Does the microcontroller have to be connected to an external crystal oscillator? Why are there two c

Most of the 51 microcontrollers I have come into contact with now do not have internal oscillators, so it is necessary to use external oscillators, and if you really want to integrate a crystal oscillator in the microcontroller, the volume of the microcontroller will become larger (think of the DS12887 chip). What I know is that the ATMEGA series microcontroller has an RC oscillator inside, which can work even when there is no external crystal oscillator.

Everyone is basically right, and I will continue to add.

The questioner should be a beginner.

I guess the "crystal oscillator" mentioned by the questioner is not the crystal oscillator in the strict sense of the word that we understand. The questioner meant that it could be used by the microcontroller, but he only knew how to use the crystal oscillator, so there was such a statement.

The inside of the crystal oscillator is a crystal that is cut according to the requirements, and it is basically impossible to do it in a single-chip microcomputer. The oscillation source inside the microcontroller is RC oscillator, and the performance cannot be compared with the crystal oscillator, but it also meets the general requirements.

The traditional 51, which is the Atmel 51 commonly used for beginners, does not have an internal RC oscillator, and some of the high-end 51s (or improved 51s) of other brands do.

Also, there is no such thing as "finished 51", and I don't know what you are trying to say.

Some of the 51 series microcontrollers have internal crystal oscillators, and some do not have internal crystal oscillators.

If there is an internal crystal oscillator, you can use an internal crystal oscillator when your system does not have high requirements for clock and timing, but if you have high clock or time requirements (such as serial port communication, electronic clock), you still use an external crystal oscillator.

Most of them have to be answered.

Like the C8051F series, there is no need.

The production process of IC and crystal oscillator is different, and it is difficult to integrate together.

In some microcontroller applications, there may be two crystal oscillators, called system clock crystals and peripheral clock crystals. This is because in a microcontroller, different modules may need to operate at different clock frequencies, so multiple crystal oscillators need to be used to meet these needs.

The system clock crystal oscillator is the main clock source of the internal system of the single-chip microcomputer, which is responsible for controlling the clock frequency of the whole system. The frequency of this crystal oscillator is generally relatively high, usually ranging from tens of MHz to hundreds of MHz. It works in tandem with the CPU and various buses.

Peripheral clock crystal oscillators are usually sensitively used in the internal peripheral modules of microcontrollers, such as timers, counters, serial ports, etc., which may need to communicate or control with external devices. The use of peripheral clock crystals allows these peripheral modules to operate at independent clock frequencies, increasing the flexibility and reliability of the entire system. Generally speaking, the frequency of peripheral clock crystal oscillators is relatively low, and the bridge keys usually range from tens of kHz to tens of MHz.

It should be noted that since different circuits are used inside the microcontroller to handle different tasks, the frequency of the peripheral clock crystal oscillator and the system clock crystal oscillator should not be the same, otherwise the system may have problems.

First, the connection method is different.

1. Internal crystal oscillator: series resonance composed of C1 and L1.

2. External crystal oscillator: parallel resonance composed of C0, C1 and L1.

Second, the characteristics are different.

1. Internal crystal oscillator: it will oscillate on one of its harmonic frequencies, which is an integer multiple of the fundamental frequency. Only odd harmonics are used, such as 3x, 5x, and 7x harmonic crystals.

2. External crystal oscillator: The capacitance on the external circuit will lower the oscillation frequency of the circuit. When designing a quartz crystal oscillation circuit, the total stray capacitance and applied capacitance on the circuit should be the same as the load capacitance used by the crystal manufacturer.

Third, the vibration frequency is different.

1. Internal crystal oscillator: quartz crystal with a frequency above 30 MHz (to 200 MHz).

2. External crystal oscillator: quartz crystal with a frequency below 30 MHz.

<> external crystal oscillator of the microcontroller can be turned off. Because the external crystal oscillator can be stopped at the right time and enter the sleep state, the power consumption can be reduced. The single-chip microcomputer is a small and perfect microcomputer system that uses ultra-large-scale integrated circuit technology to integrate the best processor CPU, random access RAM and read-only memory ROM with data processing capabilities into a high shortage of silicon wafers, which is widely used in the field of industrial control.

From the 80s of the last century, from the then 4-digit and 8-digit single-chip microcomputer socks, to the current 300m high-speed single-chip microcomputer. 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.

This can be selected by setting the programmer software fuse.

It might be useful for you to take a look here.

Without crystal oscillator, there is no clock cycle, and without clock cycle, the program cannot be executed**, and the single-chip microcomputer cannot work.

When the microcontroller works, it takes instructions from the ROM one by one and executes them step by step. The time that a microcontroller accesses a memory is called a machine cycle, which is a time benchmark. — Machine cycles include 12 clock cycles.

If a microcontroller chooses a 12MHz crystal oscillator, its clock cycle is 1 12us, and one of its machine cycles is 12 (1 12)us, which is 1us.

MCS-51 MCU all the instructions, some of them are completed relatively quickly, as long as one machine cycle is on the line, some are completed relatively many, it takes 2 machine cycles, and there are two instructions that take 4 machine cycles. In order to measure the length of time an instruction is executed, a new concept is introduced: the instruction cycle.

The so-called instruction period refers to the time it takes to execute an instruction. For example, when it is necessary to calculate the time required for the DJNZ instruction to complete, it is necessary to know the frequency of the crystal oscillator, if the crystal oscillator used is 12MHz, then a machine cycle is 1us. The DJNZ instruction is a two-cycle instruction, so it takes 2us to execute it once.

If the command needs to be executed 500 times, it is exactly 1000us, which is 1ms.

The machine cycle is not only important for command execution, but also the time benchmark for microcontroller timers and counters. For example, if a single-chip microcomputer chooses a 12MHz crystal oscillator, then when the value of the timer is added to 1, the actual elapsed time is 1us, which is the timing principle of the single-chip microcomputer.

Provide the timing of single-chip microcomputer work, in fact, it is equivalent to the principle of your computer's CPU frequency!