The AN2867 mentioned before discusses the Pierce oscillator used in STM32 and STM8 in section 3.5, which might require a serial external resistor to limit the power dissipated in the crystal. Except for 32kHz no general statement can be made as to whether Rext is necessary or not.
The external parallel resistor was used with old technologies (e.g. TTL) to set the operating point of the oscillator gate, but is no longer required with STM32 microcontrollers.
Does it answer your question?
/Peter
In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.
