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I got myself a cheap ST Nucleo-L053R8 on discount, to play around with a couple ideas.

The main STM32L053R8 only has a low-frequency 32.768 kHz crystal for RTC, but it can also be provided with an 8 MHz clock from the included ST-Link USB programmer. The LF crystal is marked 32G501, and the ST-Link's is marked 8.000Q5G.

Does anyone have any idea how accurate these two crystals are at room temperature? The documentation doesn't seem to specify.

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I found the bill of materials for this board on the product page:

It has these two descriptions for some crystals: enter image description here

So it looks like they are probably ±20 ppm. That is actually usually the tolerance over the full specified temperature range -- I'd expect even less variation at room temperature.

However, you did state that the 8MHz crystal is actually not present on this board, so I'm not sure if this is the right crystal. Even if it's not, there's a good chance they use the same one on the programmer.

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  • \$\begingroup\$ Since STLINK part of the board is detachable I don't think they would use its clock in application development. Some MCUs (e.g. STM32L4 series) have hardware auto calibration of MSI clock with LSE, for this reason HSE crystal is not populated on corresponding Nucleo boards. From the datasheet, L0 does not have this feature, although manual MSI trimming is possible. \$\endgroup\$ – Maple Jan 23 '20 at 19:38
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    \$\begingroup\$ At least on a NucleoH7 board, default CubeMX project for that board does configure the clock pins for crystal use, but to use the 8MHz external square wave clock from STLink part. However it is still not the default clock source, so it is up to user to change to use that clock. And note that the STLink crystal is 20ppm with rated 12pF load. They have put 20pF caps there, which is already 10pF load plus any pin and stray capacitance, which can easily exceed 2pF. So it may not be running at exactly right frequency if there is some tolerance in the load capacitance. \$\endgroup\$ – Justme Jan 23 '20 at 20:00
  • \$\begingroup\$ X1 is the crystal used by the ST-Link chip so it is included. X3 is not included, which would be a dedicated crystal for the target MCU, but ST-Link can provide a clock signal generated from X1. \$\endgroup\$ – relatively_random Jan 23 '20 at 20:26
  • \$\begingroup\$ @Maple I think they usually don't populate the HSE for the main MCU. Instead there is a clock signal from the programming MCU's MCO going via a solder bridge to the clock input of the main MCU. \$\endgroup\$ – Jan Dorniak Jan 24 '20 at 10:16

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