samples: data_forwarder: sample sensors only when connected

As a way to optimize current consumption, this commit introduces
gated sampling. It means that the device performs sensor sampling
only  when it can send sensor data. For the currently supported
transports it means:
- BLE NUS: sample when Bluetooth connection is active,
- serial: sample always

Ref. NCSDK-40322

Signed-off-by: Michał Grochala <michal.grochala@nordicsemi.no>

Author: grochu

samples/data_forwarder/Kconfig

@@ -104,6 +104,14 @@ config DATA_FWD_EXTRA_SENSOR_BME688
 	  Extend streamed samples with temperature, humidity, and pressure from BME688 at its native
 	  rate by upsampling (repeating the latest reading between fetches).
 
+config DATA_FWD_START_RETRY_MS
+	int "Sensor/session start retry interval (ms)"
+	range 10 10000
+	default 100
+	help
+	  Delay between retries when sensor or protocol session start fails while
+	  a transport link is up.
+
 config DATA_FWD_PROTO_MAX_CHANNELS
 	default 9 if DATA_FWD_SENSOR_BMI270 && DATA_FWD_EXTRA_SENSOR_BME688
 	default 6 if DATA_FWD_SENSOR_BMI270

samples/data_forwarder/README.rst

@@ -25,14 +25,18 @@ The default configuration on the nRF54L15 TAG reads a 6-axis BMI270 IMU and appe
 Overview
 ********
 
-The sample periodically reads data from an on-board sensor and forwards it to a host over :ref:`nrf:nus_service_readme` (default) or UART.
+The sample reads data from an on-board sensor and forwards it to a host over :ref:`nrf:nus_service_readme` (default) or UART.
 By default, the sample uses CBOR messages wrapped in COBS framing, as defined in :file:`cddl/data_forwarder.cddl`.
-Session metadata is sent periodically so a host can join an active stream.
+While a streaming session is active, session metadata is sent periodically so a host can join the stream.
 
 The sample performs the following operations:
 
-* Initializes the selected sensor driver and starts sampling at a fixed rate.
-* Encodes each sample and sends it through the configured transport.
+#. Initializes the sensor driver without starting periodic sampling.
+#. Waits until the configured transport is ready to carry data.
+#. Starts sensor sampling and a protocol session.
+#. Encodes each sample and sends it through the transport.
+
+If the sensor or the session fails to start while the link is up, the sample retries after the period configured with the ``CONFIG_DATA_FWD_START_RETRY_MS`` Kconfig option.
 
 Transport
 =========
@@ -42,9 +46,14 @@ Select the transport using the ``DATA_FWD_TRANSPORT`` Kconfig option.
 
 Bluetooth LE NUS
    Sample uses :ref:`nrf:nus_service_readme` for sending the protocol frames.
+   The sensors are sampled only while a central is connected.
+   Before that, advertising is enabled and the sensors remain stopped.
+   When the link drops, the sample stops the protocol session and the sensors, then waits for the next connection.
+   This keeps the sensors off while no host is collecting data.
 
 UART transport
    Sample uses UART for sending the protocol frames.
+   The data link is always treated as connected, so sampling starts after initialization and continues until the device is reset.
 
    .. note::
       UART transport is not available on the nRF54L15 TAG device without any external UART-to-USB converter.
@@ -185,8 +194,20 @@ Testing
       :class: highlight
 
       transport: BLE NUS transport ready
-      data_forwarder: Data forwarder started (sid 8779774)
+      data_forwarder: Data forwarder started
       transport: BLE connected
+      data_forwarder: Sampling session started (sid 8779774)
+
+   Sensor data is sent only after ``Sampling session started``.
+   When the central disconnects, the log continues with:
+
+   .. parsed-literal::
+      :class: highlight
+
+      transport: BLE disconnected
+      data_forwarder: Connection terminated
+
+   The device then advertises again and waits for the next connection before sampling.
 
 Collecting data on the host
 ===========================

samples/data_forwarder/src/main.c

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
  */
 
+#include <zephyr/kernel.h>
 #include <zephyr/logging/log.h>
 
 #include "protocol/protocol.h"
@@ -12,6 +13,87 @@
 
 LOG_MODULE_REGISTER(data_forwarder);
 
