Capacitances for external RTC crystal in LPC4088

[kruftun]

Could you recommend capacitances for external RTC crystal in LPC4088?

There is app note for it?

[xiangjun_rong]

Hi, Roman,

I think you can refer to the design of LPC43xx, the two capacitors value of RTC are 18pF, you can use the 18pF capacitors.

Hope it can help you

BR

Xiangjun Rong

[kruftun]

Hi,
Thank you for answer. For example if crystal 32.768kHz with load capacitance 12.5 pF that 18pF capacitors will be right choice? May be there is some equation for capacitors calc?

[xiangjun_rong · Accepted solution]

Hi, Roman,

I think that it is okay to use 12.5pF capacitors, the capacitor value takes effect on the crystal clock frequency slightly. But there is not a formula to calculate the capacitor effect, you have to use a frequency meter to get the actual clock frequency.

Pls refer to the application note,it discusses the capacitor effect on the crystal clock frequency with experiment.

https://www.nxp.com.cn/docs/en/application-note/AN13057.pdf

Hope it can help you

BR

XiangJun Rong