RTCC

dyeatman · Joined: 06 Sep 2003 Posts: 1981 Location: Norman, OK

Wow, that's a lot of error...
Looking at the datasheet I don't see a configuration option that
would cause that much of an error. Maybe you are running on an overtone
on the xtal.. Anyone else here got any thoughts on this?

If you have a scope, one cycle for the 32Khz oscillator
should be approximately 30.5us
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Liam_85 · Joined: 09 Nov 2014 Posts: 26 Location: Ireland

I tried measuring it with the scope but I think it was loading it because nothing really showed up!! I have a pack of 20 x 32.768kHz crystals and tried about 10 of them to see if there was a change to no avail!! I think at this stage I'm about ready to give up and just use an external DS1307 RTC that I have!!
Once again, thanks for taking the time to help me it try to resolve this problem!!
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Liam Hanmore

Liam_85 · Joined: 09 Nov 2014 Posts: 26 Location: Ireland

I managed to measure the 32.768kHz oscillator on a scope to be approx 30us which would be about right. When I tried doing the same to the primary oscillator the chip just froze and when I took the scope off the chip carried on doing whatever it was doing!!
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Liam Hanmore

Ttelmah · Joined: 11 Mar 2010 Posts: 20063

Generally on any crystal oscillator, you need to ensure you connect to the 'CLKO' pin (the one actually being driven by the oscillator chip). Otherwise the load of a scope _will_ cause massive changes. and may even introduce too much load, and stop the oscillator. This is particularly the case the higher the frequency of the oscillator. At 20MHz, a scope probe will introduce a load equivalent to a 265R resistor. At 32KHz, a few hundred KR.....