Michel Kuenemann

LPC1549 RTC goes forward 5-6 sec per hour

Discussion created by Michel Kuenemann on Jul 31, 2017
Latest reply on Aug 2, 2017 by Carlos_Mendoza

Hi all,


I have already posted this message in another thread several days ago, but I got no answer. This is why I create this new (however redundant) entry...


I am using the RTC of an LPC1549 and the clock goes forward several seconds per hour


I have 5 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.


Michel Kuenemann