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LMX2820: SPI Configuration and readback issue

Karthikeyan Selvaraj
Prodigy 20 points

Part Number: LMX2820

Tool/software:

Hi,

    I am using LMX2820 for my application. I need to configure the chip through SPI communication. As per datasheet i have taken care of pins like CE (pulled up), MUTE(Pulled down), PSYNC(pulled down). From external microcontroller i am trying to configure. I probed it and waveforms are fine for CS,SCK and Data also. To ensure communication, i just tried to read the registers. I am expecting it will give me the default value as per datasheet. But i am not received any data. I tried in both SPI mode 0 and mode 1. Any other step missed? Kindly suggest me to solve this achieve the above.

  • 0
    TI__Guru** 120060 points

    Hi There,

    To debug, after Vcc power up the device, program POWERDOWN = 1, can you see significant current drop?

  • 0 in reply to Noel Fung
    Prodigy 20 points

    Hi Noel, 

           After Power On, i tried to program register 0 with POWERDOWN = 1 and there is no change on current. I am believing that after power on, if i read any register, the default value should be out through MUXOUT pin. I tried with lesser SPI SCK  frequencies like 1 MHz and 500 KHz. Still there is no reply from LMX2820. 

  • 0 in reply to Karthikeyan Selvaraj
    TI__Guru** 120060 points

    Hi There,

    No current change with POWERDOWN = 1 means you are not able to program the device at all. 

    Make sure your SPI signal meets below requirement.

    • The LMX2820 is programmed using 24-bit shift registers.
    • The shift register consists of a R/W bit (MSB), followed by a 7-bit address field and a 16-bit data field. For the R/W bit, 0 is for write, and 1 is for read.
    • Serial data is shifted MSB first into the shift register.

    To write registers:

    –The R/W bit must be set to 0.

    –The data on SDI pin is clocked into the shift register upon the rising edge of the clocks on SCK pin. On the rising edge of the 24th clock cycle, the data is transferred from the data field into the selected register bank.

    –The CS# pin may be held high after programming, which causes the LMX2820 to ignore clock pulses.

    –If the SCK and SDI lines are toggled while the VCO is in lock, as is sometimes the case when these lines are shared between devices, the phase noise may be degraded during the time of this programming.

    To read back registers:

    –The R/W bit must be set to 1.

    –The data field contents on the SDI line are ignored.

    –The read back data on MUXOUT pin is clocked out starting from the falling edge of the 8th clock cycle.