XADC

[Sören Brinkmann]

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.

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.

	Sören