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PGA460-Q1: Threshold level offset -- how is it used to adjust threshold segment points?

Stephen Embree14
Prodigy 120 points
Part Number: PGA460-Q1

How is the threshold level offset used to adjust threshold segment points?

Quoted, from page 19 on the PGA460-Q1 datasheet, the "threshold offset is defined to allow finer adjustment of the threshold map for short-range detection."  In table 82 of the same document, I see that "Preset1 Threshold level Offset with values from 7 to -8 using signed magnitude representation with MSB as the sign bit. This bit-field powers-up un-initialized."

But I cannot find any information on how the threshold level offset is used to adjust the threshold segment points.  Are the threshold level offset bits prepended to the threshold segment point, or do the threshold level offset bits added to the threshold segment point value?  How is maximum the threshold segment value of 248 (base 10) obtained?  There are not enough bits in the threshold segment register and theshold level offset register to define a number as high as 248 (base 10). 

 

  • 0
    Prodigy 120 points

    I made a mistake in my original question.  The threshold level offset register is 4 bits.  The MSB is the sign, so that leaves three bits for the value.  

    My question still remains -- how are these three bits used to adjust the value of the threshold registers?

  • 0 in reply to Stephen Embree14
    TI__Mastermind 27950 points

    Stephen,

    The threshold offset level of -8 to +7 is applied to threshold levels L1 to L8. For example, if you were to set all threshold levels (L1 to L12) to a value of 128, with an offset of 0, then you will have a single level threshold of 128 throughout the entire record duration. However, if you were to increase the offset to +7, then L1 to L8 are now actually 128+7=135, while L9 to L12 remain at 128.

    Using this same approach, you can mask or saturate any of the L1 to L8 levels by setting the level value to 248, and then applying an offset of +7 for a 248+7=255 maximum threshold level.

    You can use the PGA460Q1EVM GUI's threshold page to get a visual representation of how the offset affects the L1 to L8 levels per preset.

  • 0 in reply to Akeem Whitehead
    Prodigy 120 points

    I think I figured it out.  To get the threshold level for a given point, you take the threshold register value for that point, left shifted by three bit positions and then add the threshold offset register value.

    I am surprised that this is not explained in the documentation for the PGA460-Q1.  It took me a couple hours to figure this out.