How many power supply methods are there for Arduino? What is the power circuit like?

Usually when there is an external power connection, it uses a P-channel MOS tube to disconnect the USB power, but there is one thing I don't understand. If there is no external power supply, what is the state of the MOS at startup? If it is turned on, the USB power supplies power to the LMV358, compares 3.3V with 0V, pulls the gate of the MOS low, and the circuit works normally (although the source of the MOS is initially voltage is floating). But if the MOS is turned off, the LMV358 has no power, so the gate voltage of the MOS is undetermined, so what will happen to the MOS? It's like which came first, the chicken or the egg.

The most commonly used development board in the world, Arduino Uno, supports multiple power supply methods, which are roughly divided into two categories: 5V direct power supply and higher voltage power supply through LDO (Low Dropout Regulator, low dropout linear regulator). Miraculously, when both power supplies are connected, the board automatically selects the latter without causing the two to conflict.

The complete schematic can be viewed here. Since the entire picture is relatively large, I pulled out the power supply part and drew it myself.

V1 and VBUS are external power supply and USB power supply respectively; S1 and S2 are used to control the connection status of the two power supplies; D1 is the anti-reverse protection diode; U1 and U2 are 5V and 3.3V LDO respectively; U3 is LMV358 low-voltage rail-to-rail The op amp plays the role of a comparator; R1 and R2 form a voltage divider circuit, and the external input voltage minus half of the diode voltage drop is used as the non-inverting input of the op amp; Q1 is a P-channel MOS tube; VCC is the output 5V power supply . (If you don’t understand the above nouns, please search them on your own.)

Let’s do a simple analysis: If S1 is closed, U1 outputs VCC5V, the op amp is powered, the inverting input is 3.3V output by U2, the forward voltage is higher than the inverting voltage, the op amp outputs 5V, and the gate and source voltages of Q1 are equal. No conduction, no matter whether S2 is closed or not, VBUS will not output current.

If S1 is disconnected and the circuit is already working normally through the VBUS power supply, then a similar analysis can show that the op amp outputs 0V, the gate voltage of Q1 is 5V lower than the source voltage, the MOS tube is turned on, VBUS outputs 5V to VCC through Q1, and the circuit continues normal work.