What is the difference between internal oscillator, passive crystal oscillator and active crystal oscillator?

The single chip computer should run , There must be a clock signal , It's like a human heartbeat . This clock signal can be provided by a passive crystal oscillator , It can be provided by active crystal oscillator , At present, most single-chip computers also integrate internal oscillators . Today, let's talk about the difference between the three .

1. Internal oscillator

The oscillator integrated inside the single chip microcomputer , It's usually RC oscillator , Many people call “ Internal crystal oscillator ”, It's not accurate .RC The principle of the oscillator is as follows ：

Its advantage is that the cost of implementation is relatively low , The disadvantage is due to the accuracy of resistance and capacitance , therefore RC The oscillation frequency error of the oscillator is large , At the same time, it is subject to temperature 、 The effect of humidity , This is related to the process of components .

When the clock accuracy is required to be high in general systems , You can't use internal RC oscillator . Of course , Now there is also a single-chip microcomputer RC The oscillator can achieve high accuracy , Meet some UART Application of communication , But compared with the external oscillator , The accuracy is still relatively low .

2. Passive crystal oscillator

English name Crystal, Strictly speaking, it should be called crystal resonator , It is not a complete oscillator , It needs to cooperate with external circuit to realize the function of oscillator . Here's the picture ：

Passive crystal oscillators are made of ceramic and quartz , The accuracy of quartz material is higher than that of ceramic material .

The accuracy of the passive crystal oscillator is generally in 10~50ppm, Better ones can get to 10ppm within , It can meet most applications .

3. Active crystal oscillator

English name Oscillator, It is a real crystal oscillator , No external devices are required , As long as the power is supplied, the oscillation signal can be output . Here's the picture ：

Compared with the passive crystal oscillator , The active crystal oscillator signal is stable , Good quality , And the connection is simple （ Mainly do a good job in power filter ）, There is no need for complex configuration circuits .

There are many kinds of active crystal oscillators ： Ordinary active crystal oscillator 、 Constant temperature crystal oscillator 、 Temperature compensated crystal oscillator 、 Voltage controlled crystal oscillator 、 Voltage controlled constant temperature crystal oscillator, etc . Some good active crystal oscillator frequency stability can be achieved ppb（10^-9） Level , It is mainly used in some occasions that require high frequency accuracy .

To sum up ：

1. Inside RC The accuracy of the oscillator is poor 、 Lowest price （ Integrated in the MCU ）.

2. The accuracy of passive crystal oscillator is slightly higher , It can meet most applications , The cost is moderate .

3. The active crystal oscillator signal is stable 、 High precision , Cost is also higher .

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