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Capacitance of OSC_32 pins

Panda
Associate II

Hello,

 

Datasheet DS14791 Rev 4 has a note on page 161 stating that PCB and pin capacitance for the OSC_IN/OSC_OUT Interface can be assumed as 4 pF combined.

I qoute: "For CL1 and CL2, it is recommended to use high-quality external ceramic capacitors,
designed for high-frequency applications, and selected to match the requirements of the
crystal or resonator (see Figure 20). CL1 and CL2 are usually the same size. The crystal
manufacturer typically specifies a load capacitance, which is the series combination of CL1
and CL2. The PCB and pin capacitance must be included (4 pF can be used as a rough
estimate of the combined pin and board capacitance)."

 

I would calculate C1 = C2 = 2*(C_LOAD - C_STRAY) and assume C_STRAY with 2 pF in this case.

- Can you confirm that this is the correct way to size C1 and C2?

 

I was wondering if the same assumption can be made for OSC32_IN and OSC32_OUT Pins. I didn't find a note on that. Otherwise I would go by the chapter 5.3.17 I/O port characteristics and assume 5 pF per pin.

- But since this a quite big difference to the 4pF combined from the note I assume that a similar assumption can be made for OSC32?

 

 

Best Regards

Lucas

 

1 ACCEPTED SOLUTION

Accepted Solutions
mƎALLEm
ST Employee

Hello,

I invite you to read this article: "How to select a compatible crystal and load capacitors for STM32 with layout guidelines" especially the section 4. Calculation of the external load capacitors value:

"Note that in most of STM32 datasheets, CIO is provided as 5 pF and this is the worst case typical condition of 5 pF which is generic to all I/Os."

And the estimation of Cs is around 3pF.

Refer also to these posts:

What is the actual STM32 pin capacitance (for the purpose of sizing an HSE crystal)?

Crystals and load capacitors for stm32l432

Hope that helps

 

 

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.

View solution in original post

2 REPLIES 2
mƎALLEm
ST Employee

Hello,

I invite you to read this article: "How to select a compatible crystal and load capacitors for STM32 with layout guidelines" especially the section 4. Calculation of the external load capacitors value:

"Note that in most of STM32 datasheets, CIO is provided as 5 pF and this is the worst case typical condition of 5 pF which is generic to all I/Os."

And the estimation of Cs is around 3pF.

Refer also to these posts:

What is the actual STM32 pin capacitance (for the purpose of sizing an HSE crystal)?

Crystals and load capacitors for stm32l432

Hope that helps

 

 

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.

Ah yes, thank you.

This was very helpful!