Now, in comparison, on the Nov 2012 version of 2.0, the ground plane is missing around that region (probably because there is not enough space between the traces to allow a ground plane). Well, in this case it did save me from potentially deep trouble. You know, I have always questioned about those ground planes and wondered whether it’s worth my effort of adding them. Below are examples of the traces on OpenSprinkler 1.4s, the more recent version 2.0, and the version in production at SeeedStudios:Īlthough they don’t match the recommended layout exactly, they are pretty good at shielding the crystal pins from picking up noise. As a result, on most PCB versions, there is indeed a ground plane around the traces from the crystal to the RTC. First of all, I usually add a ground plane to fill the empty spaces between traces. Since I didn’t know about the issue previously, and there are hundreds of boards in production at SeeedStudios, am I screwed? It turns out that the situation is not as bad as I thought. Jeese, why did’t I read the datasheet carefully!Īfter discovering the issue, I panicked a little bit.
When I designed OpenSprinkler PCBs, I had not paid attention to this recommended layout at all. If this is not implemented, the crystal may pick up noise from nearby traces and cause the clock to run significantly off the charts. Specifically, according to the datasheet (right image below), the region around traces between the 32.768kHz crystal and DS1307 should be shielded by a ground trace. After a thorough comparison to the more recent version, I discovered that the problem is actually caused by a noise coupling issue with DS1307 RTC. This helped me narrow down the search range.
Then at some point I realized all the problematic cases happened on a particular PCB version dated Nov 2012.
Initially I thought this issue may have been caused by a bad RTC crystal, which leads to incorrect timing, and which in turn causes the controller to get stuck. The symptom is that some of the units started crashing and losing network connection, and this seems to be always correlated with the timing running crazy off the charts. The background is that we have started shipping a development version of 2.0 for any recent order of the assembled OpenSprinkler. Recently a random crashing issue has surfaced on the forum regarding some of the OpenSprinkler 2.0 development boards that we sent out a few weeks ago.
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