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ADS8681: bit location of high/log flags for AVDD and Input alarm flags

Scott Allwine
Prodigy 60 points
Part Number: ADS8681

Tool/software:

Hello - I have a custom board with an ADS8681 controlled by a FPGA.  My SPI interface is working, I can set the data output reg to output 01 or 0011 and the 16 bit data returned is either x5555 or x3333.  If I ground the input to the ADC I get x8000 (+/- 3 LSb) for the ADC data.  I configure the rst_pwrctl_reg to enable both the input alarm and AVDD alarm, than set in the data output reg, I set bits 13:10 to '1111' so I can get both the high and low flags for both alarms.  My question is where will the hi and lo alarm bits be set if an alarm is triggered?  On pg 42, table 7-6 and 7-7 have the alarm flags in different bit locations.  I don't need the device address or input range or parity bits, just the adc data and 4 bit alarm flags.  These flags will be used to shutdown another part of the system.  Thank you.

  • 0
    TI__Expert 3880 points

    Hello Scott, 

    Thank you for posting on TI's E2E forum!

    The way the Output Data Word is configured is by shifting out the enabled flags in the sequence 

    Conversion Result > Device Address > ADC Input Range > AVDD Alarm Flags > Input Alarm Flags > Parity Bits > 0s to complete 32-bit output data word. 

    Table 7-6 shows all of the flags enabled, in the order previously mentioned

    If any of the flags are not enabled, then those would be skipped in the sequence and the enabled ones would be shifted left to fill the gap, also adding 0s at the end to complete the 32-bit output data word. 

    for example, if all but the device address and input alarm flags were enabled then it would look like Table 7-7

     

    In this case since the Device address was not enabled, ADC input range was left shifted and took the [15:12] spot, followed by AVDD Alarm Flags, and so on. 

    In your case, where you only want to have the conversion results and the AVDD & Input Alarm flags then it would be:

    {Conversion Result[31:16], AVDD Alarm Flags[15:14], Input Alarm Flags[13:12], 0s[11:0]}

    Please also note the other exceptions in section 7.5.3 Output Data Word, if DATA_VAL[2:0] = 1xxb, or if the READ or READ_HWORD commands are used, this will also affect the format of the data output on SDO. 

    Best regards, 

    Yolanda