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ADS8634: need clarification regarding driver, AL_PD pin

Dixit M
Prodigy 10 points
Part Number: ADS8634

Tool/software:

Hello all,

I have few questions with respect to ads8634 interfacing.

1. Is driver is mandatory to interace the ads8634 with mater device via SPI ? ( master- jetson xavier nx) 

2. For the AL_PD pin , In the 8th page of the datasheet it is metioned as below :

Digital input  : low, asynchronous power-down.
The device features an internal, weak pull-up resistor from the AL_PD pin to DVDD. The
AL_PD pin can also be floated when programmed as a power-down input.
(The default condition for this pin is programmed as a power-down input pin.

Here i want to clarify ,  "if the AL_PD pin is floating ,then is will act as power down input pin  and when it is configured as input pin then it will have a weak internal pull up " I wonder if my udnerstanding in this is correct.

Above in our case the pin is left floating. So i wonder here it will get a weak internal pullup or it will be in power down mode (default configuration). Becasue the data i am receiving  is always "0" for any register .

Best Regards,


  • 0
    TI__Guru**** 180981 points

    Hi Dixit,

    Welcome to our e2e forum!  For your first query: No - you do not need a driver from your SPI controller to interface with the ADS8634.  For Q2, your understanding is correct.  The internal pull-up should keep the ADS8634 powered on.  Please review the additional information on use of AL_PD in pages 28 through 31 and let us know how you have your registers configured.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Thanks for the response,

    I am confused, when it is left floating, will it be internally pulled up (normal working mode) or will it be in power down (default configuration)?. Please can you confirm this one.

    Best regards,
    Dixit 

  • 0 in reply to Dixit M
    TI__Guru**** 180981 points

    The AL_PD pin has two primary working functions - either as a GPIO or as the Alarm/Power Down function.  By default, the Alarm/Power Down function is active at power up.  A secondary function in the default state is for the pin to act as an alarm (which is an active high output) or as a power-down pin (an active low input).  Without doing any additional configuration, pin 22 is set by default at power up of the ADS8634 to use the power-down function.  Because there is an internal pull-up, you can leave the pin float and the ADS9634 should function normally.

    One thing to note, your HVSS appears to have a bypass cap to ground, but I do not see an HVSS source.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Thank you for the confirmation with respect AL_PD floating pin.

    As checked currently we came across problem with respect to our spi clock hardware connections, will interact with ads8634 after the corrections.
    I hope it works after these things are clarified. Appreciate your quick response. So this resolved my issue at this point of time.

    Best regards,
    Dixit M

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    We solved our clock problem with respect spi.

    Now currently i am on page 0 registers, i am writting 0x0C (for channel 0 ) to Manual Mode Register Address = 04h .
    If i read the 0x04 i am getting 0x0C , similar the case with the rest of the channels (0x2C,0x4C, 0x6C ,  C = 110 :range 0 to 5V in my case)

    But my aim it to read the digital data for any channel now. So i am not getting from which register exactly do i get that digital output code.




  • 0 in reply to Dixit M
    TI__Guru**** 180981 points

    Hi Dixit,

    Please send over o'scope or logic analyzer plots showing your SPI control lines (/CS, SCLK, SDI, SDO) and please also include and schematic changes you may have made.

  • 0 in reply to Tom Hendrick
    Prodigy 10 points

    Update: I am able to interface with it successfully. I am supposed to configure the Aux-Config: Device Auxiliary Blocks Enable/Disable Control Register (Address = 06h; Page 0) to 0x04 before reading. Now i am able to get the digital code. Thanks for the support