Help identifying VW/Skoda keyfob crystal? - Page 1
Forum section: electronics / repair

author: JPortici
author_rank: Super Contributor
timestamp: April 19, 2022, 05:41:29 pm
content: Hi.One of my car's keys (2014 skoda octavia 3) has stopped working some months ago, Don't know what happened exactly, but the lock/unlock didn't work anymore, led wouldn't blink, obviously a fresh battery didn't work.However the key can still turn on the ignition, the car doesn't complain, so the transponder is fine, it's just an issue with the unlock mechanism.So i opened (destroyed**) the enclosure to see if the issue was something i could fix easily like a cold joint, or a broken button / trace, i have repaired many keys with similar issues and in fact something was off.The crystal was exposed! Che cap was somewhere around the remains of the enclosure. A broken crystal could very well be the problem.So now i have to identify the crystal. What i've gathered so far:First, it's a crystal and not a mems and that's obvious from the circuit/pinout (no power, only two opposite terminals are soldered to something.Second, it's package its 3.2x2.5mmThird, The top had "3M" printed over itFourth, my key carrier frequency should be 434 MHzWhat could be the frequency? 3MHz? 433 MHz? I'm going to look at datasheets, hoping for the best** This bastard enclosure is not really made to be opened, so in the end i applied a bit of brute force but in the process i damaged a capacitor. It's between a pin of the silicon and battery V-, i suspect it's some sort of VCAP (internal regulator bypass) and its value should not be critical, i'll put 100n and see how it goes, or remove it altogether, put battery, look at the signal and replace again. Worst case i can only use the key manually, as i was doing before, i have already ordered a replacement enclosure anyway

author: JPortici
author_rank: Super Contributor
timestamp: April 19, 2022, 06:01:08 pm
content: QFN32 pinout simillar to ATA5795C. Many connections make sense but are shifted by one pin (e.g: crystal is not at 14-16 but at 13-15, VDD is also shifted by 1)In which case the crystal should be a 32 kHz. Yuk

author: DavidKo
author_rank: Frequent Contributor
timestamp: April 20, 2022, 07:29:47 am
content: The parts should beCapacitor - 1uF X7R SMD 0603 1μF -10 10% 16VCrystal - 13 MHz NX3225

author: DavidKo
author_rank: Frequent Contributor
timestamp: April 20, 2022, 07:43:04 am
content: 3M is probably date code Dec 2013 in case of KDS DSX321Galternate part can be NDK NX3225GA (date code does not use M)Kyocera CX3225CA also uses different marking

author: JPortici
author_rank: Super Contributor
timestamp: April 20, 2022, 09:55:18 am
content: Quote from: DavidKo on April 20, 2022, 07:29:47 amMany thanks, yesterday i made some more research and figured it could be in the 13 MHz range: with a 32x PLL, 13.56MHz x 32 = 433.92 so i ordered a 13.56 crystalIf you say it's exactly 13MHz, well i'll just order another crystal

author: DavidKo
author_rank: Frequent Contributor
timestamp: April 20, 2022, 10:57:07 am
content: One remark which I have noticed in the crystals documentations (I do not know if it is important) the frequency tolerance is 20ppm@25°C and 10ppm/10years.

author: JPortici
author_rank: Super Contributor
timestamp: April 29, 2022, 01:23:17 pm
content: Now, several days later due to various shippings i have a functioning key for about 50 euro. I confirm that 13MHz (and not 13.56) is the correct crystal, i guess there is a fractional PLL in the radio chip-15 euro for the enclosure (amazon, could have spent less with aliexpress)- 6 euro of parts (4.5 euro for one crystal at RS, wrong frequency, and it took a whole week for them to ship from EU, 1.5 euro for two crystals, right frequency, and new 1u caps because i only had 0805, at mouser, and it took just two days from the US)the remaining is all for shipping from mouserbut hey, the dealer was asking for 300 euro plus a lot of paperwork because they HAD TO replace the locks as well