STM32F103RCT6 HSE and LSE Crystal selection

[Meyup]

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.

This is HSE crystal calculation. and also link:

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

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).

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

Regards,

Mehmet

[Bouraoui Chemli]

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]

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]

Hi,

Is there anyone who can help?

Thank you,

[Bouraoui Chemli]

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