Is OSC_IN actually the output and OSC_OUT the input on STM32F030?

[mega3psp]

Can someone tell me if I’m understanding this correctly?

In the documentation for the STM32F030R8T6 chip, specifically the "RM0360 Reference Manual", there’s a diagram showing how to connect an external crystal oscillator ("Figure 12. HSE/LSE clock sources"). In this diagram, the crystal is connected directly to the OSC_IN and OSC_OUT pins, with no series resistor on the OSC_OUT line to limit the drive level.

However, on the STM32F030 DISCOVERY board, there is such a resistor – R22 – even though the crystal is not installed by default. I added the crystal I wanted to use and measured the current through it (using a current clamp), and it became clear that it was being driven beyond its specified power limits. That’s when I realized the need for the series resistor (R22). I calculated the appropriate resistance value and installed it – but it didn’t reduce the current through the crystal at all.

After further investigation, I discovered that the OSC_OUT pin on the microcontroller is actually a high-impedance input, while OSC_IN is the output. So I tried placing the resistor in series with OSC_IN instead – and, as expected, this reduced the power dissipated in the crystal.

Is my understanding correct? And if so, does that mean the diagram in the official documentation is actually incorrect?

[Andrew Neil · Accepted solution]

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@mega3psp wrote:

In the documentation for the STM32F030R8T6 chip, specifically the "RM0360 Reference Manual", there’s a diagram showing how to connect an external crystal oscillator ("Figure 12. HSE/LSE clock sources"). In this diagram, the crystal is connected directly to the OSC_IN and OSC_OUT pins,?

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No - in the case of an external oscillator (ie, an external clock), it is connected to OSC_IN only - the OSC_OUT pin is free for use as GPIO:

Both pins are only used in the case of an external crystal:

Difference between an oscillator an a crystal:

https://community.st.com/t5/stm32-mcus-products/stm32f4-osc-problem/m-p/664136/highlight/true#M241592

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@mega3psp wrote:

on the STM32F030 DISCOVERY board, ... I added the crystal I wanted to use 

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Did you also remove  SB16 et al - to disconnect the drive from the ST-Link?

[mega3psp]

The issue was indeed caused by the SB18 solder bridge being in place. After I removed it, everything started working as expected. That is, OSC_IN is the input and OSC_OUT is the output. So, to limit the drive level, the R22 resistor on the STM32F030 DISCOVERY board should be used.
Huge thanks for your help!