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LMK04828BEVM: Clock frequency is different from software calculations?

Souren Shamsinejad
Prodigy 20 points
Part Number: LMK04828BEVM
Other Parts Discussed in Thread: LMK04828, LMK04821, LMK04806

Hello all

I am using LMK04828B Evaluation Board to generate some clocks using its internal oscillator which is Crystek-CVHD 950 100M in our board.

I am using TI Code Loader to program and configure the board. I need to get 60MHZ, 16MHz and 800KHz signal from this board and I was thinking of using SDCLKout for 800KHz signal and DCLKout0 and DCLKout1 for 60MHz and 16MHz signals. I could generate some signals following instruction posted by TI, but the frequency does not match the frequency predicted by software and I have seen changing some values in CLKout tab ( like PLL2 N and prescale N, or R) won’t affect the result at all. I really appreciate it if someone can help me out and let me know how I can setup the board correctly. Why changing some parameters will not impact the output at all.

Thanks

  • 0
    TI__Mastermind 28550 points
    Hello Souren,

    Note using the LMK04828 you won't be able to get 16 MHz and 800 kHz at the same time. Assuming you use 2370 MHz of VCO0, the max divider of device clock is 32 which gets you to 74.0625 MHz.

    Normally I'd suggest the LMK04821 which has a VCO divider (2 to 8) on VCO1 which allows output of low frequencies. However to get 60 MHz and 16 MHz at the same time you need 240 MHz * n. There is no n value which lies in the VCO1 frequency range of LMK04821 which is 2920 to 3080 MHz.

    Would the LMK04806 meet your requirements?

    73,
    Timothy
  • 0 in reply to Timothy T
    Prodigy 20 points

    Hi Timothy,

    I was wondering if I can set the device VCO frequency at 480MHZ to be divided by 30 and 8 to generate 16MHz and 60MHz signals respectively.

    For 800KHz signal I was hoping to use SDCLK and SysRef and use N=600 to get 800KHz clock too. 

    Thanks,

    Souren.

  • 0 in reply to Souren Shamsinejad
    TI__Mastermind 28550 points
    Hello Souren,

    You could do this with the LMK0482x, but the 480 MHz would have to be an external VCO as 480 MHz is not covered by our internal VCOs. Let me know if you need any assistance in programming this mode.

    73,
    Timothy