Troubleshooting Crystal that wont start, how to measure load capacitance. - Page 1
Forum section: electronics / projects

author: Hugoagogo
author_rank: Newbie
timestamp: October 22, 2015, 02:18:20 am
content: I am using a MAX11060 ADC in a project, I have established SPI communications and am able to read and write registers, however I am unable to get any readings, I believe this is due to the crystal not operating correctly.This assumption is based on before enabling the oscillator, I get 0V if I probe either side of the crystal. After enabling the oscillator I see a flat 800mV or so on both sides.To backtrack a bit to fix a screw-up in our board design (wrong footprint) we have manually broken out the package and encased in epoxy (as in I REALLY don't want to re-solder this).The crystal leads are on the side with the shorter leads.I presume the extra capacitance and/or larger loop area has pushed the crystal out of its operating zone.Is there any way to measure the load capacitance and then adjust, is this as simple as using a multi-meter or LCR meter to measure between each side of the crystal and ground and adjust to match the rated value.Can this be done with the crystal in place, will this give correct readings and/or be safe to do without however the meter excites the test item causing damage.Crystal Datasheet -http://au.element14.com/fox-electronics/foxsdlf-245f-20/crystal-24-576mhz-20pf-hc-49s/dp/2058007?CMP=i-55c5-00001621ADC Datasheet -https://datasheets.maximintegrated.com/en/ds/MAX11040K-MAX11060.pdf(Also yes I understand mounted like this the chip will not be able to reach its 16 bit performance, the project has reached a it would be good if it would work at all state)Edit: First time posting here, have lurked for a while but never contributed, please let me know if this is the wrong place to ask this

author: singapol
author_rank: Frequent Contributor
timestamp: October 22, 2015, 03:35:24 am
content: It should work as per adc datasheet but first check the connections to crystal,load capacitors and adc.Are the load capacitors really 20pf ( measure them). Are the crystal pins shorted to gnd? ( solder overflow).http://www.farnell.com/datasheets/1911271.pdfhttp://www.foxonline.com/pdfs/xtaldesignnotes.pdfhttp://www.oscilent.com/spec_pages/PNDescrpt/Load_Cap.htmhttps://www.lpcware.com/content/faq/how-calculate-value-crystal-load-capacitorshttp://www.statek.com/pdf/tn33.pdfhttp://www.freescale.com/files/microcontrollers/doc/app_note/AN3208.pdf

author: Hugoagogo
author_rank: Newbie
timestamp: October 22, 2015, 05:23:54 am
content: Driving the clock from some external source is my next plan of attack, before that I wanted to try and find out what I could do to troubleshoot the crystal setup I already have.If I measure capacitance across one of the capacitors should I expect to get 20pF ? I am not sure what effect the crystal has on my measurements.How wrong can the capacitance be before it will refuse to oscillate.

author: edavid
author_rank: Super Contributor
timestamp: October 22, 2015, 05:31:40 am
content: Quote from: Hugoagogo on October 22, 2015, 05:23:54 amHugely wrong. In fact, you can try removing the caps.

author: singapol
author_rank: Frequent Contributor
timestamp: October 22, 2015, 06:58:16 am
content: Quote from: Hugoagogo on October 22, 2015, 05:23:54 amFirstly to enable external crystal adc Configeration registerXTALENmust be set to 1. ( page14).Secondly, there seems to be an error in the published schematics in which crystal load capacitors are valued at 20pf. On page 3 it saysMaximum Oscillator Load Capacitance=10pf.

author: Hugoagogo
author_rank: Newbie
timestamp: October 22, 2015, 07:03:21 am
content: Yes I am enabling the oscillator see first post.Secondly I thought the Capacitive load was defined as what is seen from the crystal 20pF || 20pF = 10pF

author: singapol
author_rank: Frequent Contributor
timestamp: October 22, 2015, 10:06:09 am
content: Quote from: Hugoagogo on October 22, 2015, 07:03:21 amOn page 14 of adc datasheet it recommends using a crystal 24.57mhz with esr less than 30 ohm of which your particular crystal listed a maximum of 30 ohm. Either you can be optimistic and believe it is or try replacing another one of the same brand/make or try another. Use smd capacitor trimmers instead of fixed values to adjust. I would try between 15pf - 17pf.

author: Dago
author_rank: Frequent Contributor
timestamp: October 22, 2015, 11:09:21 am
content: Quote from: edavid on October 22, 2015, 05:31:40 amWell I don't know what counts as "hugely wrong" but on one prototype I had 20 pF caps and the oscillator did NOT work. Swapped them to 13 pF ones and it operated just fine after that (and in production units as well).Remember that the load capacitance includes stray capacitance and capacitance of the pins of the microcontroller. With a... 3D circuit like that you are going to have quite a lot of stray capacitance.

author: Hugoagogo
author_rank: Newbie
timestamp: October 22, 2015, 09:48:45 pm
content: Thanks for the help guys I finally solved it, unfortunately like a few suggested it was a soldering problem.I somehow managed to miss-solder the easiest part, with the fewest and most widely spaced pad on the board, the crystal.I had a tiny smoosh of unmelted solder paste bridging between the pins somehow...Why it took me so long to figure it out is when I was comparing measurements with a second populated board (only without the ADC fitted) the readings were within about half an ohm. So not only did I mess up the easiest part on one board, I messed it up on two. I only noticed when I went to replace the crystal and measured its resistance not fitted in the circuit... I should have realised the 30Ohm ESR was only when running at the rated frequencyAnyway it runs a bit fast with no capacitors, with the 20pf capacitors refitted it looks to be a touch slow but much closer, I will retry with some 18pF caps I have (only other size I have that is remotely close in the same package)Thankyou for the suggestions & sorry it turned out to be a stupid mistake.Only one more ADC to bodge on ....@singapol While those links didnt help me solve the problem they have been good to look through for more refference