External resistor for my oscillator design

[Aldo_Flores_Aguayo]

Hi, I have a question about this highlighted text in the AN2867 document refering to the "calculating external resistor" paragraph 

As I understood the condition to use a Rexternal is "if the power disipated in the crystal is higher than the value specified by the crystal manufacturer", as shown above so my question is how to check the power dissipated in the crystal? how to know if I`ll need to use Rext before have a physical implementation of my MCU thanks for your help

One more extra question (not a technical one) is it normal to feel a little bit overwelmed some times doings this kind of desing? (is my first time designig the STM32 MCU lets say from scratch) 

Thanks!

[GET]

Hello @Aldo_Flores_Aguayo ,

1_ You refer to the AN2867. The way to choose external crytals in explained step by step in chapter 4, and more specific is the step 4 of 4.2 chapter about Drive Level. 

To check drive level, refer to section 3.5.1 or 3.5.2  if applicable for you. 

2_ In general, some specific characteristics of crystals and some crystals reference exists in AN2867, refer to 5.1 and 5.2 according to your need.

3_If you don't care about the Crystal supplier and don't want to spend time on verifying all parmaters , you can also opt for a simpler solution : refer to the Evaluation boards that use your MCU (in your case STM32L1). For L1, evaluation boards are MB963 and MB1136 you can access here , STM32L1 - ARM Cortex-M3 ultra-low-power MCUs - STMicroelectronics

Then check the schematics and open BOM files in "CAD Resources" folder of each board. And choose reference already mounted or reference integrated in the schematics. You can also refer to layout of each board if you need examples.

Best Regards,

[Aldo_Flores_Aguayo]

Hi @GET  thanks for your time. 

Reading the docummentation you recommend me I realized that; to know if an Rext is necessary we need the physically use an oscilloscope to measure the current trougt the crystal or the voltaje on it,  just to be sure, is this correct?? 

I thought I t would be possible to define if a Rext will be used without any physical measurement. 

So now makes more sense to refer to already used crystal in evaluation boards

Hope you can validate what I`m saying. 

Thanks. 

[GET]

Hello @Aldo_Flores_Aguayo ,

I can conclude like what you say : there are two options described in AN2867, in 3.5.1 and 3.5.2, and both need measurements. I am not aware of any other possibility or other document describing other method on this subject.

Maybe another 4th possibility, that I did not mentionned in my first answer above :

4_ For HSE crystals, which do not propose list of crystals in AN2867, you may also contact any supplier listed in AN2867 (or other preferred supplier), and check if they have made this kind of measurement and have validated a crystal it up to step 5, on any of the MBxxxx existing at ST using your STM32L1. In that way, you may re-use layout and values from this supplier as valid exemple.

Best Regards,

[Aldo_Flores_Aguayo]

Hey   @GET  I think I have a 5th possibility but before, thank you for the time you take to answer my questions.

Just want to let you know that I´m using another strategy lets say, what I did was to look for some board that implemented a crystal oscillator in the same same range as the MCU I´m triying to implement (STM32L4P5RGT), then looking at the MCU implemented in that development board, search for it´s datasheet and find the "transconducance value" of that MCU, once I verified the transconductance value was the same as the one in my application MCU, I look for the documentation of the board and look on how they design the oscillator, luckily I found this (STM32G431KB Nlucle--32 Board) 

They comment the following about the oscillator they implemneted

And what I´m planning to do is to use the same crystal model (or some one with same characteristics ESR, Freq, CL, etc) and implement it as they show on the image above but to my own MCU (that technically has the same Maximum critical crystal transconductance ) , is this correct to do? what do you think?

One more questions what does the dotted square means in between the MCU oscillator pins and the external crystal configuration??

Thank you again for your time! 

[GET]

Hello @Aldo_Flores_Aguayo ,

I would say that you're right to so such a way (do it by MCU similar family characteristics), and if crystals have the same parameters/characteristics. This can help to determine the specific Step 1 of 4.2 chapter of AN2867.

But others steps need to be checked then.

About the dotted square, I am not the designer of this board, but my assumption is : squared dots are only here to indicate those parts are specific functions of this microcontroller page. Not sure they have a real meaning here. If you have a doubt, you can refer to the related User Manual of the board. Note In the more recent STM32 schematics, there is a trend to put crystal and user buttons , Joystick , User LEds, ... on a dedicated page called "Clock and user interface".

Hope this clarifies.