Spin Stamp Crystal: 10MHz with PLL?
Phil Pilgrim (PhiPi)
Posts: 23,514
The Spin Stamp uses a 10MHz crystal. The docs state: "Existing code written for other Propeller development platforms must have the clock parameters changed to _xinfreq = 10_000_000 and _clkmode = xtal1 + pll8x to make it operate on a Spin Stamp."
From the Propeller datasheet: "The PLL’s internal frequency must be kept within 64 MHz to 128 MHz – this translates to an XIN
frequency range of 4 MHz to 8 MHz."
I assume the Spin Stamp works without a glitch, despite its oscillator being out of spec by 25%. My question, therefore, is: Is it generally considered safe to drive the PLL with a frequency above 8MHz. If so, then how high is too high? Under these conditions, is the operating temperature range more restricted?
Having looked in vain for miniature 5MHz SMD crystals, I can certainly understand the temptation to use a 10MHz crystal, which can be had in the 5 x 3.2mm form factor. But what must one beware of in doing so?
-Phil
From the Propeller datasheet: "The PLL’s internal frequency must be kept within 64 MHz to 128 MHz – this translates to an XIN
frequency range of 4 MHz to 8 MHz."
I assume the Spin Stamp works without a glitch, despite its oscillator being out of spec by 25%. My question, therefore, is: Is it generally considered safe to drive the PLL with a frequency above 8MHz. If so, then how high is too high? Under these conditions, is the operating temperature range more restricted?
Having looked in vain for miniature 5MHz SMD crystals, I can certainly understand the temptation to use a 10MHz crystal, which can be had in the 5 x 3.2mm form factor. But what must one beware of in doing so?
-Phil
Comments
If what you're looking for is a guarantee or specification, you'll have to stick with the datasheet information. The Hydra and the Spin Stamp seem to work fine with a 10MHz crystal and 80MHz clock, but have not been tested over a wide range of operating conditions. There was a Prop module attached to an FTDI USB adapter chip running off that chip's 6MHz clock output and running at 96MHz with PLL16X. I haven't heard any complaints, but that's not a specification.
On my tests I have used 14.318 000 crystal with wry good results.
But 15.000 000 Crystal had problem.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Nothing is impossible, there are only different degrees of difficulty.
Sapieha
Is it generally considered safe to drive the PLL with a frequency above 8MHz - I'd say yes, "Should generally work but not guaranteed", and also agree with Mike, this is "out of spec" as per the documentation.
I'm always hesitant to go outside the specs when designing a circuit for broad use, as is the case here. That's why I was a little surprised to see Parallax take such an excursion beyond their own published constraints — not that they had much of a choice, given the space limitations on the Spin Stamp PCB. (Of course, they are in a position to select chips that meet a broader spec, if that were to be necessary.)
Well, caveat architectus, I suppose.
-Phil
Post Edited (Phil Pilgrim (PhiPi)) : 5/18/2008 12:36:48 AM GMT
Given that the PLL appears to works at 240MHz and 160MHz is considerably less than that ( +25% over spec ) I'd personally be happy with using a 10MHz crystal for anything used in a normal 'room temperature' environment including a commercial design.
We have the official spec and the de-rating graph but it would be nice to see details of what the envelopes are for 14.318MHz and 7.3728MHz operation and the more commonly used 6MHz and 10MHz.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Paul Baker
Propeller Applications Engineer
Parallax, Inc.