XADC values read from VAUX0-15 are consistently ~10% less than they should be

XADC values read from VAUX0-15 are consistently ~10% less than they should be

[Xander Huff]

Hi Lars,

I've been working on transitioning one of our hardware devices over from using 
the out-of-tree hwmon driver that Xilinx used to provide to the xadc driver you 
added upstream, but I've been having trouble accessing our connected auxiliary 
channels. I've found that for all channels I've tested, after multiplying the 
raw value by the scale value (0.244140625), the resulting mVs are always about 
10 percent less than what we expect them to be. (2.9V instead of 3.3V, 5.4V 
instead of 6V, etc.). I'm interested in any tips you may have on how to debug 
this issue.

Thanks!

--
Xander Huff
Staff Software Engineer
National Instruments

Re: XADC values read from VAUX0-15 are consistently ~10% less than they should be

[Xander Huff]

On 5/5/2015 5:11 PM, Xander Huff wrote:
> Hi Lars,
>
> I've been working on transitioning one of our hardware devices over from using
> the out-of-tree hwmon driver that Xilinx used to provide to the xadc driver you
> added upstream, but I've been having trouble accessing our connected auxiliary
> channels. I've found that for all channels I've tested, after multiplying the
> raw value by the scale value (0.244140625), the resulting mVs are always about
> 10 percent less than what we expect them to be. (2.9V instead of 3.3V, 5.4V
> instead of 6V, etc.). I'm interested in any tips you may have on how to debug
> this issue.
Nevermind, it turns out I had somehow transcribed the scale value incorrectly 
for my testing. After modifying my test to directly use the scale values from 
our target, the resulting voltages came out as expected

-- 
Xander Huff
Staff Software Engineer
National Instruments