I'm trying to set up an HSE for my STM32f4, using this crystal. but there are 2 pins: OSC_IN and OSC_OUT, but my crystal only has one output.

What do I link OSC_IN and OUT to? enter image description here

  • \$\begingroup\$ Try here stm32f4-discovery.com \$\endgroup\$
    – Laki
    Commented Jun 2, 2015 at 14:10
  • \$\begingroup\$ Why do you have that 15pF cap on the output of your oscillator? \$\endgroup\$
    – EM Fields
    Commented Jun 2, 2015 at 15:23
  • \$\begingroup\$ The 15 pF is not needed since it is what the oscillator is capable of driving (15-50pF). \$\endgroup\$ Commented Jun 2, 2015 at 18:59

1 Answer 1


The 'crystal' you are using is actually a complete oscillator module that generates a CMOS (digital) output pulse. The two pins on the STM32 are for when you are using a bare crystal and provide access points to the built-in oscillator circuit.

As you don't need to use this you can just connect the output from your oscillator to the OSC_IN pin and leave the OSC_OUT disconnected. Note that it is very important that you configure the chip to use an external clock - not the HSE oscillator - or it won't work properly.

  • 1
    \$\begingroup\$ In STs words it's commonly called HSE bypass. \$\endgroup\$
    – Arsenal
    Commented Jun 3, 2015 at 6:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.