VL53L8CX Sensor Module
User Manual

Introduction

The purpose of this user manual is to explain how to handle the VL53L8X Time- of-Flight (ToF) sensor, using the ultra lite driver (ULD) API. It describes the main functions to program the device, the calibrations, and the output results.
Based on ST’s FlightSense technology, the VL53L8CX incorporates an efficient metasurface lens (DOE) placed on the laser emitter enabling the projection of a 45° x 45°  square FoV onto the scene.
Its multizone capability provides a matrix of 8×8 zones (64 zones) and can work at fast speeds (60 Hz) up to 400 cm.
Thanks to the autonomous mode with programmable distance threshold, the VL53L8CX is perfect for any application requiring low-power user detection. ST’s patented  algorithms and innovative module construction allow the VL53L8CX to detect, in each zone, multiple objects within the FoV with depth understanding. ST histogram  algorithms ensure cover glass crosstalk immunity beyond 60 cm.
Like all Time-of-Flight (ToF) sensors based on ST’s FlightSense technology, the VL53L8CX records, in each zone, an absolute distance regardless of the target color and  reflectance.
Housed in a miniature reflowable package that integrates a SPAD array, the VL53L8CX achieves the best ranging performance in various ambient lighting conditions, and for  a wide range of cover glass materials.
All of ST’s ToF sensors integrate a VCSEL that emits a fully invisible 940 nm IR light, which is totally safe for the eyes (Class 1 certification).

Acronyms and abbreviations

number of photons into the SPAD array)
RAM| random access memory
SCL| serial clock line
SDA| serial data
SPAD| single photon avalanche diode
ToF| Time-of-Flight
ULD| ultra lite driver
VCSEL| vertical cavity surface emitting diode
Xtalk| crosstalk

Functional description

2.1 System overview
The VL53L8CX system is composed of a hardware module and the ultra lite driver software (VL53L8CX ULD) running on a host (see figure below). The hardware module  contains the ToF sensor. STMicroelectronics delivers the software driver, which is referred to in this document as “the driver”. This document describes the functions of the  driver, which are accessible to the host. These functions control the sensor and get the ranging data.

2.2 Effective orientation
The module includes a lens over the RX aperture, which flips (horizontally and vertically) the captured image of the target. Consequently, the zone identified as zone 0, in the  bottom left of the SPAD array, is illuminated by a target located at the top right-hand side of the scene.

2.3 Schematics and I2C/SPI configuration
The communication between driver and firmware is handled by the I2C or SPI. The maximum capability of the I2C is 1 MHz, and the maximum capability of the SPI is 20  MHz. The implementation of each communication protocol requires pull ups as described into the VL53L8CX datasheet.
The VL53L8CX device has a default I2C address of 0x52. However, it is possible to change the default address to avoid conflicts with other devices, or to facilitate adding  multiple VL53L8CX modules to the system for a greater system FoV. The I2C address can be changed using the vl53l8cx_set_i2c_address() function. To use the SPI, the multisensor is wired using an independant slave configuration (the NCS pin).

To allow a device to have its I2C address changed without affecting others on the I2C bus, it is important to
disable the I2C communication of the devices not being changed. The procedure is as follows:

1. Power up the system as normal.
2. Pull down the LPn pin of the device that will not have its address changed.
3. Pull up the LPn pin of the device that has the I2C address changed.
4. Program the I2C address to the device using function set_i2c_address() function.
5. Pull up the LPn pin of the device not being reprogrammed.
   All devices should now be available on the I2C bus. Repeat the above steps for all the devices in the system that require a new I2C address.

Package content and data flow

3.1 Driver architecture and content
The VL53L8CX ULD package is composed of four folders. The driver is located in the folder /VL53L8CX_ULD_API.
The driver is composed of mandatory and optional files. Optional files are plugins used to extend ULD features.
Each plugin starts with the word “vl53l8cx_plugin” (e.g vl53l8cx_plugin_xtalk.h). If the user does not want the proposed plugins, they can be removed without impacting the  other driver features. The following figure represents the mandatory files and the optional plugins.

