ADC Crystal will not start oscillating unless I touch the scope probe to it - Page 1
Forum section: electronics / projects

author: thereza
author_rank: Contributor
timestamp: May 23, 2013, 03:34:21 am
content: I made a couple boards with the AD1258 ADC which requires a 32.768khz crystal to operate. It recommended this one -http://www.ecsxtal.com/store/pdf/ecx-306.pdf- but I used this one due to space considerations -http://www.ecsxtal.com/store/pdf/ecx-31b.pdf.I've had 2 boards where the ADC does not work, and I traced it to the xtal. I reflowed the xtal on one board, and it fixed it. No such luck on the second board.So when I touch either pin on the oscillator with my oscope probe, it starts oscillating and continues to do so. Someone recommended I add a resistor across the xtal - I added a 20Mohm one. It now only works when the probe is connected. When I touch it, you can see the waveform amplitude grow every time (which implies that it goes down to zero when it's not being touched).Not sure how to move forward.Any suggestions will be welcomeThanks,Reza

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 23, 2013, 03:39:26 am
content: Here is a recent thread about this subjecthttps://www.eevblog.com/forum/projects/pierce-oscillator-how-to-verify/msg233813/#msg233813I guess it is saying add some capacitance, as your probe lead has capacitance. Mine has around 16pf.Also check the usual, soldering, continuity etc.

author: thereza
author_rank: Contributor
timestamp: May 23, 2013, 04:01:55 am
content: I upped the capacitance by 10pf on both, and it works now but it takes a long time to startup. I added a 500ms delay after enabling it and it seems to work fine now. the values i used came from the datasheet for the ADC. the datahseet specified 35kOhm ESR max, the part they recommended was listed as 50kOhm max. The part i used had 70kOhm max. Think that could cause it? otherwise, the specs all look pretty much the same.

author: KJDS
author_rank: Super Contributor
timestamp: May 23, 2013, 07:39:19 am
content: It's not very clear from your picture, but the layout doesn't look optimum which also won't help. Having said that, if you find a cap value that works, and adding a little more or taking a little off that value still leaves it working, then accept it and move on.

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 12:17:05 pm
content: Did you add the capacitor to the circuit with one side of the 10pf cap going to ground?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 23, 2013, 04:39:25 pm
content: 32K oscs run at very low power. Startup times up to a second or so are quite normal.When probing you shoud use a x100 probe (or 100M resistance on the end of a x10 probe)

author: thereza
author_rank: Contributor
timestamp: May 23, 2013, 10:15:01 pm
content: Quote from: KJDS on May 23, 2013, 07:39:19 amHow is it not optimal? What would make it more optimal?

author: KJDS
author_rank: Super Contributor
timestamp: May 23, 2013, 10:29:17 pm
content: Not that it makes too much difference at these frequencies, but if you rotate the XTAL anticlockwise then the two lines from the chip will be shorted and similar length, rather than having one short and one long line. You've also got one cap between the XTAL and the IC and the other on the far side.

author: thereza
author_rank: Contributor
timestamp: May 23, 2013, 10:33:19 pm
content: Quote from: KJDS on May 23, 2013, 10:29:17 pmI wasn't aware that you needed equal length lines from the xtal - is this true? Also, the IC is on the bottom of the picture, neither cap is between the xtal and the IC - and if I did, how are these things bad?Reza

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 10:42:47 pm
content: you needn't worry about matching the PCB trace lengths to the chip. You've not replied if the cap you tried was one end connected to ground. Your layout looks fine. It is not the trouble. If the oscillator works when you put your probe on it then you are adding 10pf of capacitance from the probe tip applied and the solution could be as simple as tying a 10pf cap to ground at that same point.Your story seems to scream that you don't have enough loading capacitance for your xtal to oscillate or start.

author: thereza
author_rank: Contributor
timestamp: May 23, 2013, 11:07:18 pm
content: Quote from: Paul Price on May 23, 2013, 10:42:47 pmThe caps were grounded, not sure what the alternative would have been?And yes, that seems to solve it but I have 5 identical boards, two had this problem. One was fixed by reflowing the xtal. The other was fixed by adding 10pf to both caps (just stacked on top) and adding an additional 500ms delay before I start trying to talk to it. The other 3 worked fine. Just trying to understand the failure mode.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 23, 2013, 11:13:38 pm
content: Have you tried the any lower ESR caps? you mentioned this before.

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 11:16:35 pm
content: I don't know if you have tried cleaning the area around the xtal and associated caps, Your close trace routing might affect the xtal circuit if there is leakage to surrounding traces.Does the xtal require a cap on both sides of its leads? Is this what the xtal mfg recommends? Are the values that are recommended by the xtal mfg or the device those that you are using with the xtal? What are the values of these two caps?

