RTC is running to fast

[ranemyr]

Hi

I made an project with HCS08GT32 using the Processor Expert Bean Time/Date

an RTC bean. I feed the MCU with an 32.768Khz X-tal.

i put a 10Meg resistor and 2pcs 12pf cap in the osc circuit according to recommendations.

The problem is that the RTC is running to fast approx 38Seconds / 24Hour.

I changed the X-tal to other manufactures with same result.

How to trim the RTC to an lower error?

/Peter

[peg]

Hi Peter,

Your error is +0.044%, not a lot.

I presume this thing works by basing its timing on a RTI which is based directly on the xtal. Am I right?

If so short of deducting a second every so often you need to get the xtal running closer to spec.

Even though the datasheet says a series resistor of 0ohms I would try one with a value of up to 1/2 a meg.

I don't expect a very rapid response from you....

BR Peg

Just checked some crystal specs, typical seems to +/- 20ppm @ 25degC with maybe +/-50ppm for temperature so your 440ppm is quite high!

Message Edited by peg on 04-25-200610:51 PM

[ranemyr]

Hi Peg

I will try to add the Rs, however it´s a bit tricky due to space constriction in my SMD layout. I also got a offline tip to try increase the C1 to 18-22pf and see what the result will be. The other 2 option as i se it is to use an external RTC or every 
24Hour deduct the RTC with the excess seconds.

I will make some laborations and return with my result.

Thanks

/Peter