Why does an STM32 ARM Microcontroller have 2 resonators?

[Sashvat]

Hi guys,

I want to build my own Microcontroller development board from scratch, as I was checking the data sheet for the STM32F103C8T6 and STM32405RGT6 processors, there were 2 crystals specified on the data sheet. But when I look at tutorials online on how to design your own Microcontroller development board, he doesn't use a crystal at all (https://www.youtube.com/watch?v=tgUx0Gm7-RM you can skip to 18:27 to see his schematic).

And when I was watching another video on how to make your own (drone) Flight Controller this guys just uses 1 crystal than the two given on the data sheet (https://www.youtube.com/watch?v=Uu2vGFMq4Zo&list=PLoPtpxJIxgnYnPrOeGHs3rdhhPgNGIYN5&index=5 you can jump to 2:22 to see the schematic he is using.)

I am getting very confused wether I should put 2 crystals, 1, or no crystal on my board. Also, I am a 12th grader who has a passion in electronics and don't know much of the stuff as you know, so please try to make it simple for me to understand.

Thank you

[MikeDB]

Yes

[S.Ma]

I would reccomend to use STM32L496 it's a good mid-range low power part if later on you want to run on batteries.

Decent amount of on board memory as well unless using a display.

Some peripherals are 2nd gen such as the SPI with FIFO and programmable word length.

[Sashvat]

Just an out of topic question for all those who have answered here, what have you guys studied that makes you know about all this? What degree have you guys done to be able to understand all this?

[MikeDB]

We didn't have microprocessors for my first degree :) It's the 45 years of experience since that counts the most.

[Sashvat]

ohh wow! but what is your specialisation in sir?

[MikeDB]

Whatever the client wants us to be expert at :)

www.futurehorizons.com

Still three top 20 semiconductor companies haven't ever hired us for some reason ... but we're working on it :)

[Sashvat]

thank you very much for the help!

[turboscrew]

To put it more simply: There are two clocks. One is processor clock and the other is for realtime clock (and some low power peripherals). Both have RC-oscillators built in, but the accuracy is not that great. Both clocks can be switched to crystal driven oscillators or external oscillators for better accuracy.

[Sashvat]

So its not necessary to add an external oscillator, but adding one increases accuracy of timing. So there are 3 clocks in total- 1) the internal MCU's clock and the optional 8MHz and 32.768kHz crystals correct?

But which increases the accuracy of the timing? the 8MHz, or the 32.768kHz crystal?

[S.Ma]

You have to dig the spec of the external oscillator and crystal, and the datasheet electrical tables for the STM32 to know what you get.

Usually, accuracy comes at a cost, when justified. Otherwise, look at Nucleo's schematic for basic implementation.