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LMX2581 sharing MuxOut pin

Mirage
Prodigy 75 points
Other Parts Discussed in Thread: LMX2581

Hello

On my custom board, there are 3 LMX2581 PLLs sharing the DATA, CLK and MUXOUT lines of microwire bus.

I am driving them with SPI mode 0 protocol. 

Although DATA and CLK lines are working properly, I cannot manage to make readback from MUXOUT pins. 

ALL of the PLLs' MUXOUT pins are set to Open-Drain. They have no serial resistance through SPI-MISO pin of uC and only one 1K pull-up resistor to 3.3VDC.

The problem is, when all of the PLLs are in idle state, the MUXOUT line stays at LOW whereas it should be HIGH as all the pins must be OPEN. 

Whenever a readback is done, waveform changes between 0 and 1VDC only, instead of 3.3VDC. 

There is no short to ground of the board at any MUXOUT pin .

Is there a problem with sharing MUXOUT pins? Should I set the pins as High Drive Push-Pull?.

What can be the problem?

EDIT : Please find the simple block diagram of the circuitry. The file is in PDF format.

LMX2581_Muxout_Sharing.pdf

Thanks in advance.

Mirac PARLATAN

  • 0
    TI__Expert 4085 points

    Hi Mirac,

    There shouldn't be a problem if you share DATA and CLK lines.

    However, MUXOUT is an output and shouldn't be shared.

    Could you please provide a schematic?

    Thanks,
    Julian

  • 0 in reply to Julian Di Matteo
    Prodigy 75 points
    Hello Julian

    Thanks for the quick reply.
    Please find the simple block diagram of the circuitry at the attachment in edited original post.

    There is no problem with CLK and DATA lines. If MUXOUT pin cannot be shared, then how microwire is considered as compatible with SPI?


    Thank you
    Mirac Parlatan
  • 0
    TI__Mastermind 28001 points

    Parlatan,

    Theoretically, this sharing should work, but it requires individual programming.  It would seem what you would want to do is set MUXOUT_PINMODE of the unused two mux pins to "Tri-State" and then the one that you do care about set to "Push-Pull".   When the devices initially power up, there is the possibility of them shorting together before you program, so maybe put accomodations for a small series resistance if this is an issue.

    If you set all three to "Open Drain", then these are either high impedance when the output would normally be high, or ground if it would normally be low.

    Regards,
    Dean