+/**
+ * @brief Stop sensor sampling and log a warning if stop fails.
+ */
+static void sensor_stop(void)
+{
+	int err = data_fwd_sensor_stop();
+
+	if (err) {
+		LOG_WRN("Sensor stop failed (err %d)", err);
+	}
+}
+
+/**
+ * @brief Start sensors and a protocol session, retrying on failure.
+ *
+ * Retries while the transport link remains up. Returns when both starts
+ * succeed or the link drops.
+ *
+ * @param session Session configuration passed to @ref proto_start_session().
+ */
+static void session_start_retry(const struct proto_session_config *session)
+{
+	int err;
+
+	while (transport_is_connected()) {
+		err = data_fwd_sensor_start();
+		if (err) {
+			LOG_WRN("Sensor start failed (err %d), retrying in %d ms", err,
+				CONFIG_DATA_FWD_START_RETRY_MS);
+			k_sleep(K_MSEC(CONFIG_DATA_FWD_START_RETRY_MS));
+			continue;
+		}
+
+		err = proto_start_session(session);
+		if (err) {
+			LOG_WRN("Failed to start session (err %d), retrying in %d ms", err,
+				CONFIG_DATA_FWD_START_RETRY_MS);
+			sensor_stop();
+			k_sleep(K_MSEC(CONFIG_DATA_FWD_START_RETRY_MS));
+			continue;
+		}
+
+		LOG_INF("Sampling session started (sid %u)", proto_get_session_id());
+		break;
+	}
+}
+
+/**
+ * @brief Fetch sensor samples and send them until the transport disconnects.
+ */
+static void stream_samples(void)
+{
+	int err;
+
+	while (transport_is_connected()) {
+		proto_value_t values[CONFIG_DATA_FWD_PROTO_MAX_CHANNELS];
+		size_t count;
+
+		err = data_fwd_sensor_fetch(values, ARRAY_SIZE(values), &count);
+		if (err) {
+			LOG_WRN("Sample fetch failed (err %d)", err);
+			continue;
+		}
+
+		err = proto_send_samples(values, count);
+		if (err) {
+			LOG_WRN("Sample send failed (err %d)", err);
+		}
+	}
+}
+
+/**
+ * @brief Stop the protocol session and sensor sampling after a connection ends.
+ */
+static void session_stop(void)
+{
+	proto_stop_session();
+	sensor_stop();
+	LOG_INF("Connection terminated");
+}
+
 int main(void)
 {
 	int err;
@@ -47,28 +129,13 @@ int main(void)
 		return err;
 	}
 
-	err = proto_start_session(&session);
-	if (err) {
-		LOG_ERR("Failed to start session (err %d)", err);
-		return err;
-	}
-
-	LOG_INF("Data forwarder started (sid %u)", proto_get_session_id());
+	LOG_INF("Data forwarder started");
 
 	while (1) {
-		proto_value_t values[CONFIG_DATA_FWD_PROTO_MAX_CHANNELS];
-		size_t count;
-
-		err = data_fwd_sensor_fetch(values, ARRAY_SIZE(values), &count);
-		if (err) {
-			LOG_WRN("Sample fetch failed (err %d)", err);
-			continue;
-		}
-
-		err = proto_send_samples(values, count);
-		if (err) {
-			LOG_WRN("Sample send failed (err %d)", err);
-		}
+		transport_wait_connected();
+		session_start_retry(&session);
+		stream_samples();
+		session_stop();
 	}
 
 	return 0;

samples/data_forwarder/src/sensor/adxl367.c

@@ -59,6 +59,16 @@ int data_fwd_sensor_init(void)
 	return 0;
 }
 
+int data_fwd_sensor_start(void)
+{
+	return 0;
+}
+
+int data_fwd_sensor_stop(void)
+{
+	return 0;
+}
+
 int data_fwd_sensor_fetch(proto_value_t *values, const size_t values_size, size_t *count)
 {
 	int err;

samples/data_forwarder/src/sensor/bme688.c

@@ -35,13 +35,26 @@ int data_fwd_sensor_init(void)
 		return -ENODEV;
 	}
 
+	return 0;
+}
+
+int data_fwd_sensor_start(void)
+{
 	const uint32_t period_ns = Z_HZ_ns / FREQUENCY_HZ;
 
 	k_timer_start(&fetch_timer, K_NO_WAIT, K_NSEC(period_ns));
 
 	return 0;
 }
 
+int data_fwd_sensor_stop(void)
+{
+	k_timer_stop(&fetch_timer);
+	k_sem_reset(&fetch_sem);
+
+	return 0;
+}
+
 int data_fwd_sensor_fetch(proto_value_t *values, const size_t values_size, size_t *count)
 {
 	int err;

samples/data_forwarder/src/sensor/bmi270.c

@@ -69,7 +69,7 @@ static bool bmi_set_attr(enum sensor_channel chan, enum sensor_attribute attr,
 
 int data_fwd_sensor_init(void)
 {
-	struct sensor_value full_scale, sampling_freq, oversampling;
+	struct sensor_value full_scale, oversampling;
 
 	if (!device_is_ready(dev_bmi)) {
 		return -ENODEV;
@@ -84,46 +84,72 @@ int data_fwd_sensor_init(void)
 	/* Setting scale in G to match the sensor scale */
 	full_scale.val1 = 2; /* G */
 	full_scale.val2 = 0;
-	sampling_freq.val1 = FREQUENCY_HZ; /* Hz. Performance mode */
-	sampling_freq.val2 = 0;
 	oversampling.val1 = 1; /* Normal mode */
 	oversampling.val2 = 0;
 
