Dealing with a frozen microcontroller can be challenging, especially when debugging intermittent issues. Your approach to use a P-channel MOSFET for power reset is valid, and I'll provide insights on your two questions:
### 1. Power Reset Duration
Cutting power to the ATmega328P to reset it can indeed help if the MCU is frozen and not responding. The duration for which you need to cut the power depends on the specifics of your circuit and the stability of the reset condition you need. However, a few seconds is usually sufficient to ensure a complete reset. Here’s a general guideline:
- **Power Cut Duration**: A duration of 1 to 5 seconds is typically sufficient to reset most microcontrollers. This allows the MCU to fully discharge and reset any internal states. Ensure that during this period, any external components or peripherals also get a proper reset if needed.
### 2. Implementing Power Reset with a P-Channel MOSFET
Here’s a general approach for using a P-channel MOSFET to cut the power to your ATmega328P:
#### Components Needed:
- P-Channel MOSFET (e.g., IRF9540, IRF520)
- Capacitor (for time delay)
- Resistors (for pull-up/pull-down)
- NPN transistor or N-channel MOSFET (optional, for driving the gate of the P-channel MOSFET)
#### Circuit Design:
1. **P-Channel MOSFET Connection**:
- **Source (S)**: Connect to the positive power supply (e.g., 5V).
- **Drain (D)**: Connect to the VCC pin of the ATmega328P.
- **Gate (G)**: Connect to a control signal (e.g., from a GPIO pin of the ATmega328P).
2. **Gate Control**:
- Use a resistor (e.g., 10kΩ) to pull the gate of the MOSFET to the source voltage (to keep the MOSFET off by default).
- To turn off the MOSFET, the gate voltage needs to be pulled low (below the source voltage).
3. **Control Circuit**:
- Use a GPIO pin of the ATmega328P to control the gate of the P-channel MOSFET.
- When you want to cut power, set the GPIO pin to LOW, which turns off the MOSFET and disconnects the power to the ATmega328P.
4. **Power Cut Duration**:
- To ensure a reliable power cut, you might use a capacitor in conjunction with a transistor or MOSFET to delay the power restoration after the cut. The capacitor will hold the gate voltage low for a few seconds before returning it to the high state, allowing the ATmega328P to reset completely.
#### Example Schematic:
1. **Capacitor and Resistor Network**:
- Connect a capacitor (e.g., 100µF) between the gate of the MOSFET and ground. This capacitor will help in keeping the gate voltage low for a brief period after the power is cut.
- A resistor (e.g., 10kΩ) between the gate of the MOSFET and the source (5V) will ensure that the MOSFET stays off when not actively driven by the GPIO pin.
2. **Control Logic**:
- Use a simple NPN transistor or another MOSFET to drive the gate of the P-channel MOSFET if needed. The NPN transistor can be used to pull the gate of the P-channel MOSFET to ground when needed.
### Example Code to Trigger the Reset:
You can use a GPIO pin to control the gate of the MOSFET. When a reset condition is detected, you can set the pin low to cut the power.
```c
const int resetPin = 8; // Example pin connected to the gate of P-channel MOSFET
void setup() {
pinMode(resetPin, OUTPUT);
}
void loop() {
// Example condition to trigger reset
if (/* your condition */) {
digitalWrite(resetPin, LOW); // Cut the power
delay(5000); // Wait for 5 seconds
digitalWrite(resetPin, HIGH); // Restore the power
delay(1000); // Delay to ensure the MCU is fully powered up
}
}
```
### Additional Tips:
- Ensure that your power supply and MOSFET are capable of handling the current requirements of the ATmega328P and peripherals.
- Test your reset circuit thoroughly to ensure it performs reliably and does not inadvertently cause additional issues.
By implementing the above approach, you should be able to reset your ATmega328P reliably if it gets frozen, and ensure that any peripherals like the LCD and DS1307 are also properly reset.