Motion module

The motion module is responsible for generating events related to movement. The movement can be detected using the motion sensor, but it can also be sourced from GPIO pins or simulated.

Module events

Source Module	Input Event	This Module	Output Event	Sink Module
Buttons module	button_event	motion
Button simulator module
Function key module
Source modules for config_event	config_event
Power manager module	power_down_event
HID Service module	hid_report_sent_event
USB state module
HID Service module	hid_report_subscription_event
USB state module
Source modules for module_state_event	module_state_event
USB state module	usb_state_event
Bluetooth LE advertising module	wake_up_event
Buttons module
HID forward module
Motion module
Power manager module
Wheel module
			config_event	Sink modules for config_event
			module_state_event	Sink modules for module_state_event
			motion_event	HID state module
			wake_up_event	Sink modules for wake_up_event

Note

See the Application overview for more information about the event-based communication in the nRF Desktop application and about how to read this table.

Configuration

The motion module selects the source of movement based on the following configuration options:

• CONFIG_DESKTOP_MOTION_NONE - Module is disabled.

• CONFIG_DESKTOP_MOTION_SENSOR_PMW3360_ENABLE - Movement data is obtained from the gaming-grade PMW3360 motion sensor.

• CONFIG_DESKTOP_MOTION_SENSOR_PAW3212_ENABLE - Movement data is obtained from PAW3212 motion sensor.

• CONFIG_DESKTOP_MOTION_BUTTONS_ENABLE - Movement data is generated using buttons.

• CONFIG_DESKTOP_MOTION_SIMULATED_ENABLE - Movement data is simulated. The movement data generation can be controlled from Zephyr’s Shell.

See the following sections for more information.

Depending on the selected configuration option, a different implementation file is used during the build process.

You can use the CONFIG_DESKTOP_MOTION_PM_EVENTS Kconfig option to enable or disable handling of the power management events, such as power_down_event and wake_up_event. The option is enabled by default and depends on the CONFIG_CAF_PM_EVENTS Kconfig option. The option is unavailable for the motion module implementation that generates movement data using buttons (CONFIG_DESKTOP_MOTION_BUTTONS_ENABLE). The implementation does not react to the power management events.

Movement data from motion sensors

Selecting either of the motion sensors (CONFIG_DESKTOP_MOTION_SENSOR_PMW3360_ENABLE or CONFIG_DESKTOP_MOTION_SENSOR_PAW3212_ENABLE) adds the src/hw_interface/motion_sensor.c file to the compilation.

The motion sensor is sampled from the context of a dedicated thread. The option CONFIG_DESKTOP_MOTION_SENSOR_THREAD_STACK_SIZE is used to set the thread’s stack size.

The motion sensor default sensitivity and power saving switching times can be set with the following options:

• CONFIG_DESKTOP_MOTION_SENSOR_CPI - Default CPI.

• CONFIG_DESKTOP_MOTION_SENSOR_SLEEP1_TIMEOUT_MS - Sleep 1 mode default switch time.

• CONFIG_DESKTOP_MOTION_SENSOR_SLEEP2_TIMEOUT_MS - Sleep 2 mode default switch time.

• CONFIG_DESKTOP_MOTION_SENSOR_SLEEP3_TIMEOUT_MS - Sleep 3 mode default switch time.

For more information, see the sensor documentation and the Kconfig help.

Movement data from buttons

Selecting the CONFIG_DESKTOP_MOTION_BUTTONS_ENABLE option adds the src/hw_interface/motion_buttons.c file to the compilation.

The movement data is generated when pressing a button. The module detects the button presses by relying on the received button_event. Generating motion for every direction is triggered using a separate button. Key ID (button_event.key_id) of the button used to generate motion for a given direction can be configured with a dedicated Kconfig option:

• Up (CONFIG_DESKTOP_MOTION_BUTTONS_UP_KEY_ID)

• Down (CONFIG_DESKTOP_MOTION_BUTTONS_DOWN_KEY_ID)

• Left (CONFIG_DESKTOP_MOTION_BUTTONS_LEFT_KEY_ID)

• Right (CONFIG_DESKTOP_MOTION_BUTTONS_RIGHT_KEY_ID)

Pressing and holding one of the mentioned buttons results in generating data for movement in a given direction. The CONFIG_DESKTOP_MOTION_BUTTONS_MOTION_PER_SEC can be used to control a motion generated per second during a button press. By default, 1000 of motion is generated per second during a button press.

Simulated movement data

Selecting the CONFIG_DESKTOP_MOTION_SIMULATED_ENABLE option adds the src/hw_interface/motion_simulated.c file to the compilation.

The generated movement data traces a predefined octagon path. As soon as the HID subscriber connects, the motion data is sent to it automatically. The X and Y coordinates of subsequent vertexes of the octagon are defined as the coords array in the module’s source code.

You can configure the path with the following options:

• CONFIG_DESKTOP_MOTION_SIMULATED_EDGE_TIME - Sets how long each edge is traced. To speed up calculations, this Kconfig option value must be set to a power of two.

• CONFIG_DESKTOP_MOTION_SIMULATED_SCALE_FACTOR - Scales the size of the octagon. The Kconfig option’s value is used as the SCALE factor in the coords array.

Shell integration

If the Zephyr shell is enabled (meaning the CONFIG_SHELL Kconfig option is set), the motion module registers a motion_sim shell module and links to it two commands:

• start - Start sending simulated movement data to the HID subscriber.

• stop- Stop sending simulated movement data to the HID subscriber.

If the shell is enabled, motion generation no longer starts automatically when the HID subscriber connects and after system wakeup (when wake_up_event is received). The simulated movement data generation needs to be triggered using a shell command.

