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Posts: 2
Registered: ‎08-06-2017
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Driving LFXO_N with High Frequency

Can I drive LFXO_N with frequencies up to 24 MHz although we know the low energy domain is deigned for low frequencies specifically 32KHz? I need to use LETIMER0 to generate 4 MHz output and while I am searching, I found different information that does not clarify it:

1- http://community.silabs.com/t5/32-bit-MCU-Knowledge-Base/Driving-the-LFXO-with-an-external-clock-sou...

2- (3.2) https://www.silabs.com/documents/public/application-notes/AN0004.pdf

If I can drive it through  HFCORECLKLE, why can't I drive it from LFXO_N pin? You may say: go ahead and use HFCORECLKLE, but I want to be able to change HFCORECLKDIV dynamically without worrying about LF domain clocking. I intend to use CMU CLKOUTSEL0 to output fixed frequency regardless of HFCORECLKDIV.

Posts: 2,218
Registered: ‎10-14-2014

Re: Driving LFXO_N with High Frequency

[ Edited ]

As the KB stated, I think the LFXO is only designed to run at 32 kHz, so you should not use any other frequency for this crystal as high as 24MHz. But maybe there are some margin to use a higher frequency LFXO. I suggest you create a ticket in our support portal for double confirm.

http://www.silabs.com/support/Pages/default.aspx

 

My views are my own and do not necessarily represent the views of Silicon Labs

 

WeiguoLu
Posts: 2
Registered: ‎08-06-2017

Re: Driving LFXO_N with High Frequency

Hi WeiguoLu,

 

I mean driving using external CMOS square wave signal, not using a crystal.

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Posts: 492
Registered: ‎09-18-2015

Re: Driving LFXO_N with High Frequency

Hi @ibrahimMD,

 

I checked our database of support questions, and someone has previously asked about the frequency range of the LFXO.

 

We do not characterize the LFXO for operation with anything other than a 32.768 kHz crystal. Right of the top, this means you cannot input another frequency on the LFXO_N pin, even if it's not coming from a crystal.

 

Further, there is filtering on the LFXO_N pin that dampens frequencies somewhere about the 32.768 kHz nominal. I don't have the information on exactly where the cut-off is, but based on some experiments run by the customer who asked the question, I can tell you that 24 MHz will not work.

 

John