off the power. This state persists until the next time C-bar is pulled HIGH.

It’s important to note that another pin, GPIO_READ, allows a microcontroller input pin to “peek” at the state of the RTC’s interrupt even when the device is powered down. This allows for coordinated power management when the microcontroller wakes up.

Connecting the Dots: Putting it All Together

By understanding how these internal circuits interact – the constant power supply to the flip-flop, the control signals on CP and C-bar, and the ability for the microcontroller to monitor the RTC – we gain a clearer picture of how the OPEnS Power Control Board achieves its remarkable ultra-low power standby current.