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ADS8556 Busy signal and reset

Robert Rodriguez62
Prodigy 30 points
Other Parts Discussed in Thread: ADS8556, ADS8555

Hello, I'm using the ADS8556 in a board where the CONVST signals are set high once it's powered on. I notice that this causes the busy signal to trigger several times before it remains low. I see that the datasheet says this,

"Transitions high when a
conversion has been started and remains high during the entire process. Transitions low when
the conversion data of all six channels are latched to the output register and remains low
thereafter."

But I would think that resetting the ADS would remove the necessity to clear the output register.

When I actually start controlling the ADS8556, I set the CONVSTX signals low, reset the ADS by applying a pulse to the reset pin, and then begin the conversion sequence. However, the busy signal won't become high again.

I tested my code on the eval board ads855xevm and it worked. The only difference here is that the CONVST signals start high and apparently disables/breaks the busy signal. 

Can you please advise me on this? Thank you.

( I'm running the ADS in hardware/serial mode)

  • 0
    TI__Mastermind 45365 points

    Hi Robert,

    Your ADC was put into partial power-down mode, so the ADC stopped the conversion and BUSY high could not be observed. It happens by bringing the corresponding CONVST_x signal low during an ongoing conversion when the BUSY is high ( the falling edge of BUSY shown in the picture as below), the relevant channel pair can be activated again by issuing a RESET pulse. The next rising edge of the CONVST_x signal must be issued at least six conversion cycle periods after the reset pulse and starts a new conversion again. The reset signal on RESET pin can not disable this function. 

    To avoid this partial power-down mode, the CONVST_x signal of ADS8556 will have to be kept high during the ADC conversion and it can be pulled down to low after BUSY is low. Another option for you is using ADS8555 which doesn't have this partial power-down mode.

    Hopefully this is helpful for you.

    Regards

    Dale

  • 0 in reply to Dale Li
    Prodigy 30 points

    Thanks for your response.

    I tried what you suggested. I sent the reset pulse, clocked in 256 serial clocks (during which the program was idle) and pulled CONVSTx high (A, B & C are jumpered), but there was still no busy signal (and thus no conversion). Any other suggestions? I don't even know if the reason busy not working is due to it being toggled at the very beginning. Also, rest assured, standby mode is off. Thanks again.

  • 0 in reply to Robert Rodriguez62
    TI__Mastermind 45365 points

    Hi Robert,

    Can you please upload your timing screenshot and schematic? also can you explain more for "due to it being toggled at the very beginning. Also, rest assured, standby mode is off"? I don't understand very well. Thanks.

    Regards

    Dale

  • 0 in reply to Dale Li
    Prodigy 30 points
    Never mind, you were right! It was a mistake on my part and I got it up and running with your suggestion. Thank you.
  • 0 in reply to Robert Rodriguez62
    TI__Mastermind 45365 points

    Hi Robert,

    Thanks for sharing the good news to us, please let us know if you need more support.

    Regards

    Dale