LPC1768 RTC issue

[yogesh20]

We have use LPC1768 micro controller in our product,

issue observed as per below,

1) RTC increment issue (same board RTC not working),

(touch figure 32khz crystal or capacitor and RTC is working fine)

Same as touch any oscilloscope probe to capacitor RTC working fine.

but same board power on instrument RTC not working.

What is issue?

Kindly guide us to resolve this issue.

[yogesh20]

Under testing..

[yogesh20]

Ok thanks for update ,

we will check and update you.

[xiangjun_rong]

Hi,

As you said that the RTC works fine after you touch the crystal or capacitor, I suppose that the crystal or capacitor are not soldered reliably, or the capacitor value is not appropriate.

Pls consult the crystal manufacturer to get the capacitor value the crystal required, then use the formula to get the actual capacitor value.

For the following formula, Although it is based on LPC546xx, it adapts to the LPC17xx.

Hope it can help you

BR

XiangJun Rong

[yogesh20]

Dear XiangJun Rong thanks for update,

this issue only some board,

We have use 32.768Khz and capacitor value us 12pf. we resolder LPC1769 and crystal, but issue still same.

[xiangjun_rong]

Hi,

is it convenient to post the crystal part number data sheet so that we can compute the capacitor value?

BR

XiangJun Rong

[yogesh20]

830002995B
use this part

[xiangjun_rong]

Hi,

Okay, i have checked the required load capacitance is 12.5pF for the crystal, so the actual capacitor should be:

Cx1=Cx2=2*12.5-(3+1)=21pF.capacitor

Pls try to use two about 21pF capacitor for the crystal.

BR

XiangJun Rong