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I'm using the Microchip MCP79411 Real Time Clock. I can communicate with the device, read the MAC address from it, write to the EEPROM function and read back those values, but the Time and Date registers always read the same values.

I probed one of the leads of 32768Hz crystal with respect to GND and observed the following scope trace:

32kHz crystal scope trace

I'm sure I've set the ST bit to start the oscillator (or the crystal wouldn't be ringing right?). The frequency looks about right. The only thing that seems suspect to me is that the oscillator is only ringing with a peak voltage of about 250mV. I'm running the IC supply voltage at a regulated 3.3V. Is this normal? What could be causing the RTC to not be counting?

In the interest of full disclosure, the crystal I'm using is an Abracon AB26T-32.768KHZ and I have neglected to include explicit load capacitors on either leg of the crystal. Could that explain the low peak voltage? How should I debug this problem further?

For what it's worth, the Arduino sketch I'm using to test communication with the device is available for review here.


UPDATE

If I take a 22pF capacitor with one leg attached to ground and hit the other leg of the capacitor to the case of the crystal the RTC starts ticking, even after I remove the capacitor altogether. This is specifically to the case, it doesn't happen if I do it to either of the legs of the crystal... It's almost like putting some capacitance on the case to GND kick-starts the RTC. Tne benavior is the same if I simply connect the case to GND with a piece of wire (i.e. no discrete capacitor). What's the explanation for this?

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    \$\begingroup\$ So you know you didn't hook it up right but you're asking here for help anyway!!? Come back and ask again for fix this question after you've done your part. \$\endgroup\$ – Olin Lathrop Dec 10 '11 at 13:28
  • \$\begingroup\$ @OlinLathrop a little downvote-happy don't you think? How many questions do you see asked here that include software and scope traces posted? The question is asking what I should expect to see on the scope trace and whether the absent capacitors could account for it... \$\endgroup\$ – vicatcu Dec 10 '11 at 17:44
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    \$\begingroup\$ I think you should fix all the things you do know about before asking for help. Actually I don't think the lack of capacitors is causing the problem, but you hook something up against spec and then ask why the output is not as expected? I do think lack of capacitors could explain the unexpected shape of the waveform, depending on which leg of the crystal it actually came from. You neglected to mention that obviously important piece of information. \$\endgroup\$ – Olin Lathrop Dec 10 '11 at 17:54
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    \$\begingroup\$ Simply probing it with the 'scope can give a crystal adequate cap loading to start it off. \$\endgroup\$ – tyblu Dec 11 '11 at 0:09
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    \$\begingroup\$ @OlinLathrop I do agree that you should try everything you know to do before asking, but that doesn't make this a bad question by any means. Things that are obvious to you are not obvious to everyone else, you really need to learn that. We are here to be helpful and grow a database of knowledge, which you don't seem to be interested in helping with this question, so just leave it alone. \$\endgroup\$ – Kellenjb Dec 11 '11 at 6:08
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We were using the same real-time clock in one of our designs and also having problems. In our case the clock was running erratically or slowly. After discussing the problem with Microchip we were told that the low power oscillator used in this design requires very low capacitance crystals. I would double check to see if your crystal meets the requirements published in the datasheet and if not replace it.

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You shouldn't touch any pin of the oscillator with a scope probe. The probe's capacitance may disturb the oscillator, or even start it where it wasn't running before. Instead program the MFP pin to output a square wave and read the signal there.

Microchip advises against "big" capacitors, i.e. 12.5 pF. Use 8 pF caps instead. The problem may lie in the crystal, since many 32.768 kHz crystals are designed to operate with the 12.5 pF caps.

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