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

[Sashvat]

Thank you very much

[Tesla DeLorean]

#1 Skill - Self sufficiency and the ability to read technical documentation thoroughly, and infer knowledge from multiple sources/perspectives.

I have a degree in Electrical and Electronic Engineering, but was programming micro-contollers in secondary school, and building single board computers and writing compilers in sixth-form college.

>> I am a 12th grader

Lacking local context there, post secondary? What age is that like 17?

I was doing 8-bit assembler at 14, and the universe exploded from there.

The guys that designed the ARM processor were given no money, no resources and no people, which well describes the UK I grew up in.

[Tesla DeLorean]

Lot of places have pretty deep rosters of in-house talent.. and those with Einstein/Tesla level IQs working on the difficult problems.

Others have Edison's who throw crap against the wall and shout at people until things "work".

[Tesla DeLorean]

+1 on using existing boards.

If something is a close fit and does most of what you need it is better buy and experiment with that. Once you fully understand the design of the Blue-Pill, or similar boards, and their pros/cons, you'll then be in a *much* better place to design your own. It will allow you to leap-frog rather than repeat a bunch of mistakes. Some people learn by making mistakes, others learn, and avoid making mistakes. The most common cause of failure is inattentiveness to details.

[Pavel A.]

Back to the USB question: if you need a USB device (not host) function - some STM32s do not need external crystal at all. Instead, they can sync by the start-of-frame signal sent by the host side.

-- pa

[Sashvat]

I was just seeing a STM32F302CBT6 data sheet and for the LSE (32.768kHz oscillator), the capacitor values are not specified, where is it specified for this MCU?

[turboscrew]

The 8 MHz crystal or oscillator increases the accuracy of processor clock (as well as the USB and ethernet clocks). The 32 kHz crystal/oscillator increases the accuracy of realtime clock. The documents use these names:

HSE = High speed external clock = processor clock crystal/oscillator (8 MHz)

HSI = High speed internal clock = the RC-oscillator for processor clock

LSE = Low speed external clock = RTC crystal/oscillatoe (32KHz)

LSI = Low speed internal clock = the RC-oscillator for RTC clock

You can use any combination of those. Some chips can also use one crystal for both, but I have no experience how well that works.

Note, however, that when the system is set in low power mode, like standby, the processor clock is stopped.

[turboscrew]

"Almost MSc" in electrical engineering (thesis missing). Computer engineering and software engineering (my main subjects) were then under electrical engineering.

Then about 20 years of embedded systems. Different projects, different HW. From TI C6000-series to 8031.

[turboscrew]

We often start prototyping with commercial boards even when the product will eventually run on a special board we make for the customer.

[MikeDB]

I always think I'm going to make the special board with a discrete STM32, but often end up with a socket for a Blue Pill or 407 board on the PCB as often it works out cheaper than buying the individual components at the 100 off prices my customers tend to use, especially if all the other components are thru-hole.