Re: Accessing specific hardware features of mxs-lradc

[Alexandre Belloni <alexandre.belloni@free-electrons.com>]

On 02/07/2013 17:21, Lars-Peter Clausen wrote:
>>>
>>> I was looking at that yesterday as I had to had a scale for the
>>> temperature sensors. You'll have to be careful because the maximum
>>> measured voltage when DIVIDE_BY_TWO is not set is 1.85V and when it is
>>> set, it becomes VDDIO - 50mv.
> 
> I had a quick peak at the datasheet and I think that's a typo. VDDIO is 3.3V.
> This sentence also doesn't make much sense if divide by two would actually
> lower the range. "For input channels (other than BATT, 5V, or VDDIO) with
> signals larger than 1.85 V, the divide-by-two option should be set."
> 

Yeah, what I think they meant is if !DIVIDE_BY_TWO : Vmax = 1.85V else
Vmax = 3.25V. So it may not be as easy as multiplying hte scale by two.
I couldn't test it, so I didn't implement it.

>>>
>>> Also, scaling for quite a lot of channels is fixed. You'll also get
>>> channels with a fixed divider that can take DIVIDE_BY_TWO. That is quite
>>> a mess ;)
>>
>> Thank you, I checked and it's easy to add custom sysfs attributes using
>> IIO_DEVICE_ATTR(). I was thinking whether adding a divide_by_two attribute per
>> channel:
>>
>>     divide_by_two0
>>     divide_by_two1
>>     ...
>>     divide_by_two7
>>
>> or a unique divide_by_two descriptor that admits a mask value for all channels.
>> I think the first approach (one per channel) is cleaner. Opinions?
> 
> Just use the scale attribute for this. That's the standard way to handle this,
> don't invent custom interfaces if there is already a generic one.
> 

Hector, you can have a look at what I did there:

http://marc.info/?l=linux-iio&m=137269905605960&w=2

Add IIO_CHAN_INFO_SCALE to .info_mask_separate in the MXS_ADC_CHAN
macro. Then, implement the scaling in the switch case in
mxs_lradc_read_raw and you'll probably also want to implement a write_raw.


-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com