Embedded Rust Experiments: Is My STM32 MCU Running Fast?

Re:Embedded Rust Experiments: Is My STM32 MCU Running Fast? by ChuckNorris89 on Monday January 28, 2019 @03:13AM (#19015929)

TIP: You should never scope a crystal directly as that could damage it. In production, for testing crystals, we output a PWM on a pin using the crystal as a source and measure that if the MCU doesn't have a built in functionality.

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Re:Embedded Rust Experiments: Is My STM32 MCU Running Fast? by philpem on Monday January 28, 2019 @04:51AM (#19016259)

I've never heard of putting a scope probe on a crystal damaging it, though the load resistance (typically 1Megohm to ground) and capacitance (~5pF to ground) will affect the frequency.

This is why the PWM-out is useful -- it gives you a buffered, divided-down version of the crystal clock.

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Re:Embedded Rust Experiments: Is My STM32 MCU Running Fast? by berti on Monday January 28, 2019 @03:23AM (#19015967)

STM32 has MCO outputs that can be used to more directly pass various (divided) clocks through.

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Re:Embedded Rust Experiments: Is My STM32 MCU Running Fast? by edoo on Monday January 28, 2019 @10:34AM (#19018190)

Interesting, but maybe not required. You can do a programmatic check of the crystal and PLL. I've had over 200k devices fabbed and deployed without ever directly testing a single crystal. What kind of error rate do you find?

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