Re: [PATCH 0/2] iio: ti_am335x_adc: Add optional DT properties for tscadc

Hi Hannes,

On Wednesday 29 April 2015 10:36 AM, Hannes Petermaier wrote:

Hi Vignesh,

any comments on this ?
I didn't hear anything last 2 weeks from you.

Apologies...  For some reason my mail client classified your reply mails
as junk, hence I never look into it.

I agree that making SEL_INM_SWC_3_0, SEL_RFM_SWC_1_0 and SEL_RFP_SWC_2_0
configurable is good to have. But I don't think I will be able to work
on it anytime sooner.

Regards
Vignesh

best regards,
Hannes

----- Forwarded by Hannes Petermaier/Eggelsberg/AT/B&R on 29.04.2015 07:03 
-----

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From: Hannes Petermaier/Eggelsberg/AT/B&R
To: Vignesh R <redacted>
Cc: devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Dmitry Torokhov 

[off-list ref], 

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fcooper-l0cyMroinI0@public.gmane.org, Kumar Gala [off-list ref], Ian Campbell<ijc
+devicetree-KcIKpvwj1kUDXYZnReoRVg@public.gmane.org>, Jan Kardell [off-list ref], 

Johannes 

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Pointner [off-list ref], Hartmut Knaack 
[off-list ref], Karol Wrona [off-list ref], Lars-Peter Clausen 

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[off-list ref], linux-iio-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, 

linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, 

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Mark Rutland [off-list ref], Pawel Moll [off-list ref], 

Peter 

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Meerwald [off-list ref], Rob Herring [off-list ref]
Date: 15.04.2015 07:33
Subject: Re: [PATCH 0/2] iio: ti_am335x_adc: Add optional DT properties 

for tscadc

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Hi Hannes,

Hi Vignesh,
thanks for answer.

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would it be possible to add some more channel-specific settings ?

It would be nice to have allmost full control to the STEPCONFIGx 

register.

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At least we need to write the bits

SEL_RFM_SWC_1_0
SEL_INM_SWC_3_0 
SEL_RFP_SWC_2_0 

In the current mainline version only (SEL_INP_SWC_3_0) is written.
So for the other bits "0" is value is used, for my point of view 

this is 

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not correct.

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For example if we want to read a value from AIN5 the negative pin 

from 

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adc is 

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muxed allways to AIN0.

Sorry... I didn't understand what you meant by"AIN5 is muxed always 

with

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AIN0"?

Have a look to the TRM (spruh73k.pdf) Page 1740 / Figure 12-2. 

Functional Block Diagram.

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There you can see that the ADC-cell which has two inputs, one positive 

and onenegative.

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Also there are two reference inputs, one positive - one negative.

All this "pins" are muxed, because only one channel per time can be 

sampled.

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This muxes are controlled through the STEPCONFIGx registers.

If you want for example take some measurement from AIN5 the driver muxes 

the 

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positive input from the ADC to AIN5 by setting the bits for SEL_INP<3:0> 

- this is ok.

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But the bits for SEL_INM<3:0> are still 'zero'. 
In summary this results in following mux-setting (regarding page 1771 in 

TRM):

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positive-reference muxed to   VDDA
negative-reference muxed to   VSSA
positive-input muxed to       AIN5
negative-input muxed to       AIN0

From this setup we run into 2 problems:
- the negative input terminal is muxed maybe to wrong potential
In much cases we have a single-ended signal so this setup looks good at 

first,

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because the "Diff_CNTRL" bit is also false. 
In fact there is an influence to the reading if the negative 

input-terminal 

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isn't setup correctly (to VSSA or REFN). 
Maybe i interpret the "Diff_CNTRL" not correctly, there is no detailed 
description within the TRM - maybe some of your workmates can explain 

you the 

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functionality of this bit.

- reference is allways taken from VDDA/VSSA
For a precision measurement you dont't use in normal case the 

analog-supply.

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This rail brings noise, drift - all things whicht we don't need for 

accurate 

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measurement.

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In fact i can readout heavy jitter even if AIN5 is connected to 

ground - 

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after

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setting up negative adc pin within code (to use REFN)  the readout 

value 

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is 0 

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as expected without nameable jitter.
If i short AIN0 also to ground, jitter is also eliminated.

Hmmm... nobody has reported such behavior before. ADC support for
am335x-evm/beaglebone has been there for quite long time, but nobody
reported any jitter on AIN5 line. I think this may be specific to
your setup. Can you provide more info with regard to your setup?
Which kernel? Is it am335x-evm or beaglebone or a custom board?

Maybe nobody does some precision measurement with beaglebone.
For operating some touchscreen or readout a potentiometer for evaluation 

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purpose it is still good enough.

Kernel is current mainline, 4.0
Board is some custom board of my company.
But all this parameters shouldn't have some influence to the case.

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Maybe this is also some fault of TI SoC ... in normal case somebody 

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could 

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expect, that negative adc pin is equal even the Diff_CNTRL bit 

isn't set 

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- but

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in practice it isn't.

Also actually it isn't possible to make some accurate measurement 

due to 

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the 

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fact that allways VDDA_ADC is used as positive reference.

So it would be nice to have control around this bits.
Whats your opinion around that?

Sorry, I am not yet clear on your bug/use-case.

Please comment inline while replying on mailing list

okay so ?

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Regards
Vignesh

best regards,
Hannes

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