Okay, I think this is a passive crystal from Seiko-Epson. I've never seen one in this package.
Edit: https://www5.epsondevice.com/en/products/crystal_unit/fc135r.html
this post was submitted on 19 Mar 2024
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What is this thing?
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Yup looking at the components around and the layout, it does look like a quartz crystal, especially considering it's right next to the microcontroller, and the 2 beige components above are the matching capacitors.
But yeah I've never seen a package with an open window. Maybe it's an ultra high precision that has been tuned with lasers (usually done for patch antennas).
Who would need an ultra high precision quarz on an STM32 processor?
This is being used in a LoRa module (it's a Reyax RYLR896 (which is not great for my purposes ngl)). The STM32 acts as an intermediary between its UART interface and the LoRa module's SPI interface, basically doing some rough translation and some encryption, nothing major. I believe the LoRa stuff has its own crystal, so I would imagine there is no need for an external source.