Reset Conditions
The RST pin must maintain a high level for more than 2 machine cycles.
Reset Circuit
Figure 80C51 Reset Circuit
CPU State After Reset
Any microcontroller must go through a reset process before it can operate. What does reset mean? It is similar to the preparation bell we ring before class starts. Once the preparation bell rings, everyone automatically enters the classroom from the playground or other places, and during this time, there is no teacher intervention. For the microcontroller, it means that the program has not yet started executing and is still in the preparation stage. Clearly, this preparation does not take long; a reset only requires about 5ms. How is the reset performed? By applying a high level to the microcontroller’s RST pin for at least 5ms, as mentioned above. There are many ways to achieve this, and one method is provided here for reference, as shown in Figure 1. In fact, we have already seen this in the previous experiment’s diagram.
The working principle of this reset circuit is: When powered on, the capacitor is effectively short-circuited across its terminals, resulting in a high level at the RST pin. Then, the power source charges the capacitor through a resistor, causing the voltage at the RST pin to gradually decrease until it reaches a certain level, at which point it becomes a low level, and the microcontroller begins normal operation.
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