Easily disturbed crystal - Page 1
Forum section: electronics / beginners

author: fabiodl
author_rank: Frequent Contributor
timestamp: March 25, 2020, 10:51:07 am
content: I have built several circuits using atmegas without problems. Recently, in two of them, I used an 8Mhz crystal, this onehttps://abracon.com/Resonators/abm3b.pdfI noticed that when I touch the quartz (it has a metal case), and maybe its load caps, the atmega stops / jumps at random locations.I know that I probably should't touch it, but using other crystal oscillators (usually bigger, through hole,though) I never had this problem.Is there anything in the datasheet that suggest this "ease of being disturbed"?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2020, 01:59:24 pm
content: You have the recommended capacitors, right? (Note that they aren't simply the load capacitance value given, there is a calculation involved.)Try a higher drive strength in the fuse settings.Also, wouldn't hurt to ground the can, or put some other shielding around it. It sounds marginal, so it might still be that it doesn't start up under some conditions (mfg variation, low or high temp?), which the setting should help with.Tim

author: wraper
author_rank: Unknown
timestamp: March 25, 2020, 02:02:45 pm
content: It's case should be grounded as there are 2 GND pads, therefore simply touching it without touching it's signal terminals should have no effect. But disturbing oscillation by touching actual signal terminals / capacitors is normal.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 25, 2020, 02:24:58 pm
content: I agree with Tim.If the crystal drive level is too low, you could have that condition.Just FYI: We use Abracon SMT crystals extensively in our company. They are very good crystals. Essentially zero field failures.We used to employ crystals from PDI, and they would have significant field failures.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 25, 2020, 10:32:33 pm
content: How did you build your circuit?Did you make a PCB, or put the crystal on a breadboard?Problems like this are very common on breadboards. Anyone who tells you a crystal works reliably when put directly into a breadboard does not have enough experience with breadboards yet.

author: james_s
author_rank: Super Contributor
timestamp: March 26, 2020, 01:05:50 am
content: Breadboards are certainly not ideal, but I have breadboarded microcontrollers with a crystal and had it working reliably. If you use the right loading capacitors and lay it out sensibly it can work just fine. Like anything it does take some care.

author: EEVblog
author_rank: Unknown
timestamp: March 26, 2020, 01:15:43 am
content: Crystal are susceptible to shock and vibration and orientation (I've done research on this) but it's absolutely miniscule and is not the issues you are seeing here.Sounds like you have marginal cap values and the presence of your hand is upsetting the capacitance, or more likely you've got a cracked solder joint or PCB trace or via or something.

author: fabiodl
author_rank: Frequent Contributor
timestamp: March 26, 2020, 07:22:57 am
content: Thank you everybody (and a comment from Dave himself, wow!! )In both cases I noticed the problem it's on a PCB (they are completely different circuit layouts).In both cases the crystal is at less than 5mm from the microcontroller, on the same side of the board, no traces crossing under the crystal.I indeed used ceramic capacitors equal to the specified load capacitance, and did not account for other capacitance (the amega's one,traces, etc).The gnd terminals of the crystal are connected to ground. Checking with the multimeter, there's less than a ohm of resistance between case and gnd.I use the following fuses-U lfuse:w:0xff:m -U hfuse:w:0xde:m -U efuse:w:0x05:mI checkedhttps://eleccelerator.com/fusecalc/fusecalc.php?chip=atmega328pbut there seem to be no explicit reference to drive strengthOn the other hand, I never noticed the same effect when I had through hole no-name crystals, with randomly picked up 18 or 27pf ceramic caps on the breadboard, (I may have just been luckier).And, if i don't touch the board, everything works flawlessly

author: EEVblog
author_rank: Unknown
timestamp: March 26, 2020, 08:15:04 am
content: Quote from: fabiodl on March 26, 2020, 07:22:57 amWhat package are you talking about?Are they the right cut crystal?

