XMEGA External Oscillator Selection

Go To Last Post
8 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I plan to use ATxmega128A1U for an application where I need to use all 24 PWM channels. I would like to use external oscillator to improve frequency stability of my system. The CPU system clock needs to be 32 MHz with stability of 25 ppm or better. I do not need 4x peripheral clock.
My question is, what kind of external oscillator should I chose? The documentation says that I can chose anything from 0.4 MHz - 16 MHz, but the type is not specified. Do I need to use CMOS, Clipped Sine, or some other oscillator type?
If possible, please specify part number of the oscillator.

P.S. I know how to enable external clock, PLL, etc. I just need help selecting physical oscillator part.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I use a simple 8MHz crystal and 2 caps, not good enough for you?

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Get AVR042 from Atmel's site and read the XTAL section as that will help you to know what type of XTAL or oscillator to get. I don't have any specific part #s, but you'll know how to find one on Digikey, etc. after studying that doc.

1) Studio 4.18 build 716 (SP3)
2) WinAvr 20100110
3) PN, all on Doze XP... For Now
A) Avr Dragon ver. 1
B) Avr MKII ISP, 2009 model
C) MKII JTAGICE ver. 1

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Through Hole or SMD?

If, as an example, you go to Mouser Electronics and select:
All Products>Passive Components>Frequency Control & Timing Devices>Crystals and then
tic the box Stocked
and enable Smart Filtering
and select 16 MHz

Three crystals will pop up, one at 20 ppm, two at 15 ppm, ranging in price from $1.52 to $1.65 USD, Qty one.

You would set the PLL for 2 to get the 32 MHz system clock, using these 16 MHz Xtals.

JC

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

indianajones11 wrote:
Get AVR042 from Atmel's site and read the XTAL section as that will help you to know what type of XTAL or oscillator to get. I don't have any specific part #s, but you'll know how to find one on Digikey, etc. after studying that doc.

Thanks for the document, helpful.

DocJC wrote:
Through Hole or SMD?

If, as an example, you go to Mouser Electronics and select:
All Products>Passive Components>Frequency Control & Timing Devices>Crystals and then
tic the box Stocked
and enable Smart Filtering
and select 16 MHz

Three crystals will pop up, one at 20 ppm, two at 15 ppm, ranging in price from $1.52 to $1.65 USD, Qty one.

You would set the PLL for 2 to get the 32 MHz system clock, using these 16 MHz Xtals.

JC

I looked earlier at DigiKey and they had a lot of "Type" and "Output" options and I was not sure if I had to select any particular one. It seems that Mouser did not specifically sort by that category.

Then if I select 10 ppm 8 MHz oscillator, will it have 40 ppm at 32 MHz after PLL ? Am I better off selecting 16 MHz ?

Thanks

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:
Then if I select 10 ppm 8 MHz oscillator, will it have 40 ppm at 32 MHz after PLL ? Am I better off selecting 16 MHz ?

It's best to use a crystal that is directly used by the system and doesn't go through any type of xLL (DLL, PLL, DFLL, etc.) unless the cost differential justifies the engineering for using the xLL. They can only introduce noise/jitter/error.

You need to define your crystal ppm spec. Is it over temperature? Is it aging/drift? Is it part to part? Is it over lots? If you use a PLL, then the PLL specs add to the crystal specs for the output frequency.

Many people try to use a inexpensive crystal and then use a PLL to get a different frequency (for which a crystal would be expensive), but then getting burned because the PLL doesn't produce a signal is up to spec. Good/effective PLL design can be hard.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

mspeng wrote:

It's best to use a crystal that is directly used by the system and doesn't go through any type of xLL (DLL, PLL, DFLL, etc.) unless the cost differential justifies the engineering for using the xLL. They can only introduce noise/jitter/error.

You need to define your crystal ppm spec. Is it over temperature? Is it aging/drift? Is it part to part? Is it over lots? If you use a PLL, then the PLL specs add to the crystal specs for the output frequency.

In my case I don't have a choice. ATxmega will only take 16 MHz input clock while I need 32 MHz system clock. I am building a single unit, so the main concern is temperature stability. It is okay if frequency is slightly (100-200 kHz) different from nominal value. However it must be stable around that value over time.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi!

rsmorph wrote:
In my case I don't have a choice. ATxmega will only take 16 MHz input clock while I need 32 MHz system clock.

ATxmega can use 32 MHz external _generator_. We use GX0-U108I-32.00 MHz.

Ilya