 	bool ok = true;
 
 	ok &= bmi_set_attr(SENSOR_CHAN_ACCEL_XYZ, SENSOR_ATTR_FULL_SCALE, &full_scale);
 	ok &= bmi_set_attr(SENSOR_CHAN_ACCEL_XYZ, SENSOR_ATTR_OVERSAMPLING, &oversampling);
-	/* Set sampling frequency last as this also sets the appropriate power mode. If already
-	 * sampling, change to 0.0Hz before changing other attributes
-	 */
-	ok &= bmi_set_attr(SENSOR_CHAN_ACCEL_XYZ, SENSOR_ATTR_SAMPLING_FREQUENCY, &sampling_freq);
 
 	/* Setting scale in degrees/s to match the sensor scale */
 	full_scale.val1 = 500; /* dps */
 	full_scale.val2 = 0;
-	sampling_freq.val1 = FREQUENCY_HZ; /* Hz. Performance mode */
-	sampling_freq.val2 = 0;
-	oversampling.val1 = 1; /* Normal mode */
-	oversampling.val2 = 0;
 
 	ok &= bmi_set_attr(SENSOR_CHAN_GYRO_XYZ, SENSOR_ATTR_FULL_SCALE, &full_scale);
 	ok &= bmi_set_attr(SENSOR_CHAN_GYRO_XYZ, SENSOR_ATTR_OVERSAMPLING, &oversampling);
-	/* Set sampling frequency last as this also sets the appropriate power mode. If already
-	 * sampling, change sampling frequency to 0.0Hz before changing other attributes
-	 */
-	ok &= bmi_set_attr(SENSOR_CHAN_GYRO_XYZ, SENSOR_ATTR_SAMPLING_FREQUENCY, &sampling_freq);
 
 	if (!ok) {
 		return -ENOTSUP;
 	}
 
+	return 0;
+}
+
+static int bmi270_set_sampling_frequency(uint16_t frequency_hz)
+{
+	struct sensor_value sampling_freq = {
+		.val1 = frequency_hz,
+		.val2 = 0,
+	};
+	bool ok = true;
+
+	ok &= bmi_set_attr(SENSOR_CHAN_ACCEL_XYZ, SENSOR_ATTR_SAMPLING_FREQUENCY, &sampling_freq);
+	ok &= bmi_set_attr(SENSOR_CHAN_GYRO_XYZ, SENSOR_ATTR_SAMPLING_FREQUENCY, &sampling_freq);
+
+	return ok ? 0 : -ENOTSUP;
+}
+
+int data_fwd_sensor_start(void)
+{
+	int err;
 	const uint32_t period_ns = Z_HZ_ns / FREQUENCY_HZ;
 
+	err = bmi270_set_sampling_frequency(FREQUENCY_HZ);
+	if (err) {
+		return err;
+	}
+
 	k_timer_start(&fetch_timer, K_NO_WAIT, K_NSEC(period_ns));
 
 	return 0;
 }
 
+int data_fwd_sensor_stop(void)
+{
+	int err;
+
+	k_timer_stop(&fetch_timer);
+	k_sem_reset(&fetch_sem);
+
+	err = bmi270_set_sampling_frequency(0);
+	if (err) {
+		return err;
+	}
+
+	return 0;
+}
+
 int data_fwd_sensor_fetch(proto_value_t *values, const size_t values_size, size_t *count)
 {
 	int err;

samples/data_forwarder/src/sensor/data_fwd_sensor.h

@@ -38,9 +38,38 @@ extern "C" {
  */
 int data_fwd_sensor_init(void);
 
+/**
+ * @brief Start sensor sampling.
+ *
+ * Enables periodic sampling in the selected sensor driver.
+ *
+ * @retval 0 Success.
+ * @retval -errno Negative error code on failure.
+ */
+int data_fwd_sensor_start(void);
+
+/**
+ * @brief Stop sensor sampling.
+ *
+ * Disables periodic sampling in the selected sensor driver. Implementations stop
+ * the fetch timer and reset the internal sample-ready semaphore with
+ * @c k_sem_reset(), clearing any pending tick signalled before stop.
+ *
+ * @note Do not call this while @ref data_fwd_sensor_fetch() is blocked in another
+ *       context: @c k_sem_reset() aborts outstanding takes with @c -EAGAIN.
+ *
+ * @retval 0 Success.
+ * @retval -errno Negative error code on failure.
+ */
+int data_fwd_sensor_stop(void);
+
 /**
  * @brief Read one sample from the sensor into @p values.
  *
+ * Blocks until the next sample period elapses. Call @ref data_fwd_sensor_start()
+ * before fetching and @ref data_fwd_sensor_stop() when sampling is no longer
+ * needed.
+ *
  * @note This function is blocking until sensor values are available.
  *
  * @param values       Output buffer for channel values.