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 11:17:25 pm
content: Your problem is not high ESR caps.

author: thereza
author_rank: Contributor
timestamp: May 23, 2013, 11:20:09 pm
content: Quote from: Paul Price on May 23, 2013, 11:17:25 pmThe ESR is a requirement of the xtal, not the caps.And I followed the ADS1528 datahseet's recommendations. It wants a 12pF xtal, but only wants me to use 4.7pF caps on it which is a bit odd..

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 11:32:14 pm
content: You only talk of one xtal capacitor, 4.7pf. What is the purpose of the second capacitor?The 4.7pf is a reduced value because the PCB board itself has capacitance. Be aware that traces closely opposing a ground plane or a trace presenting a low capacitance relation to ground on the other side of the PCB will also affect the xtal.The traces leading to the caps and to MCU and to the xtal are themselves creating capacitive affects. The advice as to keeping the traces the same length is not so important as is keeping the leads to the xtal and the capacitors as short as possible to connect to each other and the MCU is, as well as keeping them from being too close to other traces to minimize the additional capacitance effect you gain by trace proximity.It doesn't take much to upset the applecart? Did you try cleaning the board around the xtal and its caps with a cotton swab dipped in ethanol or isopropyl alcohol?

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 24, 2013, 12:00:46 am
content: One more thing Paul, as far as layout goes, is it better to have the capacitors or the crystal closest to the MCU?It seems easier to put the capacitors closer.

author: Paul Price
author_rank: Super Contributor
timestamp: May 24, 2013, 12:06:18 am
content: I can see clearly from your PCB that trace from the left xtal 1 is running dangerously, perhaps too itimately close to the opposite end of the xtal and also to another trace to the left of it. You could do well to rotate the xtal and caps CCW 90 deg. to fix that problem, but the very close proximity of the trace coming down form xtal1 to X2 certainly provides a shunt capacitance across the xtal and a might be a path for resistive leakage if you have any residue on the board.What screams here is also that your xtal is taking far too long to start. This should not be.This could mean that you already have too much capacitance from PCB parasitic capacitance on the other side of the xtal from where you put the probe to get it to oscillate. What you may need to do then is eliminate the cap on the opposite side as the PCB board traces have already given your xtal enough cap there to work properly.

author: Paul Price
author_rank: Super Contributor
timestamp: May 24, 2013, 12:13:35 am
content: The idea is have all the leads as close as possible, but be careful not to make the traces of each side of the cap or xtal run intimately close and parallel to each other (as they are now passing the xtal pads) and keep at least a double trace width thickness of all xtal osc traces from other traces or from the ground plane. You can make the traces much thinner in width and this reduces the capacitive footprint.

author: Niklas
author_rank: Frequent Contributor
timestamp: May 24, 2013, 05:59:27 am
content: Paul: Microchip has some details about the oscillator layout in their datasheets and that shows parallell traces. A screenshot of the datasheet figure is attached. Does the "not close and parallell" you mentioned apply only to low power watch crystals or to crystal oscillators in general?

author: thereza
author_rank: Contributor
timestamp: May 24, 2013, 06:40:10 am
content: I've been given a lot of conflicting advice. Obviously, additional capacitance fixed it, so the suggestion that i have too much capacitance doesn't hold water. at 32khz, the trace lengths don't matter, and as per the recommended cap placement, parallel lines don't matter. the only thing that was out of spec was the ESR of the crystal itself. And of 5 identical boards, 1 required the additional capacitance and longer startup period. I would love to know why. In a scientific way, rather than a 'your layout isn't optimal' blanket and somewhat meaningless statement. I do appreciate the attempt though.

author: ecat
author_rank: Frequent Contributor
timestamp: May 24, 2013, 07:03:00 am
content: This may throw some light or help un-muddy the waters, or it may just add to the confusionAnyway, it is a pdf from TI so... See for yourselfhttp://www.ti.com/lit/an/slaa322b/slaa322b.pdfJust thinking out loud so to speak, sometimes the problem is not a single issue but the unfortunate conjunction of several factors.

author: millerb
author_rank: Regular Contributor
timestamp: May 24, 2013, 07:16:40 am
content: ADC datasheet specifies ECS-.327-12.5-17-TR in the "Approved Crystals" table, which is similar but has a different package from the one you have chosen. Also, have you verified PPL cap is 22nF and properly connected to ground and CLKSEL properly tied to ground?

author: thereza
author_rank: Contributor
timestamp: May 24, 2013, 07:36:21 am
content: Quote from: millerb on May 24, 2013, 07:16:40 amYeah, all the other units are working fine and it started working when I added the 10pF caps. The only difference between the approved one and the one I am using is that the approved one has a max 50kOhm ESR and the one I'm using has a max 70k ESR. And the datashseet specifies max ESR of 35K... which is strange that the approved one violates this.

