ATMEGA 328P Crystal pf - Page 1
Forum section: electronics / general

author: ianscott
author_rank: Contributor
timestamp: May 07, 2014, 02:46:18 am
content: Hi I'm ordering parts from digikey and I'm not sure what crystal I need, I'm Running a ATMEGA 328p with 22pf capacitors (399-8034-1-ND). I Have found two types of crystals a 16mhz 20pf (X1103-ND) and 16mhz 18pf (CTX1085-ND). Which one should I go for. The 18pf's are cheaper.Also Would anyone know the exact crystals the arduino uno boards use something similar to those would work for me.Thanks Ian

author: zapta
author_rank: Super Contributor
timestamp: May 07, 2014, 04:34:38 am
content: Quote from: ianscott on May 07, 2014, 02:46:18 amI had good success with 328P and resonators with internal capacitors. For examplehttp://www.digikey.com/product-detail/en/CSTCE16M0V53-R0/490-1198-1-ND/584635for smd (there are also through hole versions). Accuracy ~1% which was good for my application.As for your crystal, i think both should be fine.

author: retrolefty
author_rank: Super Contributor
timestamp: May 07, 2014, 04:58:27 am
content: Quote from: ianscott on May 07, 2014, 02:46:18 amActually the present generation of Arduino Uno and mega boards (rev3) don't utilize crystal resonators but rather use 3 terminal ceramic resonators that have internal padding caps.

author: sleemanj
author_rank: Super Contributor
timestamp: May 07, 2014, 05:03:23 am
content: Quote from: ianscott on May 07, 2014, 02:46:18 amIt's probably not going to matter a lot realistically, unless you really want to really hit the marks as it were. Either will be fine.If you want to get into specifics though, there is a formula for calculating the capacitor values, given the load capacitance of the crystal from a datasheet.CX = 2(CL - Cstray)CX is the capacitor value you use (assuming you use the same value for both caps),CL is the load capacitance specified by the crystalCstray is stray capacitance inherent in the constructionLet's say you call the stray capacitance 5pF, that means forCrystal 1 which has 20pF load, each capacitor should be 2(20-5) = 30pFCrystal 2 which has 18pF load, 2(18-5) = 26pFI'd probably use the 18pF one, because it's cheaper.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 07, 2014, 05:04:06 am
content: If you want to know more about it, and *shock* *horror* actually get an idea how to calculate this, get Atmel's application note 042.

author: casinada
author_rank: Frequent Contributor
timestamp: May 07, 2014, 06:26:14 am
content: Before you commit to Digikey check Tayda:http://www.taydaelectronics.com/crystals-resonators-oscilliators/crystals.htmlThey are based in Thailand but I get everything within a week in the USA. Somehow they ship from Colorado.For simple components is much cheaper than Digikey and Mouser. All the more rare parts I get from Mouser or DigiKey.

author: dannyf
author_rank: Super Contributor
timestamp: May 07, 2014, 10:02:18 am
content: QuoteThey are the same performance-wise. I would go with the 18pf ones because it is cheaper.QuoteThose arduino guys messed up on that one: they used ceramic resonators - a penny or so cheaper but much worse in stability / phase noise. I would either stay with a crystal or use the internal oscillator.

author: linux-works
author_rank: Super Contributor
timestamp: May 07, 2014, 03:01:02 pm
content: I've gotton nothing but low quality fakes and junk from 'tayda'. I would never send them another order.re: arduinos, I use the resonators on my atmel builds and they work fine, don't take up that much room and don't need caps. what's not to like?

author: casinada
author_rank: Frequent Contributor
timestamp: May 07, 2014, 03:26:38 pm
content: Which parts were fakes? I would like to know. Did you contact them to tell them about the fakes?

author: linux-works
author_rank: Super Contributor
timestamp: May 07, 2014, 03:55:14 pm
content: yes, I wrote back and was able to return them.I took a photo of the fake trimmers, for example. it was hilarious! every conceivable faked name you could think of. we had quite a laugh about it on some other forums...of course, there is only one BOURNS, and the others were clearly intended to look like the B-brand but I wanted no part of that BS.they were advertised as bourns brand and yet I got a mix of good and not-so-good stuff in that order.the ic sockets I ordered had plastic that would melt if you soldered to the pins. same with pin headers. I forgot what else I ordered but it was ALL junk and fakes. I have zero trust in that vendor after that.

author: westfw
author_rank: Super Contributor
timestamp: May 08, 2014, 01:37:41 am
content: QuoteProperly, one picks a crystal based on accuracy, cost, size, and etc, and then calculates the capacitors needed for that particular crystal. Picking the caps first and asking which crystal to get is backward.That said, nearly any parallel-resonant crystal will "work" in the arduino circuit.

author: Monkeh
author_rank: Super Contributor
timestamp: May 08, 2014, 01:43:38 am
content: Quote from: linux-works on May 07, 2014, 03:55:14 pmThey do not appear to be advertised as Bourns. Never seen them advertised as such. Revise your expectations.QuoteYou're too slow. I use plenty of their sockets and headers, they work fine. No, they're not expensive gold plated fairy-dust parts, but they work.QuoteCan't be fake if it doesn't claim to be 'real'.I really think you were expecting too much.

author: linux-works
author_rank: Super Contributor
timestamp: May 08, 2014, 02:12:53 am
content: I'm too slow? you've seen me solder? you know my ability, do you?(wow)go ahead and think that.but that seller sells JUNK and I know my parts quality, thankyouverymuch.

author: Monkeh
author_rank: Super Contributor
timestamp: May 08, 2014, 02:15:25 am
content: Quote from: linux-works on May 08, 2014, 02:12:53 amIf you're managing to melt the same parts I have no problems with.. yes!QuoteYes, they do sell some junk. They also sell some perfectly good, genuine, branded components.Your expectations. Revise them. You are not going to get expensive Bourns trimpots from a hobby distributor in Thailand!

author: westfw
author_rank: Super Contributor
timestamp: May 08, 2014, 03:10:19 am
content: (I think I've been buying cheap sockets in general, forever. Most of them will melt if I'm not "quick." I guess I HAVE seen SOME less melt-y varieties, but they've been clearly in the "they want HOW MUCH for a SOCKET?!" category...)

author: zapta
author_rank: Super Contributor
timestamp: May 08, 2014, 07:34:06 am
content: Quote from: sleemanj on May 07, 2014, 05:03:23 amI always assume that Cstray ~ 0, and CX = CL. No problem so far but I should spend more time reading the datasheet. ;-)

author: miguelvp
author_rank: Super Contributor
timestamp: May 08, 2014, 08:02:50 am
content: based on the previous post to yours, if Cstray is ~0 wouldn't that mean that:CX = 2*CL ?But I only read the last two posts

author: dannyf
author_rank: Super Contributor
timestamp: May 08, 2014, 12:27:47 pm
content: In a typical pierce oscillator the two capacitors are in serial and then parallel with the stray capacitance (which is not negligible vs. the load capacitance).