LPC1549 RTC goes forward 5-6 sec per hour

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LPC1549 RTC goes forward 5-6 sec per hour

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lpcware
NXP Employee
NXP Employee
Content originally posted in LPCWare by TheEngineer on Tue Feb 24 05:53:33 MST 2015
Hi,

I'm trying to use internal RTC of LPC1549 with 32Hkz cyristal and 33pf capacitors. (Capacitor value is proper according to calculation formula at UM). When i power off the controller, everything is OK, however RTC counter goes forward 5-6 sec per hour if power is on.
I can't figure out , and almost going crazy.

Any help would be very appreciated.

Thanks.
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michelkuenemann
Contributor III

Hi all,

I am currently experiencing the exactly the same issue with the LPC1549 RTC. I have several boards and each of them has got the problem:

My application:

Chip: LPC1549JBD48

Identification: ZSD14372A (Production   Year 2014 / Week 37 - Silicon revision A)

I am using a 32 KHz cylindrical Xtal with 22 pf capacitors.

Supply: all supplies are clean 3.3 V 

The RTC oscillator frequency is routed to an I/O port thanks to the CLKOUT feature.

My frequency meter indicates 32767,84 Hz - This is a -6 ppm error, yielding a  -4 seconds error per WEEK

The counter is used in basic SECONDS mode, without interrupt. I read its value once per second and I send it to my console. After a while, I see the RTC clock going forward several seconds per hour.

Errata sheet LPC15XX 2.2 - 17 february 2017 says:

3.5 RTC.1: Real Time Clock (RTC) does not work reliably.

Introduction: The RTC is a set of counters for measuring time when system power is on, and optionally when it is off. The RTC is clocked by a separate 32 kHz oscillator that produces a 1 Hz internal time reference.

Problem: The RTC COUNT should increment by one in one second. With the problem, the RTC COUNT might increment by more than one within one second, causing inaccurate time counting.

Work-around: None.

My questions:

1 - Assuming there is no direct work-around for this issue - Is there a known, simple and reliable way to use the on board 12 MHz Xtal oscillator to achieve coherence checking and remove the redundant pulses whenever they appear ?

2 - has this issue been fixed in new silicon. Are these new devices available ?

Thank you.

Regards,

Michel Kuenemann

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deucalion12
Contributor I

I have the same error. Every few minutes COUNT increased by two, and then there is no RTC_ALARM interrupt.

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lpcware
NXP Employee
NXP Employee
Content originally posted in LPCWare by 1234567890 on Wed Feb 25 04:12:29 MST 2015
O.k., that's the upper limt. But what is the supply voltage for the chip when powered, V_DD(A)?
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lpcware
NXP Employee
NXP Employee
Content originally posted in LPCWare by TheEngineer on Wed Feb 25 01:17:27 MST 2015
Thanks for your answer,

Yes, i've measured Vbat. 3.6V when power off and 3,62V when power on. 
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lpcware
NXP Employee
NXP Employee
Content originally posted in LPCWare by 1234567890 on Tue Feb 24 13:17:00 MST 2015
Have you measured V_BAT when powered off. And V_DD(A) when powered?
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