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ADS8671: Configure alarm pin for active alarm condition

H. Williams
Prodigy 20 points
Part Number: ADS8671
Other Parts Discussed in Thread: ADS8681EVM-PDK, ADS7142, ADS8681

I configured the ALARM/SDO-1/GPO pin as an alarm by setting SDO_CTL_REG bits 9-8 to 01b per table 7-14 (SDO_CTL_REG Register Field Descriptions).

I would like the GPIO pin to function as an active alarm for the alarm input and go low or high based on an active alarm condition. Unfortunately, it is functioning as a tripped alarm.  It goes high upon an alarm condition but does not go low when the alarm condition is over.  Instead, you have to read the ALARM_REG register to clear the GPIO pin low.

Is there any way to make the ALARM/SDO-1/GPO pin function as an active alarm instead of a tripped alarm?

  • 0
    TI__Guru**** 172546 points

    Hi H. Williams!

    Welcome to our e2e Forum!  Give me a day or two to look into this for you.  By the description on pages 33 and 34, it certainly sounds like the VIN can be set as an active alarm, but the block diagram of Figure 7-16 is not clear to me that it can be.  I'll discuss your query with the design team and let you know ASAP.

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    btw: There are 4 mistakes in Table 7-17. ALARM_REG Register Field Descriptions of the datasheet.  The alarm output active and tripped descriptions do not match their field labels.  The high and low are reversed on 4 of them.

  • 0 in reply to H. Williams
    TI__Guru**** 172546 points

    Hi H. Williams.

    Thank you for letting us know that!  Still waiting on confirmation of the active alarm state, hope to have more detail for you tomorrow.

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    Also do you have any simple board layouts for the ADS8671 or similar chip (Gerber, Eagle, etc.?  I just need the minimum components assuming use of the internal voltage reference only.  Thanks.

  • 0 in reply to H. Williams
    TI__Guru**** 172546 points

    Hi H. Williams,

    I heard back from the design team.  The active alarm is s sticky bit, it needs to be read to clear the register.  The only layout example I have for you is the one located in the ADS8671EVM Users Guide.  If you review the schematic on page 22 you would not need U5, U3, Q1 or any of the associated circuity with those components.  U4 is optional depending on the quality of your AVdd source.  The same is true for U2, but having a good reference is critical to accurate conversions.

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    Thanks, the schematic for the ADS8681EVM-PDK is very helpful.  

    Just one more thing, it appears that the alarm pin isn't going high when a voltage threshold is reached unless the ADC is first read.  The only way I could get the alarm pin to function properly is to continuously transmit 4 NOP commands.  I was hoping to use the ADC alarm pin as an interrupt for the microcontroller.  Is there any way to set the alarm pin to function without the need for continuous SPI communication with the ADC?  For example, could the ADC be put into some type of continuous mode where it would monitor the input voltage and then pull up the alarm pin when a voltage threshold is reached?

  • 0 in reply to H. Williams
    TI__Guru**** 172546 points

    Hi Hugh,

    The alarm function needs to do a conversion to see if it's hit the limits or not, so just applying voltage to the input without doing a conversion won't trigger the function.  Would you consider an I2C device?  We have parts like the the ADS7142 which can work in an autonomous fashion.  They can be set for hi/low level limits and they will just 'convert' in the background, alerting you if the trigger level has been hit.

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    My application requires true bipolar so the ADS7142 won't work.  Do you have any true bipolar ADC's that are as fast as the ADS8681 but with an autonomous function?

  • 0 in reply to H. Williams
    TI__Guru**** 172546 points

    Unfortunately, no.  The only parts I have that include the comparator function along with the hi/lo limit alarms are unipolar.