Custom STM32L476 based board LSE and HSE

[Stm325]

Hello everyone, so it is my first time designing a board with stm32l476rg MCU. I was using the reference design from this sheet: https://www.st.com/resource/en/application_note/an4555-getting-started-with-stm32l4-series-and-stm32l4-series-hardware-development-stmicroelectronics.pdf

When I was looking at datasheets to choose LSE and HSE I saw this note on section 7.7 of https://www.st.com/resource/en/user_manual/um1724-stm32-nucleo64-boards-mb1136-stmicroelectronics.pdf

Also, for my project I used STM32L476RG Nucleo-64 board and in clock configuration, I think only internal oscillators are used.

I am not sure if tere is a passive use of the HSE and LSE because I think those are like default for every stm32 project but I am trying to understand why are LSI RC and HSI RC not enough. Not stable with temperature changes maybe because of RC or precision, not sure. Appreciate any help. Also is the reference design on here: https://www.st.com/resource/en/application_note/an4555-getting-started-with-stm32l4-series-and-stm32l4-series-hardware-development-stmicroelectronics.pdf is enough to design my custom stm32 based board or do I need any other sheets for reference? Thank you

[TDK]

LSE and HSE are preferable for accuracy and stability. If those aren't concerns for you, HSI/LSI are sufficient. Doesn't look like you use LSI anywhere--it is typically only used on low-power projects. USART needs some amount of accuracy, typically +/- 2% or so. HSI should be sufficient here.

The AN4555 document should be all you need. The reference schematic is a bonus.

Good luck!

If you finish the schematic and want it checked, feel free to post.

[Stm325]

I am working with high frequency data, using ADC as its max speed and also SPI to send this data. the system seemed to be working without LSE or HSE but I will still try to add those oscillators if size will not be an issue. 

There are also a lot of options to choose the HSE and LSE. In Nucleo Board's guide these 2 models are mentioned:

For LSE: ABS25-32.768KHZ-6-T

For HSE: The mentioned model is not available but it says 8 MHz, 16 pF, 20 ppm for X3(Normally uses ST-Links 8MHz crystal but model is not mentioned). I couldnt find any crystal to match these exactly but I found ABLS-8.000MHZ-B2-T which is 18pF.

As I said I am not sure about using them for now but I appreciate any recommendations about choosing them, I found STM32s list about choosing oscillators and also ECS's website about choosing them but the lists are really long.

Also I will definitely share my schematic when I am done, thank you so much!