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LMX2571EVM: Register value creation for FM modulation

keiju nakanishi
Prodigy 10 points
Part Number: LMX2571EVM
Other Parts Discussed in Thread: USB2ANY

Hi Team

Let me ask you two questions.

1:

I am using burst mode to continuously change the value of register R33 using TICS pro. is it possible to change the speed of register data transmission in burst mode?

2:

Using equation (4) from the LMX2571datasheet, I obtained the register value that would result in a frequency deviation of 5 kHz and input it to R33. However, the actual frequency deviation from the carrier wave, which was checked using a spectrum analyzer, was approximately 10.5 kHz. We also created the register value representing the negative frequency deviation using equation (6), but the output frequency did not change from the carrier frequency when this value was input to register R33. Why is this happening? The conditions are described below.

fDEV = 5000 [Hz]

DEN = 2^24

fPD = 80 [MHz]

CHDIV1 = 5

CHDIV2 = 2

Prescaler = 2

Input register value (+5kHz deviation) = 147Bh
Input register value (-5kHz deviation) = EB85h

Thank you for your answers.

Best regards,

Nakanishi

  • 0
    TI__Guru* 86080 points

    Hi Nakanishi-san,

    USB2ANY SPI clock is running at 125kHz, and there is no control to the time interval between register write. You need to develop something to program the EVM if timing is critical.

    Your calculation for the deviation is correct. SPI FAST MODE should work if you have:

    1. made R34[1:0] = 0x11 (FSK_MODE_SEL1 and FSK_MODE_SEL0)

    2. made R8[10] =0x1 (FSK_EN_F1)

    Maybe you can try make DEN = 16777215, if this is a GUI problem.

  • 0 in reply to Noel Fung
    Prodigy 10 points

    Hi Noel

    Thank you for answering my question.

    The problem has been resolved. The cause was that the DEN value in TICS pro was set to [DEM = 800000], even though the DEN value in determining the value to be input to register R33 was calculated using [DEN = 2^24].

    The problem has been resolved, but a new problem has arisen, so I would like to ask one more question.

    I have done a readback of register values using TICS pro, but all register values are 0x0000h. In this case, we changed the value of register R39 to 0×11F3h so that the output of the MUXout pin is SDO. Why did this occur?

    Best regards.

    Nakanishi

  • 0 in reply to keiju nakanishi
    TI__Guru* 86080 points

    Hi Nakanishi-san,

    Right, we only need to make R39 = 0x11F3 in order to make MUXout = read back data output. 

    In the EVM, check if R52 is missing. Populate a 0 ohm resistor to connect MUXout and USB2ANY, if needed.

  • 0 in reply to Noel Fung
    Prodigy 10 points

    Hi Noel

    Thank you for answering my question.Wiring a 0Ω resistor to R52, data can now be read. Thank you very much.