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AFE5401-Q1: Voltage Level Problem on SDOUT when using one SPI interface for 2 devices

Michael K
Intellectual 370 points
Part Number: AFE5401-Q1
Other Parts Discussed in Thread: SN74LVC2G125

Hello,

I use one SPI interface to communicate with 2 AFE5401. Both have an own Chip Select, but Clock, MISO, MOSI are shared.

I can see data written to the chips at correct Voltage Level.

When reading from one device while the other device is not selected (SEN high), it looks like the 2nd device still drives the SDOUT signal.

I have two resistors in the SDOUT lines (same value 330Ohm) and connect both signals on the processor side (TI TDA2P in this case). The Voltage level is not 3.3V but 1.6V which looks to me that the other SDOUT is at Low level and the voltage is divided with the 2 resistors. I removed one resistor to disconnect the 2nd SDOUT and the level is o.k.

I can not find in the data sheet that SDOUT will go to High-Impedance when CS(=SEN) is high.

I can also see that after reading one word the Level of SDOUT stays at high or low level whatever the last bit was.

Best regards,

Michael.

  • 0
    TI__Mastermind 49296 points
    Hi Michael,

    SDOUT lines from multiple AFE5401s cannot be shorted since the AFE5401 holds the LSB bit sent on the SDOUT pin.
    In other words, SDOUT does not go to Hi-Z when SEN is high.
    You can share the SDA, SCLK lines but not SDOUT and SEN signals.
  • 0 in reply to Praveen Aroul
    Intellectual 370 points

    Hi Praveen,

    thats what it looks like. It seems that SDOUT drives the LSB level.

    Unfortunately I did not find this information in the data sheet. Or an you tell me where I can find this information?

    So I think the only way to "repair" it is to use a dual bus buffer gate with Tristate outputs like SN74LVC2G125.

    If I would use the two Chip Selects (SEN) as /OE for the SN74LVC2G125 and both SDOUTs as A inputs only one of the Y outputs would be valid.

    Do you think this would be a good workaround?

    Best regards,

    Michael.

  • 0 in reply to Michael K
    TI__Mastermind 49296 points
    Hi Michael,

    You could use the dual bus buffer with tristate output but instead of SENs as /OE, you could use a couple of GPIOs (with external pull-ups) from the MCU. With SENs, you may encounter a scenario where both SENs are low like programming both the AFEs.