Correction Request: AN2867 Oscillator design

[BarryWhit]

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 

[STTwo-32]

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 

[STOne-32]

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 ? 

• This is the only easy way to measure the famous or so called « negative resistance » by our partners of Crystal manufacturers and customers and define  another ratio so called Safety factor  that was widely used in automotive industry to determine a stable oscillation over time . But not the same as oscillation start up we define with Gm .

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

[BarryWhit]

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.