Need to learn how making frequency counter (or any time critical project) using PIC18F that display output over USB.

Having choice of 3 clock sources:

  1. ±50ppm 4.0000 MHZ crystal ATP040SM

    4.0000 MHZ 20PF SMD ±50ppm


  2. ±50 ppm 24.0MHZ external oscillator


    ±50 ppm


  3. ±100 ppm 40.0MHZ external oscillator

    MXO45-40M000 Manufacturer: CTS CORP

    40 Mhz

    ±100 ppm


To run internal PLL PIC18F will divide clock input by PLLDIV that can be setting to either 1, 6 or 10 for above options respectively.

I thinking which option providing best stability?

When thinking myself, I feel 3rd option will be best as I reasoning that ±100 ppm 40Mhz clock will be divide by 10 and hence effectively behaving like ±10 ppm 4Mhz clock?

Or is reasoning wrong and ±100 ppm 40Mhz clock after divide by 10 is effectively still ±100 ppm 4Mhz clock?

If you, experienced designing having choice of above 3 - which one you choose and why (no consider PCB space or 5V voltage issue in 3.3v world, but might consider cost only between more expensive ±50 ppm vs less expensive ±100 ppm)

  • \$\begingroup\$ I wondering why I see so many 50ppm crystals but rare 50ppm external oscillator (usually 100ppm+)? Why manufacturer not use 50ppm crystals in the external oscillator? \$\endgroup\$
    – sekharan
    Sep 1, 2011 at 6:02

1 Answer 1


You are interested in the relative error and/or stability, which is unaffected by any division. So, other things being equal, take the lowest ppm clock source for the best accuracy.

If 50 ppm is enough for you, I would choose the crystal for board space and price.

For better accuracy you could consider a better quality (tuned) crystal oscillator. Same physical format as a regular oscillator (14-pin DIP size), but higher price (~ $20).

  • 1
    \$\begingroup\$ 20% and even 10% is also available for crystals, you don't always need an external oscillator for it. That's base precision. If you want high temperature stability you may want a temperature compensated crystal oscillator (TCXO). Amazingly affordable, this one is less that USD 5.00, quantity one. \$\endgroup\$
    – stevenvh
    Aug 28, 2011 at 18:15
  • \$\begingroup\$ One question - why there are external oscillators if they are more expensive and less precise than crystal? \$\endgroup\$
    – sekharan
    Aug 29, 2011 at 0:06
  • 1
    \$\begingroup\$ An external oscillator is a complete circuit (crystal + oscillator). Feed it 5 or 3.3 V and it gives you a square wave. To use an external xtal your uC must provide the oscillator part. Most do, but there are issues with stability, startup, higher frequnecies, choice of decoupling capacitors etc. And there are circuits that don't automatically contain the analog oscillator circuit. \$\endgroup\$ Aug 29, 2011 at 6:37
  • \$\begingroup\$ @stevenvh: Could you guide how to select 48 and 75Mhz TCXO from that same site? I cliking on link on that page and it show me only 25ppm ones. \$\endgroup\$
    – sekharan
    Aug 30, 2011 at 22:24
  • 1
    \$\begingroup\$ @SGhosh - this page gets you to the TCXO selection guide, but apparently it lists neither 48 nor 75MHz oscillators :-( \$\endgroup\$
    – stevenvh
    Aug 31, 2011 at 5:15

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