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LMK04808 single pll, 0-delay, external VCO question

Andrea Gamberoni
Prodigy 40 points
Other Parts Discussed in Thread: LMK04808, CLOCKDESIGNTOOL, LMK04828, LMK04828BEVM, CODELOADER

Hi,

I would like to use LMK04808 Pll but I can't understand if mode Single Pll, External VCO, 0-delay is available. From datasheet (pag.6, 0-Delay Single Pll paragraph) I've read this phrase: "It is also possible to use an external VCO in place of PLL2's internal VCO", but in Device Mode Selection (pag.29) I can use Pll2 (Single Pll), 0-Delay only with internal VCO. In case this modality is available, could you suggest me how can I set it?

Thanks in advance.

  • 0
    TI__Mastermind 32575 points

    I suggest using MODE = 5 (Dual PLL, Ext. VCO, 0-delay w/ Internal feedback) and leaving PLL1 unused.  PLL1 charge pump can be tri-stated, PLL1 loop filter components can be omitted, and CLKin0 port can be left open.  The ref clock input can be applied to OSCin port.  The Ext. VCO output will be interfaced to Fin port and its Voltage control input will be driven by CPout2 with external loop filter.  You can use CLOCKDESIGNTOOL software to design the loop filter based on the PLL PFD frequency and VCO gain (Kv) parameters.

    Regards,
    Alan

  • 0 in reply to Alan O
    Prodigy 40 points

    Hi Alan,

    thank you for your advice but I think that using Mode=5 and leaving PLL1 unused, I can't mantain 0-delay feature. I've understood from scheme in pag.5 that the feedback from CLKoutX is unused so it's impossible to ensure 0-delay between OSCin and output clocks.

    Your solution might work if it were possible select the FBMux  input  for Mode MUX 3 (see block diagram in pag.7) but with Mode=5 the other input (from N2 Prescaler block) is set.

    Is it possible to contol Mode Mux 3 indipendently form the Mode?

    Regards,

    Andrea.

  • 0 in reply to Andrea Gamberoni
    TI__Mastermind 32575 points

    All Mode Muxes are set by the MODE register bits, so Mode Mux 3 would be selecting the output from N2 prescaler instead of from FBMux when MODE=5 and 0-delay would not be attained through PLL2.  Thanks for the correction there.

    Alternatively, you might consider the newer LMK04828 which has these Muxes separately programmable for even more flexibility.  I also has higher performance than LMK04808.

    Regards,
    Alan

  • 0 in reply to Alan O
    Prodigy 40 points

    Thank you Alan.


    I've read LMK0428's datasheet and I think it's a better solution than LMK04808.

    Regards,

    Andrea

  • 0 in reply to Andrea Gamberoni
    Prodigy 40 points

    Hi,

    I've started to test LMK04828BEVM (REV C, date 02-18-2013) but I found a problem during the initialization.

    After connecting the power supply and the USB programming cable, I launch CodeLoader 4; when I select the Device (Clock conditioners: LMK04828B) an error occurs: Run-time error '6' overflow.

    I've verified that this happens with both Windows 7 that XP.

    Do you have any suggestions for me?

    Thanks in advance.

    Andrea