Crystal oscillators on BBB

[wojtek...@yahoo.com]

Hello:

is there any particular reason why BBB uses two different crystal frequencies, 24 MHz for the CPU and 25 MHz for the Ethernet chip? Can the two be merged into a single 25 MHz frequency? There is a boot mode option of using 25 MHz for the CPU. Why is it not used?

Concerning the 12 MHz clock for the TDA12288, from the TDA12288 data sheet I understand that the OSC_IN on this chip can be driven from a free running oscillator. There is no timing relation between OSC_IN and the LCD data. Therefore, the 12 MHz clock does not need to be driven by the CPU. Is my understanding correct?

Thank you -- Wojtek

[Gerald Coley]

Because that was what all the SW was written to handle from the beginning and was the frequency that was tested. The other frequency at the time had not been tested. I went with what I knew to work. Sorry if I made a mistake.

Yes, you can add additional cost and use an external oscillator if you choose.

Gerald

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Gerald

 

ger...@beagleboard.org

http://beagleboard.org/

http://circuitco.com/support/

[wojtek...@yahoo.com]

Gerald:

thank you for the answer. From your post I understand that the 25 MHz crystal has now been tested with AM335x. Please let me know if this is not the case.

 I do not consider your design decisions as mistakes. Your explanations are very helpful while I am designing my own BBB-alike board.

Thank you -- Wojtek

[Gerald Coley]

I have not done any testing, so I have no idea if it has been done or not. Just understand that changing the base crystal will require changing the clock dividers inside the processor to keep everything working.

Gerald

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[wojtek...@yahoo.com]

Gerald:

thank you. Your answers are very helpful.

A minor point: in BBB schematics Y4 has wrong MfrNum which corresponds to 12MHz part. The correct part numbers are in the BOM.

Thank you -- Wojtek

[Gerald Coley]

OK. BOM rules.

Gerald

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