Hi everyone,

starting from the mBed schematic I produced my own design with a LPC1758, but I soon found out that the RTC does not work (yes I don't use a probe on the XTAL lines and yes it's a recent revision A which should have no problems with RTC). I tried various combinations of crystals and capacitors:

http://it.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=672-7631 http://it.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=667-6108 http://it.farnell.com/jsp/search/productdetail.jsp?SKU=1838935

but none has given me stable operation. Are there any catches with the 32.768kHz crystals and LPC17xx? Does anybody know which make and model is the mBed one? I guessed from the schematic it should be an Epson-Toyocom FC-135 but the actual package marking on the mBed does not give me a lead on which part exactly... moreover, is it normal that the FC-135 options are 7, 9, or 12.5pF and the mBed uses 18pF caps?

Sorry for the long question and thanks in advance!

Cheers, Michele

The capacitance is recomended from the crystal data sheet,

Personaly, I havent had any problems,

Daft question, do you have any shorts/Open circuits,

and are you using the correct pins?

this is one I made earlier:

Not much solidus, but I hope you find the fault.

Enjoy

Ceri

Hi Ceri,

Thank you for the swift reply... This below is my setup:

And AFAIS the pins are correct... Maybe I tried so many different combinations that something broke in the process. I will leave it for now as the RTC seems to work for anyone else than me without any problem. Still, I would be curious to understand why on the mBed they use 18pF capacitors with a crystal that does not support them.

Cheers, Michele