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6.78 MHz crystal ? — Parallax Forums

6.78 MHz crystal ?

Ken PetersonKen Peterson Posts: 806
edited 2010-05-03 05:08 in Propeller 1
Hello,

I'm toying around with a concept for short-range (< 50m) propeller to propeller communication using the 13.56 MHz ISM band.· This concept was inspired by Phil Pilgrim's AM radio.· Since you can't use a 13.56 MHz·crystal to drive the Prop, I've been looking for a 6.78 MHz crystal.· Anybody know where I can find one (in the US)?· It appears Digikey only has 13.56 MHz crystals.



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·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."

- Bjarne Stroustrup

Comments

  • LeonLeon Posts: 7,620
    edited 2008-09-26 14:58
    It isn't all that expensive to get crystals made. I use these people: www.quartslab.com/

    Leon

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  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-09-26 15:15
    Ken Peterson,

    This place used to be down the street from me when I lived in Oklahoma City... http://www.icmfg.com/

    I have dealt with them before for some custom precision crystals we had them make.· They seemed pretty reasonable with their pricing and turn around.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Ken PetersonKen Peterson Posts: 806
    edited 2008-09-26 15:33
    Thanks for the info. I think before I spend the cost for a custom crystal, I'm more likely to send a 13.56 MHz clock through a divider. I'm only going to build a few units at the most (unless it show a lot of promise as a production product).

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    ·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."

    - Bjarne Stroustrup
  • RinksCustomsRinksCustoms Posts: 531
    edited 2008-09-26 17:26
    I'm ignorant, but doesn't the spec sheet for the prop say Xin can be DC to 80MHz?

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-09-26 17:35
    RinksCustoms said...
    I'm ignorant, but doesn't the spec sheet for the prop say Xin can be DC to 80MHz?
    Yes, but if you use that input with the PLL, it has to range between 4 and 8 MHz. Also, crystal frequencies can range only to 60 MHz.

    That said, you can order programmable oscillators from DigiKey that they will program to any frequency you want. 54.24 MHz (13.56 x 4) might be a good choice for this app.

    -Phil

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    Post Edited (Phil Pilgrim (PhiPi)) : 9/26/2008 5:41:58 PM GMT
  • hippyhippy Posts: 1,981
    edited 2008-09-26 17:58
    Ken Peterson said...
    Since you can't use a 13.56 MHz crystal to drive the Prop

    Reality may be that you can, but if you want to stick within what the datasheet says then no.

    I and others have had some success with 7MHz+ and PLL16x plus 14MHz+ and PLL8x ...

    propeller.wikispaces.com/Oscillator
  • Ken PetersonKen Peterson Posts: 806
    edited 2008-09-26 18:00
    I was actually thinking of trying to overclock it up to 108.48 MHz with the 6.78 MHz crystal (x16) I know that's pushing it.. Perhaps 54.24 MHz might be enough. Any idea what those programmable oscillators cost?

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    ·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."

    - Bjarne Stroustrup
  • Ken PetersonKen Peterson Posts: 806
    edited 2008-09-26 18:04
    @Hippy: Perhaps I'll pick up a couple of the 13.56 MHz crystals and try my luck with the 8x PLL. Might be good enough for proof of concept even if it is pushing the Prop outside of its comfort zone.· Perhaps an oscillator might be more reliable than a crystal?

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    ·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."

    - Bjarne Stroustrup
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-09-26 18:05
    Ken Peterson said...
    Any idea what those programmable oscillators cost?
    Look it up: www.digikey.com

    -Phil

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  • Ken PetersonKen Peterson Posts: 806
    edited 2008-09-26 18:15
    Linear has a part that might work well:· LTC6904.·· Programmable through I2C.· $4.20 at Digikey.· roll.gif·· Only thing that worries me is whether it's precise enough to keep me within the ISM band.

    Thanks for your suggestions.

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    ·"I have always wished that my computer would be as easy to use as my telephone.· My wish has come true.· I no longer know how to use my telephone."

