I'm at my wit's end trying to get a 32.768 kHz tuning fork crystal to work and I don't know what I'm doing wrong.
First, some background: I'm using a 16MHz crystal in conjunction with an Atmega 328P-PU microcontroller and that's working fine and I'm trying to run an RTC chip off a 32.768 kHz crystal which is where I'm hopelessly stuck. I'm using 10-20 pF ceramic load capacitors and of course I'm using an oscilloscope to verify everything.
I'm trying to run this RTC chip with this inexpensive crystal. However I cannot get it to work whatsoever, either on my PCB or a solderless breadboard. I've used several different load cap values and my crystal is wired up as the RTC datasheet specifies. I've replaced the chips, the caps, and the crystals, and that didn't fix anything.
I even tried connecting the tuning fork crystal to the Atmega chip XTAL pins instead of the 16MHz crystal and that didn't help either.
Do tuning fork crystals need a special circuit to work, or did am I inadvertently damaging every single one I touch…? I even tried different crystals of a similar design from different manufacturers.
Help is greatly appreciated.
EDIT: I solved my problem by switching to a non-tuning fork crystal and getting new capacitors. I think the root cause was the use of low-quality ceramic capacitors that I had bought in a assortment kit online.
Best Answer
What exactly is going wrong? What are you expecting it to do and what is it really doing? I assume that the RTC IC won't clock and stay at time 0:00, but you can talk to it through its serial interface.
I've taken a look at the datasheet and in the control register, there's an external oscillator enable bit, which is set to low by default (disable ext. oscillator). Is it possible that you didn't set that bit?
Either way, I don't think that it's the crystal that's causing the problem. I think that (most likely), it's the RTC being set up incorrectly.
EDIT: also, according to the datasheet, the recommended load cap value is 6-9pF