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ADS8698: ADS8698: using the Alarm Output

Christian Merz
Prodigy 80 points
Part Number: ADS8698

The ADS8698 is already used for a new design in a high voltage power supply unit and works very well.  But there is one feature of this ADC i missunterstood and therefor one important feature of our PSU will not work - the software independent Alarm output. The original design idea was using the ADC for monitoring voltages and make an emergency shutoff if a value is out of range. After reading the datasheet more carefully the function of auto sequencing and alarmout is not running by the ADC itself but must be triggerd by SPI communication. So for security reason on our device when the software will hang there is nor more monitoring of the connected voltages. What was the TI idea how to use this alarm feature?  if the mikrocontroller has to read the Adc values the comparsion against high and low limits can be done in software!?
Actually we hope our software will never hang..

So what is the correct usage of the value comparsion and alarm output of ths ADC? Maybe you have a idea how we can solve our mistake? (ofcourse , reading your datasheets mor carefully can help)

Thanks,

Ch.Merz

  • 0
    TI__Mastermind 45805 points
    Hello Christian,
    Thanks for your query on E2E forum.
    When this device was designed, we considered different requirements and applications, some applications do not need Alarm feature, so the alarm feature is disabled after power up the device which is default status. If this feature is needed, a register programming to Device Features Selection Control Register (address = 03h) is required but just needs to program once after power up. After this feature is enabled, it can compare the input signal with the programmed value in Alarm automatically, the microcontroller only needs to monitor the signal on Alarm pin. If the microcontroller receives a alarm signal, it can read the status register to know which channel trips the Alarm, this mechanism can save Microcontroller's resource because microcontroller does not need to check the code continuously.
    Hence, at least once register programming is needed but you only need to monitor Alarm pin and check Alarm register status if need instead of reading ADC's conversion code of each channel.

    Thanks.
    Regards,
    Dale
  • 0 in reply to Dale Li
    Prodigy 80 points
    Hello Dale,
    what you describe is exactly what i like to have but not how the device works. Of cource the alarm feature must be enabled and configured as we did, but for the ADC internal state machine you have to generate SPI clock and chip select signals like reading from the ADC. Without SPI communication generated by the connected mikrocontroller there is no autoscan neither alarm output. In our case, if mikrocontroller stopps working, the ADC also stops with this alam feature. This ADC could be a realy good monitoring device in safety critical applications if the alarm feature will run by the ADC itself, without any SPI traffic nedded. Thats what i actually missing.
    Regards, Ch.Merz
  • 0 in reply to Christian Merz
    TI__Mastermind 45805 points

    Hello Christian,

    I totally understood your concern, but we have to follow up the procedure to use Alarm and other features for this specific device. We may consider this requirement on next product. Thanks.

    Best regards

    Dale