iio: adc: ti_am335x_tscadc: Improve accuracy of measurement

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>

diff --git a/drivers/iio/adc/ti_am335x_adc.c b/drivers/iio/adc/ti_am335x_adc.c
index cafb1dcadc488c4af2664d66409f0eb02d12d16f..9d984f2a8ba7489e2920e43af6d14a8b141a541d 100644 (file)
--- a/drivers/iio/adc/ti_am335x_adc.c
+++ b/drivers/iio/adc/ti_am335x_adc.c
@@ -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",

diff --git a/include/linux/mfd/ti_am335x_tscadc.h b/include/linux/mfd/ti_am335x_tscadc.h
index b9a53e013bff35e639830130295c065f78ea654e..483168403ae586ea1c662342375afa990ae5b394 100644 (file)
--- a/include/linux/mfd/ti_am335x_tscadc.h
+++ b/include/linux/mfd/ti_am335x_tscadc.h
@@ -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)