The clock, clock multiplier, and crystals

kt88seamp · 2009-11-25 16:17

I am using a 5Mhz crystal to drive the clock on my prop. The clock multiplier is set to multiply it up to 80Mhz. Will the prop run faster if I use an 80Mhz crystal?

Graham Stabler · 2009-11-25 16:42

No 80Mhz is 80Mhz whether generated from a multiplied 5Mhz clock or directly.

Some people over clock the prop but generally it should be avoided.

Graham

Leon · 2009-11-25 16:43

You won't find a fundamental crystal that will operate at 80 MHz, 40 MHz or so is the limit. You could use an 80 MHz oscillator module, but it won't run any faster, and will be a lot more expensive than a 5 MHz or 10 MHz crystal..

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Patrick1ab · 2009-11-26 09:14

I tried to overclock my propeller to 100 MHz, using a 25 MHz crystal and 4x Pll, but it didn't work.
It was slower than 80MHz, so I guess it used it's internal oscillator.

Is there a limit to the crystal frequency the propeller accepts or why are so many people using 6,25 MHz crystals?

dMajo · 2009-11-26 10:04

The datasheet is your best friend ... as someone stated

The PLL can be run from input frequencies from 4 to 8 M. You could supply a 25M clock but then the prop have to run at that frequency (without the PLL)

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Patrick1ab · 2009-11-26 10:32

Oh, okay. Thanks

Cluso99 · 2009-11-26 14:34

I am using 6MHz as are a number of others. 6.25MHz is also being used by quite a few. I have 6.5MHz on order. I am also going to try other higher values. Sapieha uses a 15MHz & PLLx8 = 120MHz on my TriBlade for more than 6 months. Overclocking has a lot to do with the pcb layout and power supply decoupling.

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MarkS · 2009-11-26 17:58

dMajo said...
The datasheet is your best friend ... as someone stated

The PLL can be run from input frequencies from 4 to 8 M. You could supply a 25M clock but then the prop have to run at that frequency (without the PLL)

I was about to ask this very question. The data sheet states that the maximum input frequency for the PLL is 8 MHz, and yet the Hydra uses a 10 MHz crystal with an 8x multiplier.

Leon · 2009-11-26 18:22

Lots of people use the Propeller outside the published specifications without any problems. I'm using a 10 MHz crystal on a prototype because I don't have the correct value in an SM package, and it's working OK. It's not a good idea for anything that is going into production, although it's OK for hobbyists.

Leon

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Phil Pilgrim (PhiPi) · 2009-11-28 00:39

Both the Spin Stamp and upcoming Propeller Backpack use 10 Mhz crystals (pll8x) — with impunity.

-Phil

cgracey · 2009-11-28 02:35

The clock PLL on the Propeller ALWAYS multiplies by 16, even though you may select a lower divider tap. For this reason, there is a limit to what frequency you can put into XI when using the PLL. This limit is about 10MHz, which results in an internal PLL frequency of 160MHz (the design target limit, even though we spec'd it lower at 128MHz, or 8MHz into XI). I remember from experimentation that the PLL tops out at about 230MHz, typically. This means that exceeding 230MHz/16 (14.375MHz into XI) will result in the PLL not being able to lock. When the PLL can't lock due to excessive target frequency, it just goes as fast as it can and loses all relationship to the reference clock going into XI.

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Chip Gracey
Parallax, Inc.

Phil Pilgrim (PhiPi) · 2009-11-28 03:14

The 14+some MHz top-out frequency also agrees with my observatons regarding the counter PLL modes.

-Phil

Leon · 2009-11-28 12:32

Phil Pilgrim (PhiPi) said...
Both the Spin Stamp and upcoming Propeller Backpack use 10 Mhz crystals (pll8x) — with impunity.

-Phil

The PLL upper limit is 8 MHz, though, according to the data sheet. I have an aversion to disregarding data sheet maximum ratings.

Leon

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Post Edited (Leon) : 11/28/2009 12:37:14 PM GMT

Sapieha · 2009-11-28 15:27

Hi Chip Gracey (Parallax)·and Phil Pilgrim (PhiPi)

I have it stable IN 15MHz (and litle more but not documented yet)
BUT as I write in My thread on .... Build good PCB for Overclocing.

To that Propeller need VERY stable VOLTAGES that can hold PLL's VCO in very stable states.
As Propeller is constructed for power save (Very handy in batery usage) their have one back side of this.
It give it very unstable Voltages to PLL in cases of Strting/Restarting else end from waitstates in COG's.
As that need momentary BIG curents and dry VOLTAGE's from PLL that give it in that moments very unstable function
That Needs THAT BIG caps near Propeller I describe in my thread to compensate for that VOLTAGE faults.

To that It is recommended to have good GND shield planes around XTal pins (All 4 pins -both Propellers and XTal's shielded) on both sides of PCB.

Regards
Christoffer Jönsson

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Sapieha

Post Edited (Sapieha) : 11/28/2009 7:46:26 PM GMT

Phil Pilgrim (PhiPi) · 2009-11-28 19:30

Leon said...
The PLL upper limit is 8 MHz, though, according to the data sheet. I have an aversion to disregarding data sheet maximum ratings.

So do I. But when the data sheet maxima are violated by the very people who wrote it (Parallax Spin Stamp), such exceptions seem less risky.

-Phil

Clock Loop · 2009-11-28 20:37

One thing no one has mentioned here is using a prop as a clock source.

I noticed how many people here either have trouble or can't find the proper crystal for their needs.

I would suggest a prop that generates your target clock frequency.

I did this to test my own prop and program at different clock inputs.
Its not ideal for production but works to test different clocks.

I had some pretty neat audio frequency bending happen when I tweaked the input frequency live when that prop was playing music.
Because I didn't tell that prop I was tweaking its clock input.

I regularly PWM my XI. (within limits)

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Leon · 2009-11-28 21:09

The Si570 is a nice programmable clock source.

Leon

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Amateur radio callsign: G1HSM

The clock, clock multiplier, and crystals

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