Parameter calculation of RC automatic reset circuit of single chip microcomputer

Analysis: At the moment of power-on, the voltage across the capacitor cannot change suddenly, so the voltage of RST is basically equal to Vcc (that is, the point where the blue line intersects the horizontal axis in the above figure). As time goes by, the voltage waveform of the RST pin is the same as the above figure. Assume that the voltage of the RST pin at the moment of power-on is V0 , the voltage of RST at time t is Vt , and the system power supply voltage is 5V

Then the following formula holds:

Because V0 is equal to Vcc at the moment of power-on , the following equation is obtained (set as equation 1):

In formula 1 , VCC is 5V , e is a constant of 2.71828 , RC depends on the selection of components. Taking the above figure as an example, the RC value is 0.1 , and t is the elapsed time.

Calculate the voltage of the RST pin at t=50ms: , the result is about 3.03V

Calculate how long time t has passed when the RST pin is 4V : The result t is 22ms

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AVR microcontroller RC circuit:

:

The simulation waveform in Multisim is as follows:

Analysis: The analysis method is the same as that of the 51 single-chip microcomputer, but the calculation formula is quite different. Because one end of the capacitor of this RC circuit is connected to the ground, at the moment of power-on, V0 is 0 volts, and Vt is the voltage value after power-on at time t, and the formula is as follows:

Calculate the voltage of the RST pin after t=50ms when Vcc is 5V: The calculated result Vt is about 1.96v

Calculate the value of t when Vcc is 5V and RST pin is 4V: The calculated result is about 160ms