2024-08-18 12:38 PM
Rev 19 Page 16/59:
I believe this should read "increase" instead. The purpose of Rext is to limit the current and thus lower power dissipation in the crystal. So, lowering Rext increases power dissipation.
A separate issue is that this phrase is awkward at best. First of all, on many boards Rext=0. Since it certainly can't be any lower that that, it's a strange way of phrasing the issue to say that too low a value increases power dissipation. What the appnote is trying to say is that the value of Rext should be selected to meet two competing requirements: staying below Max Drive level (which may require increasing Rext), and having sufficient Gain margin (which via the resulting change in ESR, may require decreasing Rext).
CC @STOne-32
2024-08-18 01:32 PM
Hello @BarryWhit
I will forward your rectification to the concerned team so they review it and may enter the necessary updates if they have to. Sale for the potential typo.
Best Regards.
STTwo-32
To give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.
2024-08-18 02:00 PM - edited 2024-08-18 02:02 PM
Hi @BarryWhit ,
Thanks for the valuable remark , when the first edition of this App note was written I believe in 2009 , it was a big revolution on the crystals world having STM32 HSE and especially LSE compatible with a small footprint 3215 (3,2 * 1,5mm) at that time it was the hype of IoT and consumer products battery powered. So we kept a big part of theory that was applicable for the famous tunning fork robust 12,5pF CL crystal .
Today, I supported thousands of customers and we provided xx Billions of our STM32 , it is rare to see a customer design adding a Serial Rext for LSE Crystal , it creates only issues of leakage even kept 0Ohm .
So your remark is valid !
Guess why this Rext is still maintained in litterature and design ?
crystal manufacturers and their market improved a lot today and we have a wide variety of crystals : Low power , trimmed and suitable with different sizes driven by Low power , Embedded and and Connectivity . I believe that that Rext should not exist anymore on a new design if the right crystal is selected with our LSE ( 4 drives are provided ) and also our HSE ( General Purpose or RF).
As said by @STTwo-32 we will try improve that in the AN.
Have a great end of Sunday.
STOne-32
2024-08-18 02:07 PM - edited 2024-08-18 02:30 PM
Is it also practically irrelevant nowadays to use Rext to resolve overdrive of HSE crystals? I just posted about this as a possible answer to a malfunctioning circuit, but by what you've now said perhaps I was way off.
Whether you remove the footnote or correct it doesn't matter to me, I would welcome either.
Thank you both for the very prompt response.