LPC Microcontrollers中的主题 Re: Capacitances for external RTC crystal in LPC4088

Capacitances for external RTC crystal in LPC4088

kruftun — Thu, 04 Mar 2021 05:07:19 GMT

Could you recommend capacitances for external RTC crystal in LPC4088?

There is app note for it?

Re: Capacitances for external RTC crystal in LPC4088

xiangjun_rong — Thu, 04 Mar 2021 06:40:03 GMT

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

Xiangjun Rong

Re: Capacitances for external RTC crystal in LPC4088

kruftun — Thu, 04 Mar 2021 08:37:23 GMT

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?

Re: Capacitances for external RTC crystal in LPC4088

xiangjun_rong — Thu, 04 Mar 2021 09:29:02 GMT

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

XiangJun Rong