    - Bjarne Stroustrup
  • OwenSOwenS Posts: 173
    edited 2008-09-26 20:26
    Considered one of the SiLabs ones? The Si570 (iirc) would do what you want. It's very accurate
  • LeonLeon Posts: 7,620
    edited 2008-09-26 20:32
    The Si570 is only available in large quantities, as they are made to order for particular frequency ranges. Group buys have been organised for radio amateurs: www.softrockradio.org/si570

    Leon

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  • OwenSOwenS Posts: 173
    edited 2008-09-26 21:14
    SiLabs also have a samples program. Don't know how easy it is to get samples from them though
  • SapiehaSapieha Posts: 2,964
    edited 2008-09-26 21:59
    Hi Ken.


    Read this thread.
    http://forums.parallax.com/forums/default.aspx?f=25&m=215415

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    For every stupid question there is at least one intelligent answer.
    Don't guess - ask instead.
    If you don't ask you won't know.
    If your gonna construct something, make it·as simple as·possible yet as versatile as posible.


    Sapieha
  • BamseBamse Posts: 561
    edited 2008-09-27 04:41
    I've been running a 14.3 MHz Xtal and PLL8 on my Hydra...
    My video driver seem to work fine and it reduced the necessary cogs from 5 to 3 to display my sprites... wink.gif

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    [noparse][[/noparse] ] Let's connect the motor to pin 1, it's a 6V motor so it should be fine.
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    [noparse][[/noparse] ] I dream in SX28 assembler...

    /Bamse
  • hinvhinv Posts: 1,255
    edited 2009-05-31 20:31
    Hi Ken,

    Did you get anywhere with this?
  • Ken PetersonKen Peterson Posts: 806
    edited 2009-06-01 13:05
    Unfortunately, I have been too busy with work and family and this project has been sitting on the back burner.· If I am successful, you can bet that I'll be sharing the details here.

    Ken
  • AleAle Posts: 2,363
    edited 2010-04-29 08:32
    Hei all,

    did someone use the LTC6904 ?. I'm trying to interface to it but I do not get any ACK bit and most of the time it just ignores to programming words. I'm using a 100kHz I2C clock. Any ideas ?

    thanks

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  • UnknownUnknown
    edited 2010-04-29 16:22
    This may be late but someone recommended http://www.jancrystals.com/
  • jmspaggijmspaggi Posts: 629
    edited 2010-05-01 01:21
    Can we put the crystal we want?

    Like if I get a 1.2345Mhz or a 5.4321Mhz, can I use it? Will the serial communication style works correctly?

    When you try to push the prop to 100Mhz (6.25Mhzx16), what's the impact? I will become hot?

    Can we put a heat disipator on it and push it at 120Mhz?

    Just asking. Since I can't change the crystal on the PPDB [noparse];)[/noparse]

    JM
  • AleAle Posts: 2,363
    edited 2010-05-01 07:30
    jmspaggi said...

    Can we put the crystal we want?

    Like if I get a 1.2345Mhz or a 5.4321Mhz, can I use it? Will the serial communication style works correctly?

    Yes, you can use whatever crystal you want as long as it meets the frequency and capacitance and so on specified in the datasheet. Crystals up to 15 MHz have been reported to work.
    jmspaggi said...

    When you try to push the prop to 100Mhz (6.25Mhzx16), what's the impact? I will become hot?

    The impact ? ... current needs go up some 25 %, instruction cycle decreases 25 % to 40 ns (processing power goes up to 25 MIPS per core). Decoupling is always important and at higher frequencies even more so, use a 10 uF tantalum, a 1uF and a 10 nF for each VDD pin. That should suffice.
    I cannot comment on the possibility of you becoming hot, but the propeller dissipates little power and thus it was reported not to generate much heat.
    jmspaggi said...

    Can we put a heat disipator on it and push it at 120Mhz?

    Just asking. Since I can't change the crystal on the PPDB [noparse];)[/noparse]

    JM

    Even at 120 MHz it does not need a heat sink. 120 MHz is really pushing it.

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  • AleAle Posts: 2,363
    edited 2010-05-01 07:33
    I'm still at a loss with the LTC6904 and why it does not work. I decided to use a counter to generate the square wave I need between 1 and 10 MHz.

