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Meyup
Meyup
Associate
May 31, 2020
Question

STM32F103RCT6 HSE and LSE Crystal selection

  • May 31, 2020
  • 4 replies
  • 3512 views

Hi,

I am designing a st board for the first time. First, we made all the improvements on the blue pill, which is STM32F103C8T6. When this wasn't enough, we switched to STM32F103RCT6.

I am currently drawing a schematic of this card and was confused in the choice of crystals.

I worked on AN2867 and chose some crystals for HSE and LSE. I also share how I calculate. Is there any wrong or missing in this method?

I would like to ask if these crystals I choose are correct.

0693W000001plN3QAI.jpg

This is HSE crystal calculation. and also link:

https://www.digikey.com/product-detail/en/iqd-frequency-products/LFXTAL029914REEL/1923-1523-1-ND/8634288

0693W000001plNNQAY.jpg

This is LSE crystal calculation. and also link:

https://www.digikey.com/product-detail/en/micro-crystal-ag/CM7V-T1A-32-768KHZ-6PF-20PPM-TB-QA/2195-CM7V-T1A-32-768KHZ-6PF-20PPM-TB-QACT-ND/10500164

I also see Rext resistance in some designs. I don't know why they put it or how they put it (for exp. OSC_OUT in series or between OSC_IN and OSC_OUT).

0693W000001plNmQAI.jpg

Wouldn't it be enough if I put it this way?

Regards,

Mehmet

This topic has been closed for replies.

4 replies

Bowman32
Bowman32
ST Employee
June 1, 2020

Hi @Mehmet Eyüp Arslan​ 

Both crystals are compatible with STM32F103xC.

Please refer to AN2867 Rev 12 :

  • Section 3.3 Load capacitance for the calculation of CL1 and CL2.
  • Section 3.5.3 Calculating the external resistor for Rext explanation.

Best regards

Bouraoui

Meyup
MeyupAuthor
Associate
June 2, 2020

Hi Bouraoui,

I made all my calculations following the AN2867 document.

  • You can see the calculations pictures for both LSE and HSE for CL1 and CL2.
  • I'm actually asking if Rext is needed. Because when I want to use Rext, Rext: 995 ohms according to my calculations. When I do my other calculations again, gmcirit = 5.51mA/V, gainmargin = 25/5.51 = 4.53 < 5 ( gmcirit(old) = 0.213mA/V, gainmargin(old) = 117 )

I had read in Section 3.4 that gainmargin should be greater than 5. Using Rext seems to be creating a problem, according to my design.

Would it be a problem if I don't use Rext? I am trying to understand this.

Regards,

Mehmet

Meyup
MeyupAuthor
Associate
June 5, 2020

Hi,

Is there anyone who can help?

Thank you,

Bowman32
Bowman32
ST Employee
June 24, 2020

Hi @Mehmet Eyüp Arslan​ ,

As mentioned AN2867 Rev 12 Section 3.5.3; If the power dissipated in the crystal is higher than the value specified by the crystal manufacturer, the external resistor Rext becomes mandatory to avoid overdriving the crystal.

In your case, using Rext violate the gainmargin rule. I don't recommend you to add Rext.

Best regards,

Bouraoui

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