Note:
The user also needs to implement two files located in the /Platform folder. The proposed platform is an empty shell, and must be filled with dedicated functions.
Platform.h file contains mandatory macros to use the ULD. All the file content is mandatory to correctly use the ULD.

3.2 Calibration flow
Crosstalk (Xtalk) is defined as the amount of signal received on the SPAD array, which is due to VCSEL light reflection inside the protective window (cover glass) added on  top of the module. The VL53L8CX module is self- calibrated, and can be used without any additional calibration.
Xtalk calibration may be required if the module is protected by a cover glass. The VL53L8CX is immune to Xtalk beyond 60 cm thanks to a histogram algorithm. However, at  short distances below 60 cm, Xtalk can be larger than the actual returned signal. This gives a false target reading or makes targets appear closer than they really are. All  Xtalk calibration functions are included in a Xtalk plugin (optional). The user needs to use the file ‘vl53l8cx_plugin_xtalk’.
The Xtalk can be calibrated once, and data can be saved so it can be re-used later. A target at fixed distance, with a known reflectance is required. The minimum distance  required is 600 mm, and the target must cover the whole FoV. Depending on the setup, the user can modify settings in order to adapt the Xtalk calibration, as proposed in the  following table.

Table 1. Available settings for calibration

Setting| Min| Proposed by
STMicroelectronics| Max
---|---|---|---
Distance [mm]| 600| 600| 3000
Number of samples| 1| 4| 16
Reflectance [%]| 1| 3| 99

Note:
Increasing the number of samples increases the accuracy, but it also increases the time for calibration. The time relative to the number of samples is linear, and values follow  the approximate timeout:

• 1 sample ≈ 1 second
• 4 samples ≈ 2.5 seconds
• 16 samples ≈ 8.5 seconds
  The calibration is performed using the function vl53l8cx_calibrate_xtalk(). This function can be used at any time.
  However, the sensor must be initialized first. The following figure represents the xtalk calibration flow.

Figure 7. Xtalk calibration flow

3.3 Ranging flow
The following figure represents the ranging flow used to get measurements. Xtalk calibration and optional function calls must be used before starting the ranging session. The  get/set functions cannot be used during a ranging session, and ‘on-the-fly’ programming is not supported.

Available features

The VL53L8CX ULD API includes several functions, which allow the user to tune the sensor, depending on the use case. All the functions available for the driver are  escribed in the following sections.
4.1 Initialization
Initialization must be done before using the VL53L8CX sensor. This operation requires the user to:

1. Power on the sensor (VDDIO, AVDD, CORE_1V8, and LPn pins set to High
2. Call the function vl53l8cx_init(). The function copies the firmware (~84 Kbytes) to the module. This is done by loading the code over the I2C/SPI interface, and  performing a boot routine to complete the initialization.

4.2 Sensor reset management
To reset the device, the following pins need to be toggled:

1. Set pins VDDIO, AVDD, and CORE_1V8 pins to low.
2. Wait 10 ms.
3. Set pins VDDIO, AVDD, and CORE_1V8 pins to high.

Note:
Toggling only I2C_RST pin resets the I2C communication.
4.3 Resolution
The resolution corresponds to the number of available zones. The VL53L8CX sensor has two possible resolutions: 4×4 (16 zones) and 8×8 (64 zones). By default the sensor  is programmed in 4×4.
The function vl53l8cx_set_resolution() allows the user to change the resolution. As the ranging frequency depends on the resolution, this function must be used before  updating the ranging frequency. Moreover, changing the resolution also increases the traffic size on the I2C/SPI bus when results are read.
4.4 Ranging frequency
Ranging frequency can be used to change the measurement frequency. As the maximum frequency is different between 4×4 and 8×8 resolutions, this function needs to be  used after choosing a resolution. The minimum and maximum allowed values are listed in the following table.

