fuel_gauge: composite: cached voltage for SoC

Cache the voltage queried from `FUEL_GAUGE_VOLTAGE` for immediate use
in `FUEL_GAUGE_ABSOLUTE_STATE_OF_CHARGE` if the second call occurs
within a short delay of the first call.

This eliminates small mismatches between voltage and SoC values when
queried back to back due to noise on the voltage measurement.

Signed-off-by: Jordan Yates <jordan@embeint.com>

drivers/fuel_gauge/composite/Kconfig

@@ -9,3 +9,17 @@ config FUEL_GAUGE_COMPOSITE
 	depends on DT_HAS_ZEPHYR_FUEL_GAUGE_COMPOSITE_ENABLED
 	help
 	  Enable driver for the Zephyr composite fuel gauge device.
+
+config FUEL_GAUGE_COMPOSITE_VOLTAGE_REUSE_TIMEOUT_MS
+	int "Maximum age of a cached voltage reading to use in SoC conversion"
+	depends on FUEL_GAUGE_COMPOSITE
+	default 100
+	help
+	  Since the fuel gauge API reads values one call at a time, reading the battery
+	  voltage and charge percentage are by necessity the product of two calls.
+	  Unfortunately voltage measurements can be noisy, so two calls back to back may
+	  return slightly different values. This can result in the voltage and charge
+	  percentage not exactly matching each other when querying both back to back.
+	  To mitigate this, if a charge percentage call is made within N milliseconds of
+	  a previous `FUEL_GAUGE_VOLTAGE` call, the voltage from the previous call is
+	  used directly in the percentage calulation.

drivers/fuel_gauge/composite/fuel_gauge_composite.c

@@ -21,6 +21,11 @@ struct composite_config {
 	enum battery_chemistry chemistry;
 };
 
+struct composite_data {
+	int voltage_val;
+	k_ticks_t voltage_time;
+};
+
 static int composite_read_micro(const struct device *dev, enum sensor_channel chan, int *val)
 {
 	struct sensor_value sensor_val;
@@ -51,6 +56,7 @@ static int composite_get_prop(const struct device *dev, fuel_gauge_prop_t prop,
 {
 	const struct composite_config *config = dev->config;
 	int voltage, rc = 0;
+	int voltage_age_ms;
 
 	switch (prop) {
 	case FUEL_GAUGE_FULL_CHARGE_CAPACITY:
@@ -68,13 +74,33 @@ static int composite_get_prop(const struct device *dev, fuel_gauge_prop_t prop,
 	case FUEL_GAUGE_VOLTAGE:
 		rc = composite_read_micro(config->battery_voltage, SENSOR_CHAN_VOLTAGE,
 					  &val->voltage);
+		if (rc == 0) {
+			data->voltage_val = voltage;
+			data->voltage_time = k_uptime_ticks();
+		}
 		break;
 	case FUEL_GAUGE_ABSOLUTE_STATE_OF_CHARGE:
 	case FUEL_GAUGE_RELATIVE_STATE_OF_CHARGE:
 		if (config->ocv_lookup_table[0] == -1) {
 			return -ENOTSUP;
 		}
-		rc = composite_read_micro(config->battery_voltage, SENSOR_CHAN_VOLTAGE, &voltage);
+		/* Age of the previous voltage measurement */
+		voltage_age_ms = k_ticks_to_ms_near32(k_uptime_ticks() - data->voltage_time);
+		if (data->voltage_time &&
+		    (voltage_age_ms < CONFIG_FUEL_GAUGE_COMPOSITE_VOLTAGE_REUSE_TIMEOUT_MS)) {
+			/* Re-use the latest voltage measurement rather than sampling again */
+			voltage = data->voltage_val;
+			rc = 0;
+		} else {
+			/* Measure voltage in this call */
+			rc = composite_read_micro(config->battery_voltage, SENSOR_CHAN_VOLTAGE,
+						  &voltage);
+			if (rc == 0) {
+				data->voltage_val = voltage;
+				data->voltage_time = k_uptime_ticks();
+			}
+		}
+		/* Convert voltage to state of charge */
 		val->relative_state_of_charge =
 			battery_soc_lookup(config->ocv_lookup_table, voltage) / 1000;
 		break;
@@ -106,7 +132,9 @@ static DEVICE_API(fuel_gauge, composite_api) = {
 			DT_INST_PROP_OR(inst, charge_full_design_microamp_hours, 0),               \
 		.chemistry = BATTERY_CHEMISTRY_DT_GET(inst),                                       \
 	};                                                                                         \
-	DEVICE_DT_INST_DEFINE(inst, NULL, NULL, NULL, &composite_##inst##_config, POST_KERNEL,     \
+	static struct composite_data composite_##inst##_data;                                      \
+	DEVICE_DT_INST_DEFINE(inst, NULL, NULL, &composite_##inst##_data,                          \
+			      &composite_##inst##_config, POST_KERNEL,                             \
 			      CONFIG_SENSOR_INIT_PRIORITY, &composite_api);
 
 DT_INST_FOREACH_STATUS_OKAY(COMPOSITE_INIT)