author: fabiodl
author_rank: Frequent Contributor
timestamp: March 26, 2020, 10:36:04 am
content: They are in a 5.00mm x 3.20mm package (here's the exact parthttps://www.digikey.jp/product-detail/en/abracon-llc/ABM3B-8.000MHZ-10-1-U-T/535-9721-1-ND/1873255)I hand solder them, so I probably do not respect the 260 degs the datasheet recommends for the reflow, but as reported before, they work fine, they just appear to be less robust than in my experience.After more testings, it seems indeed placing the finger on the load capacitors (0402, again hand soldered, less than 2mm from the crystal) is the most frequent case in which the microcontroller misbehaves.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 26, 2020, 02:19:44 pm
content: You could attempt to probe the crystal, but the probe's loading would affect the behavior, unless you have a sniffer type probe.Another way would be to write a little test program, on which you set an output port to toggle...... Some microcontrolers even have the capability to output a buffered replica of the crystal oscillation to a port.That way, you could monitor the oscillation frequency with a scope, while you touch the crystal. You can then observe the crystal behavior without disturbing it.

author: coppice
author_rank: Super Contributor
timestamp: March 26, 2020, 02:21:33 pm
content: Quote from: T3sl4co1l on March 25, 2020, 01:59:24 pmIts a ceramic resonator. not a quartz crystal. Extra drive strength is definitely a good idea.

author: wraper
author_rank: Unknown
timestamp: March 26, 2020, 02:23:41 pm
content: Quote from: coppice on March 26, 2020, 02:21:33 pmNope, it's a crystal. Maybe title in the datasheet confused you but it's about ceramic package.

author: coppice
author_rank: Super Contributor
timestamp: March 26, 2020, 02:29:32 pm
content: Quote from: wraper on March 26, 2020, 02:23:41 pmAh, yes. A lot of ceramic resonators get loosely described as crystals, even in data sheets, so I didn't read beyond the heading.

author: EEVblog
author_rank: Unknown
timestamp: March 29, 2020, 10:33:44 am
content: Quote from: fabiodl on March 26, 2020, 10:36:04 amOh wow, yeah, that's tricky!The URL says resonator, but they call it a "crystal" ?https://abracon.com/Resonators/abm3b.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2020, 10:53:47 am
content: A quartz crystal is still a resonator...Even if it's not AT cut α-SiO2, it seems to be rated in exactly the same way, so will respond the same in circuit.Tim

author: OZ1LQB
author_rank: Regular Contributor
timestamp: March 29, 2020, 11:18:51 am
content: Hi Allif you handsolder ceramic smd cap you might get a "microcrack"have a look herehttps://www.murata.com/en-global/support/faqs/products/capacitor/mlcc/mnt/0007

author: Manul
author_rank: Super Contributor
timestamp: March 30, 2020, 02:24:40 pm
content: I agree with OZ1LQB, that is a thing to check. It can be a symptom of a crystal with lower then normal Q.Some time ago I had UHF transceiver, which was failing at higher temperatures, because oscillator amplitude was dropping and dying out. It was JFET colpitts oscillator with hc49u crystal. I remember that moving my finger even 5mm near crystal leads was causing amplitude drop or even complete dying. It was very marginal oscillation. Then I noticed that tapping on a crystal produced random amplitude jumps. Not a surprise, after opening the crystal can and examining under microscope I found half cracked attachment point, which was causing lower and sometimes random Q.

author: fabiodl
author_rank: Frequent Contributor
timestamp: March 30, 2020, 10:04:56 pm
content: The strange thing is that I mounted two of them, on two different layout boards, and that happens in both cases. I suppose it is unlikely that both of them cracked.And it really seems that the oscillation stops when touching the load caps, not the can.

author: james_s
author_rank: Super Contributor
timestamp: April 13, 2020, 12:00:50 am
content: Have you tried a different value of loading capacitors?

author: InsertUsernameHere
author_rank: Contributor
timestamp: April 13, 2020, 03:33:13 am
content: I've seen something like this before, not too sure if this is the same issue?See if this thread is any help -https://forum.arduino.cc/index.php?topic=474264.0Check post #14 and #23.

author: fabiodl
author_rank: Frequent Contributor
timestamp: April 13, 2020, 07:33:45 am
content: Thank you, I will try setting the fuse to "full swing oscillator" (lfuse=0xF7), they are now set to "external crystal oscillator" (lfuse=0xDE)