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LMK04828: What combination of register values will provide output frequency f1 = 2GHz +/- 150MHz, and for another output - exactly f2 = f1/2?

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Replies: 4

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Part Number: LMK04828

Hello team,

Hope you are doing well and staying safe. When you get a chance please share some feedback on below:

No external VCO or crystal. CPOUT (pins 32, 46) disconnected, CLKin1 (pins34, 35) disconnected, OSCOUT (pins 40, 41) disconnected.

CLKin0 (pins 37, 38) connected to OSC XO 125.000MHZ LVDS SMD

Optionally, to OSCin (pins43, 44) can be connected anther one OSC XO 125.000MHZ LVDS SMD


Look at: PLL_CONFIG_0817(1)0.csv.

Column B: reading from default after power up.

Column C: Desirable changes. For example:

Line 323:  PLL_SPI_LOCK_LSB = 83

Line 219:  PLL_SYNC_FIX_127 = 127

Line 229:  PLL_CLKin_SEL_MODE = 0. We do not need to use external control, but pins CLKin_SEL0, CLKin_SEL1 connect to pins of Cyclone 4. There is “0”

Line224:  PLL_CLKin1_EN = 0       (PLL_CLKin0_EN=1)

Line 26:  PLL_DCLKout4_DIV = 2. The output we use for control of output Frequency. After “set PLL“, we get 593MHz. (PLL_DCLKout4_DIV = 1; 793MHz)

In this case, If line 186 PLL_Feedback_Mux = 1, PLL_DCLKout4_DIV = 2; 750 MHz. If PLL_DCLKout4_DIV = 1, it will be 1GHz

Now, after “Get PLL” we get column “D”. The question is:

  1. Line 235: PLL_CLKin_SEL0_TYPE became =4. It is output. How it is possible? It had been programmed to be input with purpose, because it is connected to another output. ?
  2. Line 219: It is Fixed register. Why it is reassigned to another number?
  3. Why it is insistently change line 224 PLL_CLKin0_EN = 1 to 0, and PLL_CLKin1_EN = 0 to 1?
  4. Why dividers lines 24 – 30 change value?


Really, we need for one output have frequency f1 = 2GHz +/- 150MHz, and for another output - exactly f2 = f1/2.  What combination of register values will provide such a frequency? 

Randhir S Kalsi

Technical Sales Engineer

  • Hi Randhir,

    Am I correct to assume you want a fixed output frequency, not tunable anywhere in the +/- 150 MHz?

    If you want 2 GHz +/- 150 MHz, the LMK04828 VCOs would not support that output frequency with its dividers.  However the LMK04821 VCO0 can tune 1930 to 2075 MHz.  A second option would be LMK04826 VCO0 which can tune 1840 to 1970 MHz.

    With the LMK04821 VCO0 for example, you could configure one output to operate at 2 GHz and another at /2 for 1 GHz.

    You can operate the LMK0482x in dual loop, single loop, or distribution mode.   Based on your description above, I think what you want to do is single loop mode using your 125 MHz XO on OSCin.  You will need to connect a loop filter to CPout2 pin (which is described as disconnected above).  This could be 2 capacitors and a resistor as illustrated on EVM.  The frequency accuracy of your output would equal the frequency accuracy of the XO.

    As for final configuration, it appears that your attachment did not come through, can you try again?  Have you used the TICS Pro software?  You can use this software to program the device and generate a register map.  Let me know if you need some help with using TICS Pro for LMK04821.



    More information Clock and Timing System products: http://www.ti.com/clock-and-timing/overview.html

  • In reply to Timothy T:

    Here is the spreadsheet PLL_CONFIG_0817(1)0.csv

  • In reply to Anat:

    Hello Anat/Randhir,

    It seems we have two items here.

    #1) After programming the LMK04828, you do not read back expected values.

    #2) You need a frequency (2 GHz +/- 150 MHz) which cannot be produced with LMK04828.  LMK04821 can achieve 2 GHz output.


    As for #1.
       - Is this occurring on the EVM or custom board?  If custom board, has more than one board been tested?
       - Is there any significant capacitance on the SPI lines?  What's the SPI data rate?  Are you able to share a plot of the SCK/SDIO/CS*?

    As for #2.
       - Can you accept the LMK04821?  If so I could provide an example programming TCS file.



    More information Clock and Timing System products: http://www.ti.com/clock-and-timing/overview.html

  • In reply to Anat:

    Hello Anat,

    We haven't heard from you in a while, so I'm going to mark this thread as resolved. Let us know if you still need assistance.


    Derek Payne

    Texas Instruments