Configuration channel

In a configuration where either CONFIG_DESKTOP_MOTION_SENSOR_PMW3360_ENABLE or CONFIG_DESKTOP_MOTION_SENSOR_PAW3212_ENABLE is used, you can configure the module through the Configuration channel. In these configurations, the module is a configuration channel listener and it provides the following configuration options:

• OPT_DESCR_MODULE_VARIANT

  This is a special, read-only option used to provide information about the module variant. For the motion sensor, the module variant is a sensor model written in lowercase, for example pmw3360 or paw3212.

  The HID configurator for nRF Desktop uses the sensor model to identify the descriptions of the configuration options. These descriptions are defined in the nrf/scripts/hid_configurator/modules/module_config.py.

• cpi

  The motion sensor CPI.

• downshift, rest1, rest2

  These firmware option names correspond to switch times of motion sensor modes, namely the sleep modes of PAW3212 and the rest modes of PMW3360. These modes are used by a motion sensor to reduce the power consumption, but they also increase the sensor’s response time when a motion is detected after a period of inactivity.

  The HID configurator for nRF Desktop refers to these mode options using names that depend on the sensor variant:

  • For PAW3212, the mode options are called respectively: sleep1_timeout, sleep2_timeout, and sleep3_timeout.

  • For PMW3360, the mode options are called respectively: downshift_run, downshift_rest1, and downshift_rest2.

Implementation details

Implementation details depend on the selected implementation. See the following sections for details specific to a given implementation. The Motion report rate section is common for all implementations and contains information useful for the HID report rate measurements.

Movement data from motion sensors

This section describes the motion module implementation for motion sensors.

Motion sensor

The motion module samples movement data from the motion sensor using the motion sensor driver.

The name of the device linked with the driver changes depending on the selected sensor. The same is true for the names of sensor configuration options. The nRF Desktop application aggregates the sensor-specific information and translates them to the application abstracts in the configuration/common/motion_sensor.h header file.

Sampling thread

The motion module uses a dedicated sampling thread to sample data from the motion sensor. The reason for using the sampling thread is the long time required for data to be ready. Using a dedicated thread also simplifies the module because the interaction with the sensor becomes sequential. The sampling thread priority is set to 0 (the highest preemptive thread priority) because it is assumed that the data sampling happens in the background.

The sampling thread stays in the unready state blocked on a semaphore. The semaphore is triggered when the motion sensor trigger sends a notification that the data is available or when other application event requires the module interaction with the sensor (for example, when configuration is submitted from the host).

Sampling pipeline

The motion module starts in STATE_DISABLED and after initialization enters STATE_DISCONNECTED. When disconnected, the module is not generating motion events, but the motion sensor is sampled to make sure its registers are cleared.

Upon connection, the following happens:

1. The motion module switches to the STATE_IDLE state, in which the module waits for the motion sensor trigger.

2. When a first motion is detected and the device is connected to a host (meaning it can transmit the HID data out), the module completes the following actions:

   1. Switches to STATE_FETCHING.

   2. Samples the motion data.

   3. Submits the motion_event.

   4. Waits for the indication that the motion_event data was transmitted to the host. This is done when the module receives the hid_report_sent_event event.

3. At this point, a next motion sampling is performed and the next motion_event sent.

The module continues to sample data until disconnection or when there is no motion detected. The motion module assumes no motion when a number of consecutive samples equal to CONFIG_DESKTOP_MOTION_SENSOR_EMPTY_SAMPLES_COUNT returns zero on both axis. In such case, the module will switch back to STATE_IDLE and wait for the motion sensor trigger.

Movement data from buttons

Motion is generated based on the total time a button is pressed. The time measurements rely on the hardware clock. If available, the module uses the k_cycle_get_64() function to read the hardware clock. Otherwise, the k_cycle_get_32() function is used for that purpose.

When a HID subscriber is connected, that is when the device is connected either over USB or Bluetooth LE, the module forwards the generated motion to the subscriber using motion_event. The first motion_event is generated when a button is pressed. The subsequent motion_event is submitted when the previously generated motion data is sent to the subscriber, that is when hid_report_sent_event is received by the module.

Simulated movement data

Current position in the generated trajectory is determined by a hardware clock. If available, the module uses the k_cycle_get_64() function to read the hardware clock when generating motion. Otherwise, the k_cycle_get_32() function is used for that purpose.

When a HID subscriber connects, that is when the device is connected either over USB or Bluetooth LE, the module submits the first motion_event containing motion calculated from the previously reported position. The subsequent motion_event is submitted when the previously generated motion data is sent to the subscriber, that is when hid_report_sent_event is received by the module. The values of motion for X and Y axes are generated from an updated timestamp and the previously reported position.

Motion report rate

If the device is polled for HID data with high frequency, that is when the hid_report_sent_event is received often by the module, the generated motion_event may contain value of 0 for both X and Y axes.

Note

A HID report with reporting motion of 0 for both axes may be ignored by host computer’s tools that analyze the HID report rate.

You can perform the following actions to ensure that non-zero motion is reported by the motion module:

• Movement data from motion sensors By constantly moving your mouse, you can ensure that the used motion sensor always reports motion for at least one of the axes. You can also increase the motion sensor’s CPI to make the sensor report bigger values of motion per every inch of distance covered by the mouse.

• Movement data from buttons Increase a motion generated per second during a button press.

• Simulated movement data Increase the value of the used scale factor or reduce time for transition between two points in the trajectory to increase the generated motion values. Keep in mind that the generated shape is periodically repeated, so the transition time between two points in the trajectory should not be too short. If it is, it might cause the same point to generate twice which would also result in submitting a motion_event with values set to 0 for both axes.

See the Configuration section for details on how to modify configuration for a given implementation of the module.