51 cause of failure of single-chip microcomputer Vibration

 

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How can I determine whether the crystal oscillator is working properly?

1. There are many judgment methods. It is most direct to view the waveform with an oscilloscope. You can also test the voltage with the voltage file of a digital multimeter. Because the duty cycle of the crystal oscillator waveform is 50%, therefore, the average voltage measured is about 1/2 VCC. For 51 single-chip microcomputer, when using external program memory, you can also test the voltage or waveform of the psen pin or P0 Port Pin, only when the crystal oscillator circuit works normally will there be signal output on those pins, but now it is seldom used to extend the memory, so the voltage or waveform at both ends of the crystal can be measured, but when the crystal oscillator circuit design is poor, the introduction of the test device may lead to vibration shutdown.

 

2. The voltage difference between the two ends of the crystal is not the average voltage difference. Although in fact, the voltage at both ends of the crystal may be different due to the influence of the external circuit, this is not the basis for judging whether the crystal oscillator is vibrating, it is not a condition for the crystal oscillator circuit to work normally. As for one high, one low, no operation means that one end is VCC or close to VCC, and the other end is 0 or close to 0. At this time, the crystal oscillator circuit certainly does not vibrate, otherwise, the duty cycle of 50% will bring the average voltage to around 1/2 VCC, but this expression is not accurate, so the technology should be described as accurately as possible.

 

3. Listen to the sound to determine whether the crystal oscillator is unreliable. The crystal oscillator frequency is far higher than the upper limit of the frequency that the human ears can hear. Sometimes it is difficult to hear it, which indicates that the crystal quality is poor, more often, normally working crystals do not sound to anyone's ears. Sometimes the sound comes from external circuit components.

4. One of the two signal inputs of the single chip microcomputer is 19 (xtal1) and the other is 18 (xtal2). The corresponding circuit inside the single chip microcomputer is a high-gain amplifier. When the crystal oscillator is connected outside, the 19-pin corresponds to the input end of the high-gain amplifier, and the 18-pin corresponds to the output end of the high-gain amplifier. Therefore, when you measure the value, there should be a signal at the output end of the high-gain amplifier, that is, the 18-pin

 

 

51 MCU oscillating circuit?

There is a high-gain reversed-phase amplifier in the MCS-51 microcontroller chip, the input end of the reversed-phase amplifier is xtal1, the output end is xtal2, the oscillator circuit and clock circuit composed of the amplifier constitute the MCU clock mode. Depending on the hardware circuit, the MCU clock connection mode can be divided into internal clock mode and external clock mode, as shown in Figure 2.11

(A) Internal Clock Circuit (B) External Clock Circuit

 

 

 

 

Internal Clock schematic diagram (a self-excited oscillator circuit)

In the internal clock circuit, the Z crystal oscillator and two fine-tuning capacitors must be connected at both ends of the xtal1 and xtal2 pins to form an oscillating circuit. Generally, C1 and C2 take 30pf, the crystal oscillator frequency ranges from 1 to 2 MHz ~ Between 12 MHz. For external clock circuits, xtal1 is required to be grounded and xtal2 is connected to the external clock. There are no special requirements for external clock signals, as long as a certain pulse width is ensured, the clock frequency is lower than 12 MHz.

The oscillator's oscillator signal is sent from the xtal2 end to the internal clock circuit. It splits the oscillator signal into two groups and generates a two-phase clock signal P1 and P2 for MCU use. The period of the clock signal is called the state time s, which is twice the period of the oscillation. The P1 signal is valid in the first half of each State, and the P2 signal is valid in the second half of each State. The CPU is the two-phase clock P1 and P2 as the basic cycle to coordinate the effective work of each part of the microcontroller.