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iio: adc: ti_am335x_tscadc: Improve accuracy of measurement

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When performing single ended measurements with TSCADC, its recommended
to set negative input (SEL_INM_SWC_3_0) of ADC step to ADC's VREFN in the
corresponding STEP_CONFIGx register.

Also, the positive(SEL_RFP_SWC_2_0) and negative(SEL_RFM_SWC_1_0)
reference voltage for ADC step needs to be set to VREFP and VREFN
respectively in STEP_CONFIGx register.
Without these changes, there may be variation of as much as ~2% in the
ADC's digital output which is bad for precise measurement.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
This commit is contained in:
Vignesh R 2018-12-03 13:31:18 +05:30 committed by Lee Jones
parent b40ee006fe
commit 4b3ab9372f
2 changed files with 8 additions and 1 deletions
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5
drivers/iio/adc/ti_am335x_adc.c
View File

@ -142,7 +142,10 @@ static void tiadc_step_config(struct iio_dev *indio_dev)
stepconfig |= STEPCONFIG_MODE_SWCNT;
tiadc_writel(adc_dev, REG_STEPCONFIG(steps),
stepconfig | STEPCONFIG_INP(chan));
stepconfig | STEPCONFIG_INP(chan) |
STEPCONFIG_INM_ADCREFM |
STEPCONFIG_RFP_VREFP |
STEPCONFIG_RFM_VREFN);
if (adc_dev->open_delay[i] > STEPDELAY_OPEN_MASK) {
dev_warn(dev, "chan %d open delay truncating to 0x3FFFF\n",

4
include/linux/mfd/ti_am335x_tscadc.h
View File

@ -78,6 +78,8 @@
#define STEPCONFIG_YNN BIT(8)
#define STEPCONFIG_XNP BIT(9)
#define STEPCONFIG_YPN BIT(10)
#define STEPCONFIG_RFP(val) ((val) << 12)
#define STEPCONFIG_RFP_VREFP (0x3 << 12)
#define STEPCONFIG_INM_MASK (0xF << 15)
#define STEPCONFIG_INM(val) ((val) << 15)
#define STEPCONFIG_INM_ADCREFM STEPCONFIG_INM(8)
@ -86,6 +88,8 @@
#define STEPCONFIG_INP_AN4 STEPCONFIG_INP(4)
#define STEPCONFIG_INP_ADCREFM STEPCONFIG_INP(8)
#define STEPCONFIG_FIFO1 BIT(26)
#define STEPCONFIG_RFM(val) ((val) << 23)
#define STEPCONFIG_RFM_VREFN (0x3 << 23)
/* Delay register */
#define STEPDELAY_OPEN_MASK (0x3FFFF << 0)