51. Why should the crystal oscillator of the single chip microcomputer touch it to vibrate?

Definitely a matter of reset. After the development board is completely powered off for a period of time, can it work properly when it is powered on? I guess it should work.

What you are talking about is abnormal, it should be turned on immediately after the development board is turned off, and then the microcontroller is not normal. Right? Actually, the reason is very simple.

A typical reset circuit is a resistor connected in series with a capacitor. This reset circuit is effective when the initial power is on. Because there is less charge on the capacitor at this point, a reset pulse can be formed.

But when you turn off the board and turn it on again immediately, the charge on the capacitor will not disappear quickly due to the resistance in the reset circuit. But the microcontroller has stopped working because there is no voltage. However, when you turn it on again, the capacitor in the reset circuit has not completely disappeared because the charge has not completely disappeared, so the reset circuit will not work at this time.

Therefore, the microcontroller cannot work normally because the reset is unsuccessful. PP proverb.

It is possible that the capacitors at both ends of the crystal oscillator have poor contact or the capacitance value deviates too much, and the contact changes the circuit parameters to achieve the activation effect.

The crystal oscillator load is wrong, take a capacitor or increase a capacitor, generally about 30 47pf, if you can't reduce it, try it.

This may be if you don't have a reset.

First of all, it needs to be clear: 51 single-chip microcomputer does not have to use a 12m crystal oscillator, it is OK, 18m is OK, that is, generally speaking, the passive crystal oscillator (two-legged) within 35m can be.

Why the common 51 single-chip microcomputer should use a 12m crystal oscillator, because we often say that the 51 single-chip microcomputer is a 12-way frequency, if the 12m crystal oscillator is selected, if it is a single instruction period statement, it is just 1us, and other statements are like integer multiples of 1us, so it is very convenient to calculate the instruction time.

For the use of 51 single-chip microcomputer for serial port communication, the crystal oscillator is generally selected, because for the baud rate of 9600 and 4800, the crystal oscillator can be calculated to obtain an integer.

Microcontrollers (microcontrollers) is a kind of integrated circuit chip, which is a small and perfect microcomputer system composed of a small and perfect microcomputer system composed of a silicon wafer that uses ultra-large-scale integrated circuit technology to integrate the best processor CPU, random access RAM, read-only memory ROM, a variety of IO ports and interrupt systems, timer counters and other functions (which may also include display drive circuits, pulse width modulation circuits, analog multiplexers, AD converters and other circuits) into a silicon waferIt is widely used in the field of industrial control. From the 80s of the last century, from the 4-bit and 8-bit single-chip microcomputers at that time, to the current 300m high-speed single-chip microcomputer.

A single-chip microcomputer, also known as a single-chip microcontroller, is not a chip that completes a certain logic function, but integrates a computer system into a chip. It is equivalent to a miniature computer, and compared with a computer, the microcontroller only lacks the IO device. In a nutshell:

A chip becomes a computer. It is small in size, light in weight and cheap, which provides convenient conditions for learning, application and development. At the same time, learning to use a microcontroller is the best choice to understand the principles and structure of computers.

The use of single-chip microcomputer has been very wide, such as smart meters, real-time industrial control, communication equipment, navigation systems, household appliances, etc. Once a variety of products are used in single-chip microcomputers, they can play a role in upgrading the product, and often preceded by adjectives in front of the product name - "intelligent", such as intelligent washing machines.

It is possible to do without a 12m crystal oscillator. When a 12m crystal oscillator is used, the clock goes through a 12-way generated machine cycle, which is exactly 1 microsecond.

For this 1 microsecond, other frequencies, if you want to time accurately, it will be very cumbersome to calculate.

You can also use 6m, but the serial port baud rate is more accurate.

Is it okay to have a 6m crystal oscillator, which is determined by the device itself?

The function of the crystal oscillator circuit is to pass the clock signal flow for the microcontroller. If you have studied digital circuits, you will know that the single-chip microcomputer circuit is composed of countless gate circuits, and the gate circuit needs a clock signal as a trigger when it works. Therefore, in the case of the same circuit, the higher the pulse frequency, the higher the performance of the single-chip microcomputer.

Single-chip microcomputer, also known as microcontroller, is a microcomputer that integrates processors, memory, timing counters, and various input and output interfaces on an integrated circuit chip. Compared with general-purpose microprocessors used in personal computers, it emphasizes self-efficiency (without external hardware) and cost savings.

Its biggest advantage is that it is small in size and can be placed inside the instrument, but the storage capacity is small, the input and output interface is simple, and the function is low. Due to its rapid development, the definition of the old microcontroller can no longer be satisfied, so it is called a wider range of microcontrollers in many applications; From the 80s of the last century, from the 4-bit and 8-bit single-chip microcomputers at that time, to the current 32-bit 300m high-speed single-chip microcomputer.

The core circuit of the entire single-chip microcomputer circuit, the heart of the entire circuit, what happens if a person has no heart or the heart does not beat? You know. The crystal oscillator provides a basic clock in the timing of the entire circuit, and the clock source is indispensable in the timing circuit.

For example, the human heart provides a reference clock pulse on which all the actions of the microcontroller are carried out.

1. Analysis of the reasons why the single-chip microcomputer crystal does not vibrate.

It is a common phenomenon that the crystal oscillator does not vibrate, so what are the reasons for the crystal oscillation?

1) The PCB board is incorrectly wiring;

2) There is a problem with the quality of the single-chip microcomputer;

3) There is a problem with the quality of the crystal oscillator;

4) The load capacitance or matching capacitance does not match the crystal oscillator or the capacitor quality is defective;

5) The PCB board is damp and hits the vertical field, resulting in impedance mismatch and cannot vibrate;

6) The wiring of the crystal oscillator and the small fiber circuit is too long;

7) There is a wire between the feet of the crystal oscillator;

Just my question was for the calculation of the baud rate.