Confusion About an Oscillator for MCU

[Mtech1]

Hello Community,

I am trying to understand how to choose the right oscillator for my any microcontroller. While exploring, In general I found multiple options like internal oscillators, external oscillators, crystal oscillators, and quartz crystals.

I believe Microcontrollers require a clock signal to operate, and this clock signal is typically a square wave generated by an oscillator circuit

I would like to know:

What are the key differences between using a quartz crystal , crystal oscillator, internal oscillator and an external oscillator?

 

[KlausST]

Hi,

every microcontroller has different options. Thus I don´t think it´s a good idea to talk about this topic in "general".
Every micrcontroller datasheet lists all the specifications, requirements, features ... of the different options.
What for one microcontroller may be true.. it is not necessarily true for a different microcontroller.

Thus I recommend to focus on a specific microcontroller. Read it´s datasheet about the options and their specifications ... and on any confusion:
* post a link to the datasheet
* refer to the section / page / figure ....
* and ask a specific question.

I mean some "general features" should be obvious: like that an internal oscillator needs no external oscillator components, thus it is cheaper and needs less PCB space.

Usually: I don´t look for "what a microcontroller can offer ... and how to use it", but the other way round: I decide what my application needs and look for a microcontroller that can satisfy this.

Klaus

 

[danadakk]

General differences :

Xtal Based typically < 100 ppm inaccuracy over T & V. If external module/hybrid type 20 ppm or better.

Internal RC based, factory trimmed, ~ 5% accurate, some 2% range.

Check datasheets for more complete data/characterization info.

Other considerations jitter, startup delay.


Regards, Dana.

 

[Aussie Susan]

What @KlausST says about the use of a microcontroller also applies to the selection of the clock source.
For example, a UART typically will allow for about 2-3% variation in the clock for reliable interpretation of the incoming signal (and also the generation of the outgoing signal).
On the other hand, if you are using SPI or some other communication that includes a clock signal then jut about anything will do.
If you are trying to make (say) a digital clock, then you need to think how accurate it needs to be and over what length of time. This will tell you what accuracy of the internal timers/delays are needed and hence the accuracy of the appropriate clock source.
Therefore the key consideration is what you are trying to do, and then design the rest of the system accordingly.
Susan

 

[dick_freebird]

In general I favor buying a CMOS-levels "clock in a can" oscillator, good to go. Even a uC which "can" use a bare xtal, will be less painful if you feed it from one of those.

 

[FvM]

Using external oscillator is convienant and avoids frequency errors by wrongly selected load capacitors, agreed. But modern uC have EMC friendly low level oscillator circuits. Using a crystal keeps a possible interference source inside the chip.

 

[danadakk]

UART Clock Accuracy requirements :

https://www.analog.com/en/resources/technical-articles/determining-clock-accuracy-requirements-for-uart-communications.html