sensor: voltage_divider: delay sampling after power-on

Enforce some minimum delay between enabling the voltage divider with a
GPIO and sampling the analog voltage. Without this delay the ADC can
easily sample the transient power up curve instead of the steady state
output.

Signed-off-by: Jordan Yates <[email protected]>

Author: JordanYates

drivers/sensor/voltage_divider/voltage.c

@@ -18,10 +18,12 @@ LOG_MODULE_REGISTER(voltage, CONFIG_SENSOR_LOG_LEVEL);
 struct voltage_config {
 	struct voltage_divider_dt_spec voltage;
 	struct gpio_dt_spec gpio_power;
+	uint32_t sample_delay_us;
 };
 
 struct voltage_data {
 	struct adc_sequence sequence;
+	k_timeout_t earliest_sample;
 	uint16_t raw;
 };
 
@@ -35,6 +37,9 @@ static int fetch(const struct device *dev, enum sensor_channel chan)
 		return -ENOTSUP;
 	}
 
+	/* Wait until sampling is valid */
+	k_sleep(data->earliest_sample);
+
 	ret = adc_read(config->voltage.port.dev, &data->sequence);
 	if (ret != 0) {
 		LOG_ERR("adc_read: %d", ret);
@@ -90,6 +95,7 @@ static const struct sensor_driver_api voltage_api = {
 static int pm_action(const struct device *dev, enum pm_device_action action)
 {
 	const struct voltage_config *config = dev->config;
+	struct voltage_data *data = dev->data;
 	int ret = 0;
 
 	if (config->gpio_power.port == NULL) {
@@ -109,6 +115,8 @@ static int pm_action(const struct device *dev, enum pm_device_action action)
 		if (ret != 0) {
 			LOG_ERR("failed to set GPIO for PM resume");
 		}
+		data->earliest_sample = K_TIMEOUT_ABS_TICKS(
+			k_uptime_ticks() + k_us_to_ticks_ceil32(config->sample_delay_us));
 		break;
 #ifdef CONFIG_PM_DEVICE
 	case PM_DEVICE_ACTION_SUSPEND:
@@ -133,6 +141,9 @@ static int voltage_init(const struct device *dev)
 	struct voltage_data *data = dev->data;
 	int ret;
 
+	/* Default value to use if `power-gpios` does not exist */
+	data->earliest_sample = K_TIMEOUT_ABS_TICKS(0);
+
 	if (!adc_is_ready_dt(&config->voltage.port)) {
 		LOG_ERR("ADC is not ready");
 		return -ENODEV;
@@ -169,6 +180,7 @@ static int voltage_init(const struct device *dev)
 	static const struct voltage_config voltage_##inst##_config = {                             \
 		.voltage = VOLTAGE_DIVIDER_DT_SPEC_GET(DT_DRV_INST(inst)),                         \
 		.gpio_power = GPIO_DT_SPEC_INST_GET_OR(inst, power_gpios, {0}),                    \
+		.sample_delay_us = DT_INST_PROP(inst, power_on_sample_delay_us),                   \
 	};                                                                                         \
                                                                                                    \
 	PM_DEVICE_DT_INST_DEFINE(inst, pm_action);                                                 \

dts/bindings/iio/afe/voltage-divider.yaml

@@ -36,3 +36,12 @@ properties:
 
       If present the corresponding GPIO must be set to an active level
       to enable the divider input.
+
+  power-on-sample-delay-us:
+    type: int
+    default: 100
+    description: |
+      Duration to delay sampling after enabling the circuitry with
+      `power-gpios`. In most cases the switched voltage rail will
+      require some non-zero time to settle to its final value. The
+      default value of 100us should be sufficient in most situations.