Question about RTC trimming of LPC1756

[lpcware]

Content originally posted in LPCWare by wookjina on Mon Apr 02 04:57:48 MST 2012
Hi all,
I know the RTC of LPC1756 have to calibrate according to AN10849
(link http://ics.nxp.com/support/documents/microcontrollers/zip/an10849.zip)
But I wonder how to calibrate each boards on MP stage?
The AN10849 shows how to determine only one board for example.
Actually, I have serveral LPC1756 boards and each boards has an error within maximum 10 minutes per day.
Who have any idea or something about it? please advice me.
Regards,
Wookjin

[lpcware]

Content originally posted in LPCWare by wookjina on Tue Apr 10 01:51:52 MST 2012
Thanks for your opinion. I also agree your mention. I'll try it. But It's hard to understand there is calibrating register for RTC. I don't have seen it in any other RTC. Thanks.

[lpcware]

Content originally posted in LPCWare by wmues on Wed Apr 04 06:09:43 MST 2012
10 Minutes per day is WAY ahead of what is possible with a 32 KHz XTAL!
Are you sure that the XTAL oscillator is running OK?

A deviation of 100ppm are 8 seconds/day!

So you have to fix a hardware problem.

[lpcware]

Content originally posted in LPCWare by wookjina on Wed Apr 04 02:18:18 MST 2012
Hi Lien,
Thanks for reply.
LPC175x series are 80 TQFP and there is no CLKOUT pin of P1.27.
Therefore I think this measure of RTC clock out is not possible.
Most of all, though I fixed one of these boards to calibrate, it seems to be hard to calibrate all.
Any further idea will br appreciated for me.
Regards,
Wookjin

[lpcware]

Content originally posted in LPCWare by Lien.Nguyen on Tue Apr 03 19:43:52 MST 2012
Hi Wookjin,
In section 27.6.5 of UM10360, I saw a guide for calibration as below. Did you try to use it?
"A recommended method for determining the calibration value is to use the CLKOUT
feature to unintrusively observe the RTC oscillator frequency under the conditions it is to
be trimmed for, and calculating the number of clocks that will be seen before the time is off
by one second. That value is used to determine CALVAL.
If the RTC oscillator is trimmed externally, the same method of unintrusively observing the
RTC oscillator frequency may be helpful in that process."

Best Regards,
Lien