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New To Propeller - Please Help Clarify XTAL Selection — Parallax Forums

New To Propeller - Please Help Clarify XTAL Selection

soshimososhimo Posts: 215
edited 2008-12-16 02:48 in Propeller 1
Okay, I did the prerequisite google search and forum search and while there is plenty of information out there I would have to desiminate quite a bit just to get the answer I'm looking for. I'm not new to uChips and have worked with PICs for about 8 years now so I'm not unfamiliar with the concepts, I'm just confused about how the Propeller treats the external oscillator and the differences between external and internal oscillators (as it pertains to the propeller application). Specifically, I'm confused about why the specs say that the propeller will run up to 50Mhz yet it also states that the external oscillator can be any where between 5-8MHz. If the PLL multiplier is 16x (which I think I read somewhere, but that may be off too) then even at 5MHz you are over the 50MHz threshold (if using 16x PLL). I didn't read too much into the details of the PLL register, and maybe I should (I guess I'm used to just setting a FUSE for HS, XT, or RC and being done with it), but I am hoping for something more straightforward in my hardware design than relying on a software switch.
Here is what I am looking for - if I used a 5MHz crystal what is the highest PLL setting I can use and therefore the highest clockspeed the Propeller will run at? And secondarily, what how many MIPS does the Propeller execute at that setting?

Comments

  • Nick McClickNick McClick Posts: 1,003
    edited 2008-12-15 21:42
    Your confusion is based on 50Mhz. The prop is rated at 80Mhz, which is a 5Mhz xtal * a 16xPLL. It's 20MIPS per cog for a total of 160mips.

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  • PhilldapillPhilldapill Posts: 1,283
    edited 2008-12-16 01:02
    There are some brave souls that use a 6MHz crystal with a 16X PLL, for a total speed of 96MHz, but I'm not one of those and it isn't usually recommended. However, the usual 5MHz crystal with a 16xPLL, like Nick said, is sure to give reliable operation.
  • soshimososhimo Posts: 215
    edited 2008-12-16 02:48
    Okay, that makes much more sense. I'm not sure why the number 50Mhz got stuck in my head - but I'm in firehose mode trying to learn as much about this guy as I can.
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