XADC

[Sören Brinkmann]

On Thu, 2014-06-05 at 10:26AM +0200, Lars-Peter Clausen wrote:
> On 06/04/2014 04:27 PM, Sören Brinkmann wrote:
> >Hi Lars, Peter,
> >
> >On Wed, 2014-06-04 at 09:27AM +0200, Lars-Peter Clausen wrote:
> >>On 06/04/2014 08:47 AM, Peter Meerwald wrote:
> >>>Hello,
> >>>
> >>>>I recently found that you developed a driver for XADC and I thought I
> >>>>enable it (see patch below). But it seems something is not working
> >>>>correctly. The values I tested don't seem to be right and also fixed.
> >>>>I.e. they don't change over time or across boards:
> >>>>
> >>>>   # cat in_temp0_scale
> >>>>   123.040771484
> >>>>   # cat in_voltage0_vccint_scale
> >>>>   0.732421875
> >>>>   # cat in_voltage0_vccint_raw
> >>>>   1384
> >>>>   # cat in_voltage1_vccaux_s[ 1203.660637] random: nonblocking pool is initialized
> >>>>   # cat in_voltage1_vccaux_scale
> >>>>   0.732421875
> >>>>   # cat in_voltage3_vccpint_scale
> >>>>   0.732421875
> >>>
> >>>the _scale values are supposed to be constant; their purpose is to scale a
> >>>measurement so that the result is in a given unit
> >>>
> >>>e.g. in_temp0_scale scales in_temp0_raw so that the result
> >>>(in_temp0_raw * in_temp0_scale) in milli Celsius
> >>>
> >>>in_voltage0_vccint_raw * in_voltage0_vccint_scale is supposed to give
> >>>milli Volts
> >>>
> >>>so in_voltage0_vccint_raw should vary accross time / boards (and voltage
> >>>hopefully :), but not the _scales
> >>
> >>Yep, as Peter said scale and offset will be constant, raw should
> >>change though, e.g. both voltage and temperature should a slight
> >>jitter if you repeatedly read them. If not something is broken. We
> >>had the driver in our images for Zynq board for a while now and so
> >>far it worked fine. I think Mrinal and Radhey from Xilinx also
> >>tested the driver.
> >
> >Okay, thanks for the answers. Since the _scale value was the only one
> >close to a Celsius or Fahrenheit value I thought that would be the
> >final temperature. Anyway, the raw values do show some jitter. So far,
> >so good, but the math doesn't add up I think:
> >
> >  # cat in_temp0_raw in_temp0_scale
> >  2559
> >  123.040771484
> >
> >multiplying those values gives
> >2559 * 123.040771484 = 304156.787108
> >
> >Even if that is mC, that would mean my chip is at 304 degrees C. I'm pretty
> >sure my chip's not on fire :)
> >(I have the feeling I just did another beginner's mistake here though).
> >A 10x error might be in there somewhere. 30 degrees sounds realistic.
> 
> There is also a in_temp0_offset attribute which needs to be added to
> in_temp0_raw
> 
> temperature = (in_temp0_raw + in_temp0_offset) * in_temp0_scale.
> 
> In your case that is: (2559 - 2219) * 123.040771484 = 41833.86
> 
> >
> >I didn't test all voltages, but those look better.
> >
> >Also, is there a way to directly obtain a reading in a reasonable scale?
> >This approach needs to read two sysfs files, one of those even giving a
> >float.
> 
> No.

That explains things. Thanks a lot. 

	Sören