[RESEND v4 13/15] iio: adc: aspeed: Add compensation phase.

Billy Tsai
2021-08-24T09:12:41Z

This patch adds a compensation phase to improve the accurate of ADC
measurement. This is the built-in function though input half of the
reference voltage to get the ADC offset.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
---
drivers/iio/adc/aspeed_adc.c | 54 +++++++++++++++++++++++++++++++++++-
1 file changed, 53 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/adc/aspeed_adc.c b/drivers/iio/adc/aspeed_adc.c
index 4d979dd7fe88..20caf28dff18 100644
--- a/drivers/iio/adc/aspeed_adc.c
+++ b/drivers/iio/adc/aspeed_adc.c
@@ -95,6 +95,7 @@ struct aspeed_adc_data {
struct reset_control *rst;
int vref;
u32 sample_period_ns;
+ int cv;
};

#define ASPEED_CHAN(_idx, _data_reg_addr) { \
@@ -104,7 +105,8 @@ struct aspeed_adc_data {
.address = (_data_reg_addr), \
.info_mask_separate = BIT(IIO_CHAN_INFO_RAW), \
.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE) | \
- BIT(IIO_CHAN_INFO_SAMP_FREQ), \
+ BIT(IIO_CHAN_INFO_SAMP_FREQ) | \
+ BIT(IIO_CHAN_INFO_OFFSET), \
}

static const struct iio_chan_spec aspeed_adc_iio_channels[] = {
@@ -126,6 +128,51 @@ static const struct iio_chan_spec aspeed_adc_iio_channels[] = {
ASPEED_CHAN(15, 0x2E),
};

+static int aspeed_adc_compensation(struct iio_dev *indio_dev)
+{
+ struct aspeed_adc_data *data = iio_priv(indio_dev);
+ u32 index, adc_raw = 0;
+ u32 adc_engine_control_reg_val;
+
+ adc_engine_control_reg_val =
+ readl(data->base + ASPEED_REG_ENGINE_CONTROL);
+ adc_engine_control_reg_val &= ~ASPEED_ADC_OP_MODE;
+ adc_engine_control_reg_val |=
+ (FIELD_PREP(ASPEED_ADC_OP_MODE, ASPEED_ADC_OP_MODE_NORMAL) |
+ ASPEED_ADC_ENGINE_ENABLE);
+ /*
+ * Enable compensating sensing:
+ * After that, the input voltage of ADC will force to half of the reference
+ * voltage. So the expected reading raw data will become half of the max
+ * value. We can get compensating value = 0x200 - ADC read raw value.
+ * It is recommended to average at least 10 samples to get a final CV.
+ */
+ writel(adc_engine_control_reg_val | ASPEED_ADC_CTRL_COMPENSATION |
+ ASPEED_ADC_CTRL_CHANNEL_ENABLE(0),
+ data->base + ASPEED_REG_ENGINE_CONTROL);
+ /*
+ * After enable compensating sensing mode need to wait some time for ADC stable
+ * Experiment result is 1ms.
+ */
+ mdelay(1);
+
+ for (index = 0; index < 16; index++) {
+ /*
+ * Waiting for the sampling period ensures that the value acquired
+ * is fresh each time.
+ */
+ ndelay(data->sample_period_ns);
+ adc_raw += readw(data->base + aspeed_adc_iio_channels[0].address);
+ }
+ adc_raw >>= 4;
+ data->cv = BIT(ASPEED_RESOLUTION_BITS - 1) - adc_raw;
+ writel(adc_engine_control_reg_val,
+ data->base + ASPEED_REG_ENGINE_CONTROL);
+ dev_dbg(data->dev, "Compensating value = %d\n", data->cv);
+
+ return 0;
+}
+
static int aspeed_adc_set_sampling_rate(struct iio_dev *indio_dev, u32 rate)
{
struct aspeed_adc_data *data = iio_priv(indio_dev);
@@ -155,6 +202,10 @@ static int aspeed_adc_read_raw(struct iio_dev *indio_dev,
*val = readw(data->base + chan->address);
return IIO_VAL_INT;

+ case IIO_CHAN_INFO_OFFSET:
+ *val = data->cv;
+ return IIO_VAL_INT;
+
case IIO_CHAN_INFO_SCALE:
*val = data->vref;
*val2 = ASPEED_RESOLUTION_BITS;
@@ -444,6 +495,7 @@ static int aspeed_adc_probe(struct platform_device *pdev)
return ret;
}

+ aspeed_adc_compensation(indio_dev);
/* Start all channels in normal mode. */
adc_engine_control_reg_val =
readl(data->base + ASPEED_REG_ENGINE_CONTROL);
--
2.25.1