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  • LeonLeon Posts: 7,620
    edited 2010-05-01 11:40
    I checked the data sheet; it isn't very accurate or stable. You probably need a crystal.

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  • jmspaggijmspaggi Posts: 629
    edited 2010-05-01 13:31
    Ale said...

    I cannot comment on the possibility of you becoming hot.


    Hahaha! I did not figured I wrote that! [noparse];)[/noparse]

    Thanks for your reply. I will try to play a little with the Crystal on my next Prop to see if I can have it working fine with another Prop using a different Crystal.

    What's the max frequency ever tried on propeller?

    Thanks,

    JM

    Message Edité (jmspaggi) : 5/1/2010 1:47:49 PM GMT
  • Cluso99Cluso99 Posts: 18,069
    edited 2010-05-02 02:44
    There is a thread somewhere for overclocking. I think Coley owns the record but I cannot record what speed he got. Sapieha has been running a 15MHz xtal at 120MHz on a TriBlade for over a year now. I run 6.5MHz (104MHz) and ship 13.5MHz (108MHz) with my RamBlade. However you must ensure good design practices and decoupling to get these speeds without problems. I have briefly tried 14.318MHz on the RamBlade and it worked, but it was not an exhaustive test.

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  • jmspaggijmspaggi Posts: 629
    edited 2010-05-02 11:49
    Thanks for your feedback.

    I will do some testing to see what's the result.

    When you are talking about "decoupling", are you talking about the same thing Ale said?
    Ale said...

    Decoupling is always important and at higher frequencies even more so, use a 10 uF tantalum, a 1uF and a 10 nF for each VDD pin. That should suffice.

    Or is there anything else I need to do for this decoupling? Also, there is 3 capacitors here. Should I put all of them in series between the Prop and Vdd? Or in parallele? Or just one of them?

    Also, if I back Crystals, except the frequency, is there any other caracteristic I should take care of?

    Thanks,

    JM
  • Cluso99Cluso99 Posts: 18,069
    edited 2010-05-02 12:41
    Decoupling is a term used for capacitors across the power supply pins Vdd to Vss(Gnd). You need 100nF across each Vdd/Vss pair (so 4 on a QFP) plus a 10nF and 10uF is also recommended for overclocking. You will also require a good pcb layout. There is way too much to explain here.

    What do you mean "back Crystals"??

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    Links to other interesting threads:

    · Home of the MultiBladeProps: TriBlade,·RamBlade,·SixBlade, website
    · Single Board Computer:·3 Propeller ICs·and a·TriBladeProp board (ZiCog Z80 Emulator)
    · Prop Tools under Development or Completed (Index)
    · Emulators: CPUs Z80 etc; Micros Altair etc;· Terminals·VT100 etc; (Index) ZiCog (Z80) , MoCog (6809)·
    · Prop OS: SphinxOS·, PropDos , PropCmd··· Search the Propeller forums·(uses advanced Google search)
    My cruising website is: ·www.bluemagic.biz·· MultiBlade Props: www.cluso.bluemagic.biz
  • jmspaggijmspaggi Posts: 629
    edited 2010-05-02 12:55
    Cluso99 said...
    What do you mean "back Crystals"??
    I should have wrote "if I buy crystals" [noparse];)[/noparse]
    Cluso99 said...
    There is way too much to explain here.
    Where can I found more information for that?
    Cluso99 said...
    plus a 10nF and 10uF
    Where should I place those 2 ones?

    Thanks,

    JM
  • AleAle Posts: 2,363
    edited 2010-05-03 05:08
    About decoupling there is extensive information. I have seen application notes by some semiconductor manufacturers like Analog and National. OTOH, Beau Schwabe hat posted information about the topic some time ago. Try searching the site via google. Did you already check the wiki ?.
    Note: I did not have the time to make the changes you sent me. I'll add them as soon as I can.

    Ale

    Edit: Decoupling capacitors should be placed as close to the power pins as possible ( < 8 mm away and with thick traces, like 0.4 mm or thicker). Larger caps like 10 uF do not need to be that close (a few cm apart is okay).

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