[PATCH 08/11] staging: iio: ad7746: Add comments

Hernán Gonzalez
2018-03-21T14:30:36Z

Add comments to clarify some of the calculations made, specially when
reading or writing values.

Signed-off-by: Hernán Gonzalez <hernan@vanguardiasur.com.ar>
---
drivers/staging/iio/cdc/ad7746.c | 34 ++++++++++++++++++++++++++++------
1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/drivers/staging/iio/cdc/ad7746.c b/drivers/staging/iio/cdc/ad7746.c
index 8abba71..b6b99e2 100644
--- a/drivers/staging/iio/cdc/ad7746.c
+++ b/drivers/staging/iio/cdc/ad7746.c
@@ -420,6 +420,7 @@ static int ad7746_write_raw(struct iio_dev *indio_dev,
goto out;
}

+ /* 2^16 in micro */
val = (val2 * 1024) / 15625;

switch (chan->type) {
@@ -546,6 +547,13 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
if (ret < 0)
goto out;

+ /*
+ * Either for Capacitance, Voltage or Temperature,
+ * the 0x000000 code represents negative full scale,
+ * the 0x800000 code represents zero scale, and
+ * the 0xFFFFFF code represents positive full scale.
+ */
+
*val = (be32_to_cpu(chip->data.d32) & 0xFFFFFF) - 0x800000;

switch (chan->type) {
@@ -557,7 +565,13 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
*val = (*val * 125) / 256;
break;
case IIO_VOLTAGE:
- if (chan->channel == 1) /* supply_raw*/
+
+ /*
+ * The voltage from the VDD pin is internally
+ * attenuated by 6.
+ */
+
+ if (chan->channel == 1) /* supply_raw */
*val = *val * 6;
break;
default:
@@ -598,21 +612,28 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
ret = IIO_VAL_INT;
break;
case IIO_CHAN_INFO_OFFSET:
+
+ /*
+ * CAPDAC Scale = 21pF_typ / 127
+ * CIN Scale = 8.192pF / 2^24
+ * Offset Scale = CAPDAC Scale / CIN Scale = 338646
+ */
+
*val = AD7746_CAPDAC_DACP(chip->capdac[chan->channel]
- [chan->differential]) * 338646;
+ [chan->differential]) * 338646;

ret = IIO_VAL_INT;
break;
case IIO_CHAN_INFO_SCALE:
switch (chan->type) {
case IIO_CAPACITANCE:
- /* 8.192pf / 2^24 */
+ /* CIN Scale: 8.192pf / 2^24 */
*val = 0;
*val2 = 488;
ret = IIO_VAL_INT_PLUS_NANO;
break;
case IIO_VOLTAGE:
- /* 1170mV / 2^23 */
+ /* VIN Scale: 1170mV / 2^23 */
*val = 1170;
*val2 = 23;
ret = IIO_VAL_FRACTIONAL_LOG2;
@@ -666,7 +687,8 @@ static struct ad7746_platform_data *ad7746_parse_dt(struct device *dev)
unsigned int tmp;
int ret;

- /* The default excitation outputs are not inverted, it should be stated
+ /*
+ * The default excitation outputs are not inverted, it should be stated
* in the dt if needed.
*/

@@ -678,7 +700,7 @@ static struct ad7746_platform_data *ad7746_parse_dt(struct device *dev)
ret = of_property_read_u32(np, "adi,exclvl", &tmp);
if (ret || tmp > 3) {
dev_warn(dev, "Wrong exclvl value, using default\n");
- pdata->exclvl = 3; /* default value */
+ pdata->exclvl = 3;
} else {
pdata->exclvl = tmp;
}
--
2.7.4