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What's with the CDCE72010 MISO output during write cycles?

David Burton
Expert 1330 points
Other Parts Discussed in Thread: CDCE72010

Hi,

I've notice the CDCE72010 MISO output is active and outputting data during register write cycles.

The data makes no sense to me.  it's predictable, but I would expect it to match the value of the previous write, and sometimes it does, or is similar.

Is this data meaningful at all?  Is there a logic diagram of the SPI port circuitry somewhere?

I do understand to actually directly read a specific register contents I need to do a read command with the address followed by the read cycle.  I'd just like to know what all the junk I see on MISO is during a write.

 

Thanks.

  • 0
    TI__Expert 4150 points

    Hi David,

    I am not sure about the answer myself, and the device designer is out until sometime in earlier January. However, you should ignore what the device writes back as it is not specified in the data sheet. I suspect it's some sort of acknowledgment or maybe the register content from the past write action, or maybe even from the past read action. There is definitely nothing written back that you would be required to monitor.

    Sorry for not having a more comprehensive answer but hope this still addresses your concerns. Best regards.

    Falk Alicke

  • 0 in reply to Fritz
    Expert 1330 points

    OK, I'll ignore the MISO data except after a read CMD.

     

    When I do a write of E8 C0 41 D7 to reg 7 and then a read command and read it back I get 68 C0 41 D7.  Reversing these to decipher the register contents shows I wrote to an address of reg 7, but when I read that location back it gave an address of reg 6.  Why is that?

     

    Thanks.

  • 0 in reply to David Burton
    TI__Expert 4150 points

    David,

    on page 20 of the CDCE72010 data sheet in the footnotes of table 4 it mentions to ignore the address field on read instructions. This is because the device has one errata item. Address bit A0 is not set properly when the device reports back the register value.

     

    Thank you. BR. Falk Alicke

    PS: I think in your response above you meant to say write CMD.

  • 0 in reply to Fritz
    Expert 1330 points

    Thanks,  I think I read that once before and forgot about it.

     

    I did mean read CMD.  I have to write a read CMD to address 1110 and then I can read the register on the next transaction.

  • 0 in reply to David Burton
    TI__Expert 4150 points

    ok, now now I see why you said read CMD.