LPC51U68 RTC Crystal

[jplathuile]

Hi

Concerning a LPC51U68 and the RTC clock.
For using the deep sleep mode, I use the RTC clock, with the crystal I have tried I got weird result. With some crystal a huge jitter, with other an annoying instability the oscillator stall.

The datasheet are not of a great help, it seems on some other nxp device that some capacitor are already included in the chip, but no mention of what to connect.

Can you give me one or two crystal references with the appropriate capacitor, that I can order at Mouser.

Tks

[ZhangJennie · Accepted solution]

HI

in DS, there is Cx formula as below. 

Cx depends on :

CL - Crystal load capacitance, please check your crystal manual for it. 
CPad - Pad capacitance of the RTCXIN and RTCXOUT pins (~3 pF).

CParasitic – Parasitic or stray capacitance of external circuit.

on LPCXpresso51U68 demo board, the Cx is 18pF. Crystal is SSPT7F-12.5pF20-R

see below.

Have a great day,
Jun Zhang

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[jplathuile]

Hi,
Thanks for the answer, you have information I don't have, from your site I downloaded,UM11071User manual Rev. 1.2 — 13 August 2019, and there is no mention of the page you gave me.

Ok for the demo board schematic, and I go for that solution.

Regards

JP

[ZhangJennie]

My typo. it's on LPC51U68 datasheet. Thanks for the reminder.

I will revise my original answer *UM* --> "DS"

https://www.nxp.com/docs/en/data-sheet/LPC51U68.pdf 

[jplathuile]

Hi,

Ok,

with the crystals i tried ( cylindrical case) and different capacitor i can't have a stable oscillation.
I hope on the reference you gave me.

Regards

JP