The RTC of the LPC1227 doesn't start

[lpcware]

Content originally posted in LPCWare by alubeiro on Wed Mar 11 07:25:58 MST 2015
Hi everybody

I have done a hardware desing using a PR601 and there is an issue with the RTC of LPC1227. The RTC doesn't start.
First of all i must say that i have based my desing in the EBV of the PRH601, so I have used the same crystal and load capacitors used in these EBV.
When I realised that this issue was happening in my hardware, I checked it out in the EBV. I was rather surprised this issue was also happening there.
After taking a look to the crystal datasheet i saw that the recomended load capacitors for these crystal were 12.5pF instead of the 10pF which are mounted in the EBV.
Then, I changed the original capacitors by some of 13pf and the RTC of LPC1227 started to work properly both at in the EBV and in my desing.
But now i have more devices manufactured and for some of them this 13pF seems not to be enought, up to a point that I have had to mount capacitors of 18pF.

Have you got any idea why of this issue????
I am not sure if I should mount capacitors of 18pF in all devices, because maybe there are devices that would not work properly with this value. Remember that the Crystal's datasheet says that the recomended capacitors are of 12.5pF.

I don't know what to do. Any help will be apreciated.

Thanks and regards

[lpcware]

Content originally posted in LPCWare by alubeiro on Thu Mar 12 02:02:22 MST 2015
Hi Wolfgang,

Thank you very much!
Your replay was very usefull for me.

Best regards

[lpcware]

Content originally posted in LPCWare by wmues on Wed Mar 11 08:33:09 MST 2015
If you have a crystal with a load capacitance of 12,5pF, you will need to add 2 capacitors of 25pF each.
This is because these 2 capacitors are connected in series.

The calculation is:

cap = (2 * cap_load) - cap_pcb - cap_rtc

cap_load = 12.5pF
cap_pcb = stray capacitance of your layout
cap_rtc = capacitance of the RTC input/output pins

cap_pcb + cap_rtc is approx. 7pF in most designs.

so cap = (2 * 12,5) - 7 = 18pF

best regards
Wolfgang