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EMI issue...

Ty Wood
Prodigy 30 points
Other Parts Discussed in Thread: TMS320VC5509A

 I have a customer that is working on an EMI problem.

Their product uses a TMS320VC5509A , Fixed Point DSP chip.
 
One of the problem frequencies is 192MHz , originating from the DSP PLL .
 
Can you refer me to a TI App Note that could be used with this part for Clock Dithering ?
Many Thanks !
 
Ty.
  • 0
    TI__Guru*** 125430 points

    Can you tell me more about the clocks in use in their system?  For example:

    • Are they using a crystal + internal oscillator or are they inputting a 3.3V clock?
    • What speed crystal/clock?
    • What are their PLL multipliers and dividers (all clock domains)?
    • At what speed are they clocking the EMIF?
    • Have they disabled the CLKOUT pin by setting EBSR.BIT15=1?

     Has he read this article?

  • 0 in reply to Brad Griffis
    TI__Prodigy 30 points

    Thanks Brad,

    Here is more information from the customer...

    The external 12 MHz crystal is attached at "X2/CLKiN" ; pin F2   (Using internal oscillator ).

    192MHz PLL has an overall multiply ratio of 16, referred to crystal .
     
    What the customer feels that he needs to do as part of an overall solution...is dither 192 MHz by 1 to 2 % .
     
    Does the TI part have any built in capability , or do we need to design in an external clock source and dither it a factor of 16 less ?
     
    An external Dithered source is too much of a change for the Customer's threshold of pain . An internal solution is the only option that they will consider.
     
    The customer will check on your suggestion of "Have they disabled the CLKOUT pin by setting EBSR.BIT15=1? ".
    The customer is seeing EMI effects from the outputs of downstream devices as well . Emissions are not limited to the TI part , even though it appears to be the source.
    Appreciate your HELP !
    Regards,
    Ty.

     

     

  • 0 in reply to Ty Wood
    Intellectual 390 points

    Hello Ty,

    I don't think that dithering the internally-generated clock is possible. All you have access to is the external clock/crystal input. You can dither the external clock input frequency, but I think you would need to provide a nominal 192 MHz clock. Such a high frequency external clock is likely to make your situation worse, rather than better.  If you dither a 12 MHz external clock, the internal PLL will attempt to remove the dither unless its modulation rate is slow enough for the PLL to track. That might be too slow to help with the RF compliance problem.

    Honestly, the real solution is probably a more careful circuit board layout. I built a board based around a 5509A running at 144 MHz  (from a 12 MHz crystal) that came in 14 dB under the RF emissions limit. But it was a six-layer board with internal stripline routing of high-speed traces. I kept all outer-layer digital traces as short as possible.

    Best of luck,

    David

  • 0 in reply to David Rick
    Intellectual 300 points

    I have a kind of same problem. Please see my post below

     

    We have a custom design c5509a board. When we set the clkmd to 2512, everything is ok but when we set the value to 0x2892 ( and we have to run the code at this speed) the code sometimes run but sometimes hangs on. I saw some suggestions on internet to disable the clkout pin on http://e2e.ti.com/forums/p/12107/47323.aspx and also in http://www.dsprelated.com/groups/c55x/show/979.php.

    Why do we have to disable it. And in general do you have any ideas about this problem. First I thought that our core voltage is not enough to run on this speed and increased it a little to 1.72Volt but the problem is still there.

    Is there a problem on c5509a about the pll or something.

    Target : c5509a
    Crystal : 12 Mhz
    Emulator : xds510 usb plus
    CC Studio 3.1
    DSP/BIOS was USED in the project
    CLKMD value : 0x2892
    Used peripherals in design : EMIF for sdram, codec processing, SPI, I2C nearly all of the peripherals

    waiting for your urgent reply

  • 0 in reply to saltan
    TI__Guru*** 125430 points

    Are you having an EMI issue?  If not, you should start a new thread.  I recommended turning off CLKOUT because the OP was having EMI issues.  Having a pin toggling needlessly at high speeds is not going to help EMI.

  • 0 in reply to Brad Griffis
    Intellectual 300 points

    I'm not sure the source of my problem. It may be because of the EMI. I posted a new thread also http://e2e.ti.com/forums/p/12457/48495.aspx#48495