author: millerb
author_rank: Regular Contributor
timestamp: May 24, 2013, 07:50:22 am
content: Quote from: thereza on May 24, 2013, 07:36:21 amThat is weird that they would have a part in their recommended list that violates a rule they defined. Never hurts to contact TI and ask about it.Have you scoured over their eval board for this part as well? Here's a PDF of it:http://www.ti.com/lit/ug/sbau126c/sbau126c.pdf.Their 4.7pF caps on the xtal are :4.7pF C25, C26 Capacitor, C0G Ceramic, 4.7pF TDK C1608C0G1H047JTPerhaps compare those to the ones you sourced and see if there are any clues there.

author: Paul Price
author_rank: Super Contributor
timestamp: May 24, 2013, 02:49:28 pm
content: This could mean that you already have too much capacitance from PCB parasitic capacitance on the other side of the xtal from where you put the probe to get it to oscillate. What you may need to do then is eliminate the cap on the opposite side as the PCB board traces have already given your xtal enough cap there(at that side of the xtal) to work properly.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 24, 2013, 04:21:19 pm
content: Another potential problem is that your crystal is locking up or failing to start due to too much noise. One of the ways to 'fix' (kludge) this is to use two different capacitor sizes--which is what you are doing by placing the probe tip on one of the caps.The proper remedy for this type of problem (and the best practice, generally) is to use a guard ring around the oscillator signals as shown in the document ecat linked to earlier.The problem is that, due to the crystal's high ESR, the oscillator signals are very susceptible to noise being coupled in from nearby traces. Since--as Mike pointed out-- watch crystals start so slowly, you will likely have a lot of other signals making noise when you first start a circuit up. The guard ring is meant to protect the oscillator traces from these other noisy signals. The guard ring is typically GND but can be any low impedance, low noise signal, such as any of the power supply rails.

author: Paul Price
author_rank: Super Contributor
timestamp: May 24, 2013, 11:39:46 pm
content: It applies to xtal oscillators in general. You'll notice that the layout of the suggested diagrams use parallel traces with the shortest leads and a ground traces in between.

author: Niklas
author_rank: Frequent Contributor
timestamp: May 26, 2013, 07:50:43 am
content: Quote from: Paul Price on May 24, 2013, 11:39:46 pmI was perhaps a bit too quick to read and misread about parallell. But, I do not see a GND trace between the two oscillator traces in the picture from the appnote. A recommendation seems to be a GND guard around to crystal oscillator together with a GND fill at another layer covering the oscillator area. Almost like building a GND box.thereza: Was it that just 2 out of a batch of PCB were not working? Have you considered to change both the crystal and the 3 capacitors (2 for crystal and 1 for the Tank PLL). Unsolder, clean the board and then put some new components there? At least change the capacitors.

author: ftransform
author_rank: Frequent Contributor
timestamp: May 26, 2013, 08:40:05 am
content: I thought that guard traces were bullshit in regards to removing noise. I thought the only use of guard traces was to prevent leakage in very high impedance circuits (think electrometers).I read an article somewhere that said guard traces have the same effect as physical separation, and the reason why they are said to reduce noise is because they add physical seperation...

author: MacAttak
author_rank: Unknown
timestamp: May 26, 2013, 06:24:29 pm
content: By that logic, wouldn't that mean that metal shielding would also be snake oil? I mean, wouldn't a metal enclosure be just the same effect as physical spacing via an air gap?And so the Agilent Spectrum Analyzer that Dave tore down recently would be engineered the way it is just to increase cost/price (tons of machined RF shields and thick heavily-stitched ground traces to separate circuit blocks)?For some reason I am inclined to believe that the ground traces are in fact useful for noise protection.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 26, 2013, 06:52:49 pm
content: QuoteCapacitance between the 2 xtal pins can definitely be a problem - I've seen it cause startup issues before.

author: PA4TIM
author_rank: Super Contributor
timestamp: May 26, 2013, 07:41:49 pm
content: http://www.google.nl/url?sa=t&rct=j&q=intell%20oscillators&source=web&cd=1&cad=rja&ved=0CC4QFjAA&url=http%3A%2F%2Fecee.colorado.edu%2F~mcclurel%2Fiap155.pdf&ei=EWSiUbiFMKLA0QXO0IGgAg&usg=AFQjCNF3HRxczqoB6h5dui2WSqB0ckwbOw&sig2=UpFFT_Fhj1rQ8dH3itgJMA&bvm=bv.47008514,d.d2kInteresting article about oscillators. About vapacitance between legs, case etc.Just made an 1Hz oscillator with a 4060 and watch Xtal, first on a breadboard andthat worked fine, then on single sided pcb smd style using through hole. I needed around 17 pF on one leg and 25 on the other to get it running at 1Hz precise. But it oscillated with everything between 15 and 40 pF (have not tried more values)