51 MCU crystal oscillator and reset common typical circuit-EEWORLD

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1. Typical circuit of internal oscillation.

Theoretically, the higher the oscillation frequency, the faster the microcontroller runs, but at the same time, the higher the requirements for the memory speed and printed circuit board. Just like the barrel principle. At the same time, the performance of the microcontroller is not only related to the CPU computing speed, but also has a lot to do with the memory speed, peripheral speed, etc. Therefore, 6~12MHZ is generally selected. The parallel resonant circuit does not have strict requirements on the value of the capacitor, but it will affect the stability of the oscillator, the frequency of the oscillator, the rapidity of oscillation, etc. Therefore, C1 and C2 are generally selected to be 20~100pF, and the oscillator has a higher frequency stability at 60~70pF. Ceramic package capacitors can further improve temperature stability.

51 MCU<wbr>Crystal Oscillator and Reset Commonly Used Typical Circuits

Internal oscillation typical circuit

2. Typical circuits for power-on reset and button reset.

(Excerpt from Baidu Knows) The 51 MCU is reset at a high level, so first look at the situation when the MCU is powered on with a 5V power supply (power on): At this time, the capacitor charging is equivalent to a short circuit (capacitor characteristics: AC, DC, power on is equivalent to AC), you can think that the voltage on RST is VCC, this is the reset state of the MCU. As time goes by, the voltage across the capacitor increases, which causes the voltage on RST to decrease. When it drops to the threshold voltage, the reset process is completed.

If SW is pressed (the hat button in the key reset), the button short-circuits C. At this time, the capacitor is discharged and the voltage at both ends is VCC, that is, the voltage of the RST pin is VCC. If the specified reset time is exceeded, the microcontroller is reset. When the button is released, the voltage of the RST pin is 0, and the microcontroller is in operation.

The reset requirement for the 51 microcontroller is: the high level time on RST is greater than 2 machine cycles. You are using a 12MHz crystal oscillator, so one machine cycle is 1us. To reset, just add a high level for 2us.

The RC constant in the figure is 51K×1uF=51ms (this is a calculation from Baidu's picture, which can be used to deduce the values of R and C. The values are for reference only, and common values of components are preferred), that is, 51 milliseconds. This constant is large enough.

Power-on reset typical circuit

Typical circuit for button reset (it seems that R2 can be smaller than R1?)

Reference address：51 MCU crystal oscillator and reset common typical circuit

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