STM32U5 HSE Crystal

[AMars.4]

I'm currently at schematic stage designing a board based around the STM32U5G9ZJT6Q. The PCB will be produced and assembled by JLCPCB, so I had a look for a HSE crystal they have in stock and found this:

https://jlcpcb.com/partdetail/YangxingTech-X322516MLB4SI/C13738

https://jlcpcb.com/api/file/downloadByFileSystemAccessId/8579709258350247936

Calculations seem to check out OK (Gm_crit < Gm_critMax), I'm just a bit unsure about the Rext resistor and whether it is required for this part. I have put an 0603 0R resistor in for now, is there a way to calculate whether I will need some resistance there, or does it require measurements to be taken on the circuit, then adjust accordingly?

Many Thanks,

Anthony

[Peter BENSCH · Accepted solution]

For STM32U5 and your crystal (16MHz, C0=7pF, CL=9pF, ESR=80ohms, R19=0) I'm calculating g_margin = 9.059, which is much larger than the minimum of 5.

With R19=64ohms or greater, your g_margin will be lower than the required value of 5, so leave it at zero ohms.

Please check the layout recommendations in AN2867, as they are at least as important as the load caps.

Regards
/Peter

[AMars.4]

Glad to know it should be OK, I will take a look at the layout recommendations.

Would you mind sharing the formula / calculation you used to arrive at g_margin = 9.059?

Thank you :)

[AMars.4]

Actually I understand now, get Gm from (GmCritMax * 5), then do (gm / Gmcrit) to arrive at your value.

I missed the sentence on pg.14 of AN2867:

The conversion between the oscillator transconductance (gm) and the oscillation loop
maximal critical transconductance (Gm_crit_max) is given by Gm_crit_max = gm / 5.