RTC time error in lpc546j512bd208

[prasannanaik]

Hi,

I am using rtc of lpc546j512bd208 with an external crystal 32.768khz of 20 ppm(crystal capacitance 6pF) and load capacitances CL1=CL2= 6pF. Load resistance is not used.

I am getting a lag of 30 seconds in one day.

How to minimize this error. I can't use a better ppm crystal because of cost constraint.

Thanks and best regards,

Prasanna

[xiangjun_rong]

Hi, Naik,

I have to say it is difficult to figure out the capacitor value to generate 32.768KHz clock because of parasitic capacitor for the pins. But the larger the load capacitor is, the lower the crystal clock frequency is.

Because your phenomenon is "I am getting a lag of 30 seconds in one day", so I suppose you have to increase the crystal clock frequency or decrease the load capacitor for example 5pF or 4pF. Pls have a try.

BR

XiangJun Rong

[prasannanaik]

Hi xiangjun,

      This suggestion may work for a particular board which is acceptable during development but during production this may cause problem because of tolerance of components.

Please suggest an alternative method suitable for production samples.

Thanks & best regards,

Prasanna Naik

[xiangjun_rong]

Hi, Prasanna Naik,

Unfortunately, I have not idea to improve the external crystal accuracy for production samples.

Sorry for not helping you

BR

XiangJun Rong