[Question] How to calculate the value of capacitor C1 in the microcontroller reset circuit? I need the specific formula and explanation! Urgently needed!
[Incorrect Answer]
Taking the 51 microcontroller as an example: The reliable reset time for the microcontroller must be greater than 2 machine cycles. If using a 12MHz crystal oscillator,
the reset time must be greater than 2 microseconds.
According to the charging time of the capacitor in the power-on reset circuit: t=3RC=3*10*10^-6*10*10^3=0.3 seconds, which can ensure the normal reset of the microcontroller.
[Correct Answer]
(3~5)RC is the time to fully charge. In fact, it is far from fully charged; the voltage across the resistor is considered low level.
This algorithm is incorrect. You should calculate how long the high level can last in this circuit.
This doesn’t even need to be calculated!
The microcontroller chip manufacturers have already specified and published the values, so there is no need to calculate them. In the “Incorrect Answer,” using the “charging time of the capacitor” to replace the “reliable reset time” is clearly a concept switch. At the moment of power-on (or pressing the reset button), a high level can appear across resistor R instantly. Afterwards, capacitor C will charge, and the voltage across resistor R will decrease exponentially. (3~5)RC is the time to fully charge, at which point the voltage across resistor R will drop to 0 V. Anyone who has studied digital circuits knows: the voltage does not need to drop to 0 V to be considered low level. What voltage level at the RST pin is considered low level? This is determined by the manufacturer. The chip manufacturer also provides the required values for R and C in the reset circuit. Using the values recommended by the manufacturer ensures that the high level is maintained for 2us. This does not require the user to calculate it themselves.
