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Can STM32L071KZU6 use HSE oscillator?

Question asked by Aaron Gage on May 2, 2018
Latest reply on May 3, 2018 by waclawek.jan

The STM32L071KZU6 is a fairly new part in a compact no-lead (UFQFPN32) package.  I am trying to determine if a high-speed external oscillator can be used with this part (in bypass), but the documentation is inconsistent and unclear.


Arguments for being able to use a high-speed oscillator with STM32L071KZU6:

  • Table 2 in the datasheet shows "Clocks: HSE/LSE/HSI/MSI/LSI" as "1/1/1/1/1" for all parts and packages, including this one, which strongly implies that one HSE is available.
  • In the reference manual, section 7.2.1 (HSE Clock), for HSE bypass, it says: "In this mode, an external clock source must be provided.  It can have a frequency of up to 32MHz...On devices where OSC_IN and OSC_OUT pins are available: the OSC_IN pin must be driven...Otherwise, the CK_IN pin must be driven."  This implies that there is always an HSE input, but it may be called "OSC_IN" or "CK_IN".


Arguments against being able to use a high-speed oscillator with STM32L0717KZU6:

  • The package pinout diagram (Figure 10) in the datasheet shows pin 1 as "PC14-OSC_IN" and pin 2 as "PC15-OSC32_OUT".  There is no "OSC32_IN" to match "OSC32_OUT" so it is not clear whether the marking for PC14 is correct.
  • Table 15 in the datasheet describes pin 1 for the UFQFPN32 package as "PC14-OSC32_IN (PC14)" with additional function of "OSC32_IN".  There is no pin shown on this package for "OSC_IN" which is where I would expect the HSE to connect.  "OSC32_" has always meant the 32.768kHz (LSE) clock input on STM32 parts.
  • The datasheet shows no pin marked "CK_IN" as mentioned in the reference manual.  "CK_IN" does not appear anywhere in the datasheet.
  • In other STM32 parts, PC14 has always been tied to "OSC32_IN". It isn't clear if the role of PC14 has changed just for this package.  Also, Figure 1 in the datasheet shows that the OSC32_IN only goes to the LSE input and OSC_IN only goes to the HSE input, so it does not appear that one pin can do both roles.


So which is it?  Can the STM32L071KZU6 use an external high-speed (20MHz) oscillator in HSE bypass mode or not?  My guess is that it cannot, but I would like to be sure since this this is how we would prefer to use the part.


Can anyone from ST, or anyone who has used this part in this way, provide some clarification?  It would be nice if the documentation made this more explicit.