Table 2. Minimum and maximum ranging frequencies

Resolution| Min ranging frequency [Hz]| Max ranging frequency [Hz]
---|---|---
4×4| 1| 60
8×8| 1| 15

Ranging frequency can be updated using function vl53l8cx_set_ranging_frequency_hz(). By default, the ranging frequency is set to 1 Hz.

4.5 Ranging mode
Ranging mode allows the user to choose between ranging in high performance or low power consumption.
There are two modes proposed:

• Continuous: The device continuously grabs frames with a ranging frequency defined by user. The VCSEL is enabled during all ranging, so maximum ranging distance  and ambient immunity are better. This mode is advised for fast ranging measurements or high performances.
• Autonomous: This is the default mode. The device continuously grabs frames with a ranging frequency defined by the user. The VCSEL is enabled during a period  defined by the user, using function vl53l8cx_set_integration_time_ms(). As the VCSEL is not always enabled, the power consumption is reduced. The benefits are more  obvious with a reduced ranging frequency. This mode is advised for low power applications.
  The ranging mode can be changed using function vl53l8cx_set_ranging_mode().

4.6 Integration time
Integration time is a feature only available using Autonomous ranging mode (refer to Section 4.5 Ranging mode).
It allows the user to change the time while VCSEL is enabled. Changing integration time if Ranging mode is set to continuous has no effect. The default integration time is  set to 5 ms.
The effect of integration time is different for 4×4 and 8×8 resolutions. Resolution 4×4 is composed of one integration time, and 8×8 resolution is composed of four  integration times. The following figures represent the VCSEL emission for both resolutions.

The sum of all integration times + 1 ms overhead must be lower than the measurement period. Otherwise the ranging period is automatically increased.

4.7 Power modes
Power modes can be used to reduce the power consumption when the device is not used. The VL53L8CX can operate in one of the following power modes:

• Wake-up: The device is set in HP idle (high power), waiting for instructions.
• Sleep: The device is set in LP idle (low power), the low power state. The device cannot be used until set in wake-up mode. This mode retains the firmware and the configuration.
  The power mode can be changed using function vl53l8cx_set_power_mode(). The default mode is wake-up.
  Note:
  If the user wants to change the power mode, the device must not be in a ranging state.

4.8 Sharpener
The signal returned from a target is not a clean pulse with sharp edges. The edges slope away and may affect the distances reported in adjacent zones. The sharpener is used  to remove some or all of the signal caused by veiling glare.
The example shown in the following figure represents a close target at 100 mm centered in the FoV, and another target, further behind at 500 mm. Depending on the  sharpener value, the close target may appear in more zones than the real one.

Figure 11. Example of scene using several sharpener values

Sharpener can be changed using function vl53l8cx_set_sharpener_percent(). The allowed values are between 0 % and 99 %. The default value is 5 %.

4.9 Target order
The VL53L8CX can measure several targets per zone. Thanks to the histogram processing, the host is able to choose the order of reported targets. There are two options:

• Closest: The closest target is the first reported
• Strongest: The strongest target is the first reported
  The target order can be changed using function vl53l8cx_set_target_order(). The default order is Strongest.
  The example in the following figure represents the detection of two targets. One at 100 mm with a low reflectance, and one at 700 mm with a high reflectance.

4.10 Multiple targets per zone
The VL53L8CX can measure up to four targets per zone. The user can configure the number of targets returned by the sensor.
Note:
The minimum distance between two targets to be detected is 600 mm.
The selection is not possible from the driver; it has to be done in the ‘platform.h’ file. The macro
VL53L8CXNB TARGET_PER_ZONE needs to be set to a value between 1 and 4. The target order described in Section 4.9 Target order directly impacts the order of  detected target. By default, the sensor only outputs a maximum of one target per zone.
Note:
An increased number of targets per zone increases the required RAM size.
4.11 Xtalk margin
The Xtalk margin is an additional feature only available using the plugin Xtalk. The .c and .f files ‘vl53l8cx_plugin_xtalk’ need to be used.
The margin is used to change the detection threshold when a cover glass is present on the top of the sensor. The threshold can be increased to ensure that the cover glass is  never detected, after setting Xtalk calibration data.
For example, the user can run a Xtalk calibration on one single device, and re-use the same calibration data for all other devices. The Xtalk margin can be used to tune the  Xtalk correction. The figure below represents the Xtalk margin.

