Questions about the crystal oscillator of the microcontroller. Why are there two crystal oscillators

1.Not all microcontrollers require crystal oscillators.

The crystal oscillator provides an accurate frequency, but it can also be connected to an off-the-shelf clock signal, using a ceramic oscillator, using an RC oscillator, setting it to use an internal oscillator (usually an internal RC), etc.

2.The xtal foot, as its name suggests, is designed to connect the crystal oscillator.

3.Some single-chip microcomputer xtal pins and i o pins share physical pins, if set to do i o use, it is no longer xtal pin, not for crystal oscillator.

4.The active crystal oscillator is a circuit module, which contains a complete oscillation circuit, including crystal and load capacitors, as well as active devices such as transistors or integrated circuits, etc., so it needs to be connected to the power supply, and the oscillation signal can be output after the power supply.

The passive crystal oscillator is just a simple quartz crystal, and various other devices must be added to form a vibrating soup circuit. Passive crystal oscillators are passive devices and do not have power pins. In addition, some passive crystal oscillators are also encapsulated with two capacitors inside, and a total of three pins are outside, in this case, there are fewer quartz crystal oscillators, and ceramic magnetic crystal oscillators are easier to see.

1.Do all microcontrollers need crystal oscillators?

The clock pulse of the single-chip microcomputer either relies on the crystal oscillator or the external pulse.

Is the foot just for the crystal oscillator? Yes.

3.The difference between active crystal oscillator and passive crystal oscillator is the presence or absence of load capacitance? There is no such statement.

The capacitor is to filter the crystal oscillator clutter, the excitation signal is called active, and the passive is to oscillate by self-excitation.

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.

According to the needs of use, for example: if you want to generate a standard serial port baud rate, you should use, if you want to let the 51 single-chip microcomputer generate an integer clock frequency, you can use 12MHz or 24MHz single-chip microcomputer.

In addition, according to the parameters of the microcontroller itself, do not choose too high frequency, otherwise it will work unstable. For example: ATMEGA8L-8PU, the meaning of an 8 after this single-chip microcomputer is that it is recommended that the maximum working frequency should not exceed 8MHz, if it exceeds 8MHz and is not greater than 16MHz, you can choose ATMEGA8L-16PU.

Fetch an instruction from memory and indicate where the next instruction is in memory. Instructions are decoded and tested, and corresponding operation control signals are generated to facilitate the execution of specified actions. Command and control the direction of data flow between CPU, memory, and I/O devices.

The PC is used to determine the address of the next instruction to ensure that the program can be executed continuously, so it is often referred to as the instruction address counter. Before the program can start executing, the address of the memory unit (i.e., the first address of the program) must be sent to the PC so that it always points to the address of the next instruction.

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.

Summary. A crystal oscillator circuit is a small electronic component consisting of a transistor, multiple lattices, upper and lower capacitors, and two coupling structures. The function of the crystal oscillator circuit is to generate a sinusoidal wave of a fixed frequency, which is often used as an input signal for a phase-locked loop or as a circuit clock signal.

Crystal oscillator circuit is very common in the electronics industry and has a wide range of uses, because it is an important sequencer in electronic circuits, and has its important role in many electronic circuits such as signaling control systems, radio frequency circuits, radios, digital systems, household appliances, etc.

The crystal oscillator circuit is the core of the minimum system of the single-chip microcomputer, it plays the role of the generation and synchronization of the clock cycle, is the core of the system of the town, it can provide a clock with accurate frequency, and provide clock control for each module of the single-chip microcomputer, such as CPU, memory, external IO, etc., and at the same time provide the correct time series for the system to ensure the normal operation of the system.

A crystal oscillator circuit is a small electronic component consisting of a crystal hand wide body tube, multiple crystal lattices, upper and lower capacitors, and two coupling structures. The function of the crystal oscillator circuit is to generate a sinusoidal wave of a fixed frequency, which is often used as an input signal for a phase-locked loop or as a circuit clock signal. Crystal oscillator circuit is very common in the electronics industry and has a wide range of uses, because it is an important sequencer in electronic circuits, and plays an important role in many electronic circuits such as signaling control systems, frequency change circuits, radios, digital systems, household appliances, etc.

First, the role of crystal oscillator:

The clock source of microcontroller can be divided into two categories: clock sources based on mechanical resonant devices, such as crystal oscillator and ceramic resonant groove; RC (resistive, capacitive) oscillator. One is the Peirce oscillator configuration, which is suitable for both crystal and ceramic resonant grooves.

The other is a simple discrete RC oscillator. Oscillators based on crystal and ceramic resonant grooves typically provide very high initial accuracy and low temperature coefficients. RC oscillators are quick to start up and inexpensive, but are typically inaccurate over the entire temperature and operating supply voltage range, varying from 5% to 50% of the nominal output frequency.

However, its performance is affected by environmental conditions and the choice of circuit components. Careful attention needs to be paid to the selection of components and the layout of the circuit board for the oscillator circuit. When in use, the ceramic resonant groove and the corresponding load capacitance must be optimized for a specific logic family.

Crystal oscillators with a high Q value are not sensitive to amplifier selection, but can easily cause frequency drift (and possibly even damage) when overdriven. Environmental factors that affect the operation of an oscillator are: electromagnetic interference (EMI), mechanical vibration and shock, humidity and temperature.

These factors can increase changes in output frequency, increase instability, and, in some cases, cause oscillator stalling. Most of the above problems can be avoided by using the oscillator module. These modules come with their own oscillators, provide low-impedance square wave outputs, and are capable of guaranteed operation under certain conditions.

The two most commonly used types are crystal oscillator modules and integrated RC oscillators (silicon oscillators). The crystal oscillator module provides the same accuracy as the discrete crystal oscillator. Silicon oscillators are more accurate than discrete RC oscillators and in most cases can provide accuracy comparable to ceramic resonant grooves.

2. A brief introduction to crystal oscillator:

The full name of crystal oscillator is crystal oscillator refers to cutting a thin slice (referred to as a wafer) from a quartz crystal at a certain azimuth angle, quartz crystal resonator, referred to as quartz crystal or crystal, crystal oscillator; The crystal element that adds an IC to the inside of the package to form an oscillation circuit is called a crystal oscillator. Its products are generally packaged in metal shells, but also in glass shells, ceramic or plastic packages.

What is a crystal oscillator and what role can it play next to the microcontroller? Today is a long time to see.

Input the clock signal to the single-chip microcomputer, and the single-chip microcomputer executes the instruction period, and the period is determined by the crystal oscillator.

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!