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I'm looking for a 25MHz crystal oscillator for the STM32F411. I spent a couple hours today going through the AN2867 Oscillator design guide from ST but still having trouble understanding it and finding something that fits the required parameters. There are suggested LSE oscillator part numbers in the application notes but nothing for the HSE oscillators. I posted a picture of some specs below but when I filter for 25MHZ oscillators on Digikey, I cannot find filter options shown in the picture below.

Can someone suggest a 25MHz crystal oscillator, or a better way to filter/find parts on digikey/mouser/etc that would fit these specs?

enter image description here

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    \$\begingroup\$ Are you looking for a "crystal", a two pin passive piece of quartz that connects to both OSC pins, or for a "crystal oscillator", an active device that is fed with power and it gives out square wave clock that connects only to OSC_IN? Big difference. \$\endgroup\$
    – Justme
    Apr 8, 2020 at 6:57
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    \$\begingroup\$ Usually for high frequencies (>16MHz) a MEMS Oscillator is used instead of quartz crystal. For 16Mhz there are recommendations on Page 40 on the AN. A suitable 25MHz Mems Oscillator would be NZ2520SB-25.00M, its used on the STM32 Evaluation boards. \$\endgroup\$
    – sgt_johnny
    Apr 8, 2020 at 7:22
  • \$\begingroup\$ At least the STM32F2 eval board has a standard 25 MHz crystal. The datasheet of the part you mentioned says that it is a standard crystal clock oscillator, not a MEMS oscillator. \$\endgroup\$
    – Justme
    Apr 8, 2020 at 8:24
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    \$\begingroup\$ I also have to ask why to specifically want a 25 MHz crystal for this? Why not 8 MHz? Higher frequency crystals and smaller crystal packages tend to have larger ESR so it will be more difficult to find a suitable 25 MHz crystal. The only reason I see to use 25 MHz crystal would be Ethernet, and the F411 does not have it. \$\endgroup\$
    – Justme
    Apr 8, 2020 at 9:37
  • \$\begingroup\$ most new micro controllers have plls or dlls which can use the external or internal oscillator to generate a clock of an "arbitrary" frequency, so you can actually use a lower frequency oscillator or crystal and then step it up, note: be careful when using oscillators on low power designs, as you will have to shut it down somehow when going low power \$\endgroup\$
    – diegogmx
    Apr 8, 2020 at 11:45

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The parameters in the picture you posted are not parameters of the crystal. They are the parameters of the STM32 built-in oscillator, namely transconductance, and the critical limit the crystal circuit must not require for good operation. That is why you can't find crystals with those parameters. But you will need parameters of both devices to find out if they are compatible or not.

With the parameters of both the crystal and MCU, and the equations in the appnote, you can calculate if the gain margin is larger than 5, which is a generally accepted safe value that the crystal and oscillator work together.

Basically the equations say that higher frequency, higher load capacitance rating, and higher ESR rating of the crystal need more transconductance from the oscillator to work, so all those bring down the gain margin. To put it another way, lower frequency, lower load capacitance rating, and lower ESR rating make it easier to achieve good gain margin.

In your case, if you don't have any specific need (such as Ethernet or some audio master clock) to use exactly 25 MHz crystal, I suggest to use lower frequency crystal like for example 8 MHz. These may come in slightly larger packages than the 25 MHz crystals. Since you want 100 MHz CPU frequency, you must use the PLL, and the crystal frequency must be prescaled down to the 1 MHz to 2 MHz range before it is fed to the the PLL input. So the PLL does not know if the 1 MHz was 25 MHz divided by 25, or 8 MHz divided by 8.

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