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Subject[PATCH 07/10] iio: adc: mcp3911: use correct formula for AD conversion
Date
The ADC conversion is actually not rail-to-rail but with a factor 1.5.
Make use of this factor when calculating actual voltage.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
---
drivers/iio/adc/mcp3911.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/adc/mcp3911.c b/drivers/iio/adc/mcp3911.c
index 65831bef12f6..9e365947d285 100644
--- a/drivers/iio/adc/mcp3911.c
+++ b/drivers/iio/adc/mcp3911.c
@@ -48,8 +48,8 @@
#define MCP3911_CHANNEL(x) (MCP3911_REG_CHANNEL0 + x * 3)
#define MCP3911_OFFCAL(x) (MCP3911_REG_OFFCAL_CH0 + x * 6)

-/* Internal voltage reference in uV */
-#define MCP3911_INT_VREF_UV 1200000
+/* Internal voltage reference in mV */
+#define MCP3911_INT_VREF_MV 1200

#define MCP3911_REG_READ(reg, id) ((((reg) << 1) | ((id) << 5) | (1 << 0)) & 0xff)
#define MCP3911_REG_WRITE(reg, id) ((((reg) << 1) | ((id) << 5) | (0 << 0)) & 0xff)
@@ -178,11 +178,18 @@ static int mcp3911_read_raw(struct iio_dev *indio_dev,

*val = ret / 1000;
} else {
- *val = MCP3911_INT_VREF_UV;
+ *val = MCP3911_INT_VREF_MV;
}

- *val2 = 24;
- ret = IIO_VAL_FRACTIONAL_LOG2;
+ /*
+ * For 24bit Conversion
+ * Raw = ((Voltage)/(Vref) * 2^23 * Gain * 1.5
+ * Voltage = Raw * (Vref)/(2^23 * Gain * 1.5)
+ */
+
+ /* val2 = (2^23 * 1.5) */
+ *val2 = 12582912;
+ ret = IIO_VAL_FRACTIONAL;
break;
}

--
2.36.1
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