LMX2820: PLL Doesn't Respond to Serial Commands

Jaco du Preez

Prodigy 60 points

Part Number: LMX2820
Other Parts Discussed in Thread: USB2ANY, MSP-EXP430FR6989, LMX2594

Hi,

I'm using the LMX2820EVM, and testing with two different setups:

Setup 1: TICS Pro / USB2ANY / LMX2820EVM

Setup 2: MSP-EXP430FR6989 w/ custom SW / LMX2820EVM

I cannot get the PLL to respond to any commands when using Setup 2.

For example, on power-on, if I trigger a calibration via TICS I can measure the SPI bus:

And I see a perfect spectrum at 5.65GHz.

When I try to do the same with Setup 2, I don't get any response from the PLL, and there is no spectrum. Here is the bus capture:

I'm failing to see the difference between these to sequences.

Moreover, when I perform the recommended startup sequence from the LMX2594 datasheet:

Nothing happens on the PLL either, and there is no output. Here's a snippet from the first few bytes of the default sequence:

Perhaps there are some other things I can check and/or am missing?

Thanks.

over 1 year ago

0 Jaco du Preez over 1 year ago

Prodigy 60 points

I should also add that my SPI configuration in the MSP SW is:

Master clock = 8MHz
SPI_CLK_DIV_64 (125kHz)
SPI mode 0
MSB first
Active low / idle high CS

0 vicente flores prado over 1 year ago in reply to Jaco du Preez

TI__Mastermind 29542 points

Hi Jaco,
I believe this is the same case with Ruan?
https://e2e.ti.com/support/clock-timing-group/clock-and-timing/f/clock-timing-forum/1348007/lmx2820-fpga-spi-protocol-from-fpga-to-lmx2820

Regards,

Vicente

0 Jaco du Preez over 1 year ago in reply to vicente flores prado

Prodigy 60 points

Hello Vincente, it is a similar case, he is debugging our custom board and I'm working on the EVM, altough while he's programming registers from the FPGA, I'm doing so from the MSP in a test setup. We're not quite sure if the issues are related, however.

0 vicente flores prado over 1 year ago in reply to Jaco du Preez

TI__Mastermind 29542 points

Hi Jaco,
Understood. So you're using the EVM and when you use USB2ANY + TICSpro - no issue which is case 1 & case 2 is when you use your own controller and that is when you encounter issues.

I still have the same question, have you checked to ensure your controller is meeting all the SPI timing requirements? Is your controller sharing data and clock lines with another device? Are you following the recommended start up sequence?

Regards,

Vicente

+1 Jaco du Preez over 1 year ago in reply to vicente flores prado

Prodigy 60 points

Hello Vincente, thanks for getting back to me.

I have found the issue, and it's related to the chip enable input. I'm following the recommended startup procedure to the letter.

On our EVM, the CE jumper is set to be pulled high by the 1k resistor, which works perfectly fine with TICS. However, when the MSP is starting up and setting up pins, there seems to be something about that that the PLL is not happy with, especially since some of the SPI pins are toggled after a reset.

If I drive the CE on the EVM from the MSP and only assert it after the MSP's setup is completed, then it works perfectly fine, as expected.

0 vicente flores prado over 1 year ago in reply to Jaco du Preez

TI__Mastermind 29542 points

Glad to know you have found the issue! I recommend only asserting after MSP has finished setting up.
Feel free to reach out if you have any other questions.

Regards,

Vicente

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LMX2820: PLL Doesn't Respond to Serial Commands