What Is The Maximum Allowable Trace Length From The Crystal To The Propeller Chip?
idbruce
Posts: 6,197
Hello Everyone
I remember reading it someplace that the crystal should be as close to the XI and XO pins of the Propeller chip as possible. In a worst case scenario, what would be the maximum trace length of these connections and still expect it all to function properly?
Bruce
I remember reading it someplace that the crystal should be as close to the XI and XO pins of the Propeller chip as possible. In a worst case scenario, what would be the maximum trace length of these connections and still expect it all to function properly?
Bruce
Comments
Since it creates square waves, the nasty harmonics are radiated from it. The faster you go, the more power you need to use to drive it and the bigger the problem. Generally, under 20Mhz or so, it is not an issue. And driving at 3.3v is less of an issue than at 5.0v.
If you look at the old SX chip Proto Boards, the Xtal configuration was quite far away (inches maybe) and they clocked up to 80Mhz without functional problems. But on a bad day, the FCC might come knocking on your door to ask what exactly you are doing. First time offenders are politely asked to just stop and do no more.
Some good info here, with a few references as well.
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=printview&t=81294&start=0
And *look* it even includes our beloved Leon.
C.W.
I like to keep the xtal traces within 300 mils and definitely all other traces with fast transitions angled away. I usually leave the ground plane under those traces and allow for a couple extra pF in the loading. One nice thing about the Prop is that you don't have to find a place to put the capacitors or where to ground them.
Bruce
EDIT: Additionally, this is a one layer design.
Yea that is a nice thought, but I am trying to keep it at bare bones minimum. However, if I don't have enough room to add the other components and route the necessary traces, that definitely sounds like a quick remedy. I have read about that trick somewhere before, but I had forgotten about it. Thanks for reminding me Mike.
Bruce
Okay, but internally - the first thing that happens to that sine wave is it is converted to a square wave and then passed down the line. What you imply is that only the fundamental frequency will be emitted from the crystal leads. Since these are driven by inverted logic, I thought that in might be closer to a triangular wave that is commonly seen in relaxation oscillators.
Use a 3 pin surface mount resonator if space is at a premium. No additional capacitors and very small. And if it is a single sided circuit board, that will all go on the underside very nicely. Or does the Propeller have to use a 2 pin resonator? That would be even easier.
If its just a crystal mounted at right angles to the prop (.5" trace length), I do this often and have never had an issue across several designs.
That is unlikely to be a problem.
A purist might spin the Xtal 180' so the XI lead is the shortest, as the impedance of that is nominally higher than XO,but those are both short leads.
Well that's is good to here! That is just the answer I was looking for
Bruce
As it happened, I was looking at a lot of crystal signals today, not from the Prop, but from a 32kHz xtal on the timer1 LP oscillator on a PIC12F675. Lots of variation in startup time and amplitude on 25 boards.