[PATCH v1 5/9] iio: afe: rescale: add support for temperature sensors

Liam Beguin
2021-05-30T00:59:48Z

From: Liam Beguin <lvb@xiphos.com>

Add support for various linear temperature sensors.

temperature-sense-rtd is used when the measured temperature is a
function of the sensor's resistance (like RTD sensors).

temperature-sense-current is used when the measured temperature is a
function of the sensor's output current (like the AD590)

temperature-sense-amplifier is used when the measured temperature is a
function of the sensor's voltage (like the LTC2997)

Signed-off-by: Liam Beguin <lvb@xiphos.com>
---
drivers/iio/afe/iio-rescale.c | 141 ++++++++++++++++++++++++++++++++++
1 file changed, 141 insertions(+)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 3bd1f11f21db..eb53d833bf7c 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c
@@ -222,10 +222,133 @@ static int rescale_voltage_divider_props(struct device *dev,
return 0;
}

+static int rescale_temp_sense_rtd_props(struct device *dev,
+ struct rescale *rescale)
+{
+ u32 factor;
+ u32 alpha;
+ u32 iexc;
+ u32 tmp;
+ int ret;
+ u32 r0;
+
+ ret = device_property_read_u32(dev, "excitation-current-microamp",
+ &iexc);
+ if (ret) {
+ dev_err(dev, "failed to read excitation-current-microamp: %d\n",
+ ret);
+ return ret;
+ }
+
+ ret = device_property_read_u32(dev, "alpha-micro-ohms-per-ohm-celsius",
+ &alpha);
+ if (ret) {
+ dev_err(dev, "failed to read alpha-micro-ohms-per-celsius: %d\n",
+ ret);
+ return ret;
+ }
+
+ ret = device_property_read_u32(dev, "r-naught-ohms", &r0);
+ if (ret) {
+ dev_err(dev, "failed to read r-naught-ohms: %d\n", ret);
+ return ret;
+ }
+
+ /*
+ * The transfer function:
+ *
+ * - V(T) = R(T) * iexc
+ * - R(T) = r0 * (1 + alpha * T)
+ *
+ * T = 1 / (alpha * r0 * iexc) * (V - r0 * iexc)
+ */
+ tmp = r0 * iexc * alpha / 1000000;
+ factor = gcd(tmp, 1000000);
+ rescale->numerator = 1000000 / factor;
+ rescale->denominator = tmp / factor;
+
+ rescale->offset = -1 * ((r0 * iexc) / 1000);
+
+ return 0;
+}
+
+static int rescale_temp_sense_current_props(struct device *dev,
+ struct rescale *rescale)
+{
+ u32 alpha;
+ u32 sense;
+ int ret;
+
+ ret = device_property_read_u32(dev, "sense-resistor-ohms", &sense);
+ if (ret) {
+ dev_err(dev, "failed to read the sense resistance: %d\n", ret);
+ return ret;
+ }
+
+ ret = device_property_read_u32(dev, "alpha-micro-amps-per-degree",
+ &alpha);
+ if (ret) {
+ dev_err(dev, "failed to read alpha-micro-amps-per-degree: %d\n",
+ ret);
+ return ret;
+ }
+
+ /*
+ * The transfer function:
+ *
+ * - V(K) = Rsense * Isense(K)
+ * - K = Isense(K) / alpha
+ * - C = K - 273.15
+ *
+ * C = 1 / (Rsense * alpha) * (V - 273.15 * Rsense * alpha)
+ */
+ rescale->numerator = 1000000;
+ rescale->denominator = alpha * sense;
+
+ if (device_property_read_bool(dev, "use-kelvin-scale"))
+ rescale->offset = -1 * ((27315 * alpha * sense) / 100000);
+
+ return 0;
+}
+
+static int rescale_temp_sense_amplifier_props(struct device *dev,
+ struct rescale *rescale)
+{
+ u32 alpha;
+ int ret;
+
+ ret = device_property_read_u32(dev, "alpha-micro-volts-per-degree",
+ &alpha);
+ if (ret) {
+ dev_err(dev, "failed to read alpha-micro-volts-per-degree: %d\n",
+ ret);
+ return ret;
+ }
+
+ /*
+ * The transfer function:
+ *
+ * - K = V(K) / alpha
+ * - C = K - 273.15
+ *
+ * C = 1 / (alpha) * (V - 273.15 * alpha)
+ */
+ rescale->numerator = 1000000;
+ rescale->denominator = alpha;
+
+ if (device_property_read_bool(dev, "use-kelvin-scale"))
+ rescale->offset = -1 * ((27315 * alpha) / 100000);
+
+ return 0;
+}
+
enum rescale_variant {
CURRENT_SENSE_AMPLIFIER,
CURRENT_SENSE_SHUNT,
VOLTAGE_DIVIDER,
+ TEMP_SENSE_RTD,
+ TEMP_SENSE_CURRENT,
+ TEMP_SENSE_AMPLIFIER,
};

static const struct rescale_cfg rescale_cfg[] = {
@@ -241,6 +364,18 @@ static const struct rescale_cfg rescale_cfg[] = {
.type = IIO_VOLTAGE,
.props = rescale_voltage_divider_props,
},
+ [TEMP_SENSE_RTD] = {
+ .type = IIO_TEMP,
+ .props = rescale_temp_sense_rtd_props,
+ },
+ [TEMP_SENSE_CURRENT] = {
+ .type = IIO_TEMP,
+ .props = rescale_temp_sense_current_props,
+ },
+ [TEMP_SENSE_AMPLIFIER] = {
+ .type = IIO_TEMP,
+ .props = rescale_temp_sense_amplifier_props,
+ },
};

static const struct of_device_id rescale_match[] = {
@@ -250,6 +385,12 @@ static const struct of_device_id rescale_match[] = {
.data = &rescale_cfg[CURRENT_SENSE_SHUNT], },
{ .compatible = "voltage-divider",
.data = &rescale_cfg[VOLTAGE_DIVIDER], },
+ { .compatible = "temperature-sense-rtd",
+ .data = &rescale_cfg[TEMP_SENSE_RTD], },
+ { .compatible = "temperature-sense-current",
+ .data = &rescale_cfg[TEMP_SENSE_CURRENT], },
+ { .compatible = "temperature-sense-amplifier",
+ .data = &rescale_cfg[TEMP_SENSE_AMPLIFIER], },
{ /* sentinel */ }
};
MODULE_DEVICE_TABLE(of, rescale_match);
--
2.30.1.489.g328c10930387