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AFE5818EVM：Problem of using DIG_SPI_EN pin

Ruichen Yuan
Prodigy 10 points
Part Number: AFE5818EVM
Other Parts Discussed in Thread: AFE5818, AFE58JD18EVM, AFE58JD18

I have a AFE5818EVM, and I encountered a problem while testing SPI communication .

According to the schematic diagram and data sheet, DIG_ SPI_EN  pin is not required on AFE5818 when testing SPI communication ,and it is only requied on AFE58JD18. However, after testing, it is found that if the DIG_SPI pin is ignored, SPI communication of AFE5818 will not be able to proceed, and DIG_ SPI must be  pulled high to communicate successfully.（Of course, I'm sure what I tested is AFE5818EVM, not AFE58JD18EVM）

This indicates that this pin is necessary for SPI communication of AFE5818, which is inconsistent with data sheet and schematic diagram.In AFE58JD18,this pin is K16,but in AFE5818,there is no DIG_SPI_EN pin, and AFE5818's K16 pin is 'NC'.

Now I plan to use AFE5818 for related design, but I don't know how to deal with this ‘’nonexistent‘’ pin——DIG_SPI_EN, which is necessary for SPI communication. Or , maybe I should call it K16?

  • 0
    TI__Mastermind 49986 points

    Hello Ruichen,

    The DIG_SPI_EN signal is not used by the AFE5818 device and hence the signal is not necessary for SPI communication in AFE5818.

    The same board is used for multiple devices and AFE5818 is one such variant. DIG_SPI_EN is used by device AFE58JD18.

    In your design you can leave pin K16 of AFE5818 unconnected.

    Can you please elaborate on your statement below? What do you mean by DIG_SPI_EN Is ignored?

    Ruichen Yuan said:
    if the DIG_SPI pin is ignored, SPI communication of AFE5818 will not be able to proceed

  • 0 in reply to Praveen Aroul
    Prodigy 10 points
    Ruichen Yuan said:
    if the DIG_SPI pin is ignored, SPI communication of AFE5818 will not be able to proceed

    It means, if I leave the DIG_SPI_EN UNCONNECTED or pull to LOW， the SPI communication will be FAILED.

    And once I pull it to HIGH，the SPI communication  SUCCEEDS.

    So what you said " the signal is not necessary for SPI communication in AFE5818" is NOT CORRECT.At least my test results are like this.

    SCLK,SEN,SDATA,and SDOUT are connect to FPGA pins, using the external interface J4. If leave the DIG_SPI_EN UNCONNECTED or pull to LOW，FAILED.   If pull  to HIGH，SUCCEED(The success is judged by reading back the register).The DIG_SPI_EN is the only variable .

    I have tested with afe5818evm many times, and the DIG_SPI_EN must be raised to carry out SPI communication，at least that's the case with my afe5818evm. So, can you do some tests with AFE5818EVM and tell me the result? I'm not sure if my afe5818evm is special，or the content about SPI communication of the data sheet  of AFE5818 is wrong.

    I think I have made it clear and detailed.

  • 0 in reply to Ruichen Yuan
    TI__Intellectual 1590 points

    Hi Ruichen,

    Internally in the AFE5818 device, K16 is pulled up with 16kohm resistor to DVDD_1P8 supply. That's why we recommend in the datasheet to leave this pin NC as already it's being pulled by device. If K16 is driven to GND, the SPI doesn't work. This pin can be kept NC or can be pulled HIGH externally for the SPI to work.

    I could think of one reason why in your experiments you see SPI is not working even with DIG_SPI_3.3 connection made floating by removing R16 resistor.

    As you can see in above picture, DIG_SPI_3.3 is also being driven by Port OUTB of digital isolator. Though DIG_SPI_3.3 is made float near J4 connector, there can be still chance that it is being driven low by the digital isolator. So, in your experiments have you removed U6 component also ?

    Regards,

    Kalyan

  • 0 in reply to Bellala Kalyan
    Prodigy 10 points

    Thank you.Maybe I got it.

    In fact I have removed the isolator because its' outputs are internally pulled high even if the inputs are made float, that will cause a conflict when I try to use J4 for SPI communication.

    And what you said that ''K16 is internally pulled up with 16kohm resistor to DVDD_1P8 supply'' is the key point. It let me aware that the element pulled the DIG_SPI_EN low is not the isolater ,but the level converter.After test, I found that the level converter 's outputs are internally pulled low when inputs are made float.

    That's the answer,thank you!