20Mhz crystals

[PsyKotyk]

I've been working on my PICStep clone, and am having lots of issues with oscillation. I'm using 20Mhz crystals with a couple 27pf caps. I found that on one I needed a 1M resistor in series with OSC2, but I think it's still somewhat noisy. On the other I made, I can't get it to run stable. It sort of runs if I put my finger on the cap connected to OSC2.

I realize that most people use ceramic resonators (which I wish I could find), but I woud like to know why I can half-ass get the crystal to work.

Anything obvious? Unfortunately I don't own a scope, so I can't really see what's coming out.

My electronics knowledge is scattered, so this falls outside my realm of knowing what to try next.

[xyz100]

PICs require use of "parallel cut" crystals. Serial cut crystals will be unstable. The difference is in how crystal is manufactured.

[BCwanderer]

You can often find resonators on E-bay
Different frequencies are listed on different days.
The three legged resonators include the capacitors and are easier to use.
They are not expensive.

[PsyKotyk]

I lucked out and found some 20Mhz parallel cut xtals at Computer Recyclers here in Ottawa. 2/$3 so I scooped most of them. Digi-key had ceramic resonators .... $5 for 10 ... and then $20 for shipping & handling. I couldn't bring myself to pay 3x in shipping more than the part.

[pastera]

Why use a crystal at all? I use oscillators almost exclusively. You can get a 20MHz oscillator for less than $2 in singles (Digikey).
I know a crystal will cost about a quarter of that but unless you are making units by the 1000s, the saved time is worth the money

Aaron

[sswitaj]

I'm with pastera on this one.

Use an off-the shelf oscillator if you can.

Crystals can start to get pretty picky above a few MHz, largely because once you get much higher than that they start to run in harmonic modes instead of fundamental modes, and the resultant tuned circuit becomes much more sensitive to the impedance of it's components.

Don't forget, this is an analog circuit, and the analog effects of things like component drift, temprature, wire routing, and fingers is substantial at 20Mhz.

Canned oscillators also have the advantage that they will start and run cleanly, discrete oscillators can come up in funky modes when run on harmonics.

[tfmacz]

In Alans design the DACRef pin (pin-14) is connected to 5V is this required as it is not connected to anything in the clone board????

[Smertrios]

http://www.mouser.com/catalog/627/727.pdf $0.59 each

[davesaudio]

couple of sources for canadians:
Randy -glitchbuster.com ships reasonably to Canada
Speff on ebay - is in toronto and has resonators (none listed atm so ask)

[smarbaga]

xtals and oscilators are much more accurate than resinators.
put the xtal as close as possible to the osc pins (0.1 to 0.2").
put the 22-30 pF caps real close to the xtal ( same 0.1 to 0.2").
use a 0.1 uf bypass cap as close as possible to the power pins.
maybe a 10 uf electrolytic as well, in the vicinity.
reflow your solder from the gnd, xtal caps, xtal and osc pins.
use ciramic capacitors.
the only thing i could see would be a resistive ground trace to the xtal caps or/ and the chip. i assume you are using a 7805 voltage regulator to power the chip., or maybe the osc configuration setup bite.