Hello Marek,
On 07/23/2013 12:36 AM, Marek Vasut wrote:
Dear Hector Palacios,
Adds in_voltageX_scale_available file for every channel to read
the different available scales.
There are two scales per channel:
[0] = divider_by_two disabled (default)
[1] = divider_by_two enabled
The scale is a struct made of integer and nano parts to build
a long decimal number.
Signed-off-by: Hector Palacios <hector.palac...@digi.com>
---
[...]
@@ -1008,6 +1098,26 @@ static int mxs_lradc_probe(struct platform_device
*pdev) if (ret)
goto err_trig;
+ /* Populate available ADC input ranges */
+ for (i = 0; i < LRADC_MAX_TOTAL_CHANS; i++) {
+ for (s = 0; s < ARRAY_SIZE(lradc->scale_avail[i]); s++) {
+ /*
+ * [s=0] = optional divider by two disabled (default)
+ * [s=1] = optional divider by two enabled
+ *
+ * The scale is calculated by doing:
+ * Vref >> (realbits - s)
+ * which multiplies by two on the second component
+ * of the array.
+ */
+ scale_uv = ((u64)lradc->vref_mv[i] * 100000000) >>
+ (iio->channels[i].scan_type.realbits - s);
Thinking about this, this is basically
vref_mv[CHANNEL] * 100 000 000
scale_uv = --------------------------------
2^(12 - s)
Where s can be either 0 or 1.
Why do you multiply it by 100000000 I don't quite understand, but maybe it's
fully obvious. It should be documented though.
I copied Michael Hennerich in CC who was the author of this math in
drivers/iio/adc/ad7793.c
drivers/staging/iio/adc/ad7192.c
which I copied. Maybe he can explain. It looks like it is just a question of having a
intermediate large enough number to later do the do_div operation, rather than having
a meaningful figure.
+ lradc->scale_avail[i][s].nano =
+ do_div(scale_uv, 100000000) * 10;
Are we not loosing precission here?
Yes, but the do_div is modifying the scale_uv parameter with the integer part so I
guess it might be somewhat justified.
As I said before, I tried to do my own math without success so I just copied what was
in other drivers.
+ lradc->scale_avail[i][s].integer = scale_uv;
+ }
+ }
+
/* Configure the hardware. */
mxs_lradc_hw_init(lradc);
Best regards,
Marek Vasut
Best regards,
--
Hector Palacios
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