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ADS1015: Some read-only bits can be overwritten...advice please?

Darren (FAE)
Expert 5483 points
Part Number: ADS1015
Other Parts Discussed in Thread: ADS1115

I have a customer who has confirmed that they can overwrite the bits[3:0] in the Lo_thresh and Hi_thresh registers.

According to Figure 22 and 23 in the DS, these bits are labeled as R-0h.

But they have written data over I2C from the Master to these registers, and the data overwrote these bits.

1. Usually these type of bits (R-0h) reject writes, but is this not the case for this device?

2. If any of these bits are overwritten and become 1, is there any possibility of the device malfunctioning somehow?

Thanks,

Darren

  • 0
    TI__Guru* 94285 points

    Darren,


    Just to be clear, the four LSb of the Lo-threshold is listed as 0s and the Hi-threshold is listed as 1s.

    I don't think this is a problem, but I'll need to check with the design group. The ADS1015 is basically the same as the ADS1115, with different digital functions enabled or disabled. Because the ADS1115 uses the same registers I'm not sure if these bits are truly locked out as the datasheet implies.

    Is your customer having problems with this device? Is it giving bad data or is the data formatted differently?


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Expert 5483 points

    Hi Joseph,

    I really appreciate the quick reply.

    You are right, the Hi-thresh is specified as R-1h in the ADS1015 DS.

    Understood...in the case they aren't locked out, would modifying them on the ADS1015 cause any foreseeable issues?

    Also, I see in the DS setting these bits can setup the RDY function for when data has finished converting, but could you explain a little about how to go about choosing which bits to set HI/LOW for the comparator function? I can't quite understand what this does...how the threshold is set.

    EDIT: I looked at Figure 13 again. It looks like it is simply that you can set the "digital code" threshold. So you could set the low threshold to the digital code 1000 by setting the bits to 3E8h, and the high threshold to the digital code 60000 by setting the bits to EA60h. So, the ADS1115 is a 16-bit device, and allows setting these bits up to 16bits of resolution.

    Then the ADS1015 is a 12-bit device, and the comparator settings can only handle 12 bits, and this is why the last 4 bits of the threshold registers are listed as Read-only. Because they shouldn't affect the conversion...right? Only 12-bits can be set for the 12-bit comparator.

    Does it mean the "data transfer" can handle as many bits as are set high in the threshold register? Or is it something else?

  • 0 in reply to Darren (FAE)
    TI__Guru* 94285 points

    Darren,


    I checked with the designer. The last four bits of the threshold registers are forced to specific values at the start, but they are not blocked from programming. They are not used for comparison in the threshold and the values for those bits should not affect the threshold or ALERT/RDY functions.

    For the RDY function, this works whenever a conversion is completed. If the conversion output data is above the hi_threshold or if it is below the low_threshold, then the ALERT/RDY is indicated.

    Additionally, if you set the hi_threshold below the lo_threshold, then every conversion trips the comparator and every conversion would set the ALERT/RDY and you could use it as a data ready indication.


    Joseph Wu