In electronic circuits, PMOS tubes only support unidirectional level conversion, while bidirectional NMOS circuits have the ability to conduct in both positive and negative directions. The working characteristics of bidirectional NMOS circuits are similar to those of mechanical switches, which are suitable for frequent switching scenarios. It has a large on-resistance, is suitable for low-frequency signal level conversion, has low cost, and has a smaller voltage drop after conduction than triodes, which is suitable for scenarios with high voltage requirements. However, its specific implementation will vary depending on the application scenario and device characteristics. In actual applications, appropriate circuit topology and device parameters should be selected as needed.

Let's analyze how it works:

 When S (TXD_1V8) outputs a high level, Vgs of MOS tube Q4 = 0, the MOS tube is in the off state, and D (UART1_RXD) is pulled up to 3.3V through resistor R42;

When S (TXD_1V8) outputs a low level, Q4's Vgs = 1.8V, exceeding the turn-on voltage threshold, the MOS tube is turned on, and Net2 is pulled down to a low level through the MOS tube;

When D (UART1_RXD) outputs a high level, the Vgs of Q4 remains unchanged and continues to be in the off state, and S (TXD_1V8) is pulled up to 1.8V by resistor R37;

When D (UART1_RXD) outputs a low level, Q4 is not turned on at first, and the body diode first pulls S (TXD_1V8) down to a low level. At this time, Vgs ≈ 1.8V, and the MOS tube is turned on immediately, further pulling down the voltage of S (TXD_1V8).

In addition, when designing this circuit, it is also necessary to meet the high voltage > low voltage – 0.7V, otherwise there will be problems when D→S transmits a high level, that is, Vs = Vd + 0.7V, resulting in Vs < VCC. At the same time, circuits involving 1.8V should pay attention to the Vgs conduction voltage of the MOS tube and be cautious in device selection.