Figure 13. Xtalk margin

4.12 Detection thresholds
In addition to the regular ranging capabilities, the sensor can be programmed to detect an object under certain predefined criteria. This feature is available using the plugin  “detection thresholds”, which is an option not included by default in the API. The files called ‘vl53l8cx_plugin_detection_thresholds’ need to be used.
The feature can be used to trigger an interrupt to pin A1 (INT) when conditions defined by the user are met. There are three possible configurations:

• Resolution 4×4: using 1 threshold per zone (total of 16 thresholds)

• Resolution 4×4: using 2 thresholds per zone (total of 32 thresholds)

• Resolution 8×8: using 1 threshold per zone (total of 64 thresholds)
  Whatever the configuration used, the procedure for creating thresholds and the RAM size are the same. For each threshold combination, several fields need to be filled:

• Zone id: id of the selected zone (refer to Section 2.2 Effective orientation)

• Measurement: measurement to catch (distance, signal, number of SPADs, …)

• Type: windows of measurements (in windows, out of windows, below low threshold, …)

• Low threshold: low threshold user for trigger. User does not need to set the format, it is automatically handled by the API.

• High threshold: high threshold user for trigger. User does not need to set the format, it is automatically handled by the API.

• Mathematic operation: only used for 4×4 – 2 threshold combinations per zone. The user can set a combination using several thresholds in one zone.

4.13 Interrupt autostop
The interrupt autostop feature is used to abort the ranging session during a measurement. By default, the sensor cannot be stopped during a measurement, because the frame  measurements need to be completed. However, by using autostop, the frame measurements are aborted when an interrupt is triggered.
The autostop feature is useful when it is combined with a detection threshold. When a target is detected, the current measurement is automatically aborted. Autostop can be  used in a customer state machine to switch quickly to another sensor configuration.
An interrupt autostop feature can be enabled by using the function vl53l8cx_set_detection_threshold_auto_stop().
After a measurement is aborted, it is recommended to stop the sensor by using the function vl53l8cx_stop_ranging().
4.14 Motion indicator
The VL53L8CX sensor has an embedded Firmware feature allowing motion detection in a scene. The motion indicator is computed between sequential frames. This option  is available using the plugin ‘vl53l8cx_plugin_motion_indicator’.
The motion indicator is initialized using the vl53l8cx_motion_indicator_init() function. If the user wants to change the sensor resolution, he must update the motion indicator  resolution using the dedicated function: vl53l8cx_motion_indicator_set_resolution().
The user may also change the minimum and maximum distances for detecting motion. The difference between the minimum and maximum distances cannot be greater than  1500 mm. By default, distances are initialized with values between 400 mm and 1500 mm.
Results are stored in the field ‘motion_indicator’. In this field, the array ‘motion’ gives a value containing the motion intensity per zone. A high value indicates high motion  variation between frames. A typical movement gives a value between 100 and 500. This sensitivity depends on the integration time, target distance, and target reflectance.
An ideal combination for low power applications is the use of the motion indicator with Autonomous ranging mode, and detection thresholds programmed on the motion. This allows detection of movement variations in the FoV with minimum power consumption.

4.15 External synchronization pin
An external trigger source can be used to synchronize acquisitions. When the external synchronization is enabled, the VL53L8CX waits for an interrupt on the SYNC pin to  start the next acquisition. To use this feature, the SYNC pin (B1) needs to be connected as described in the product datasheet.
There are no specific requirements for using the external synchronization. However, the VL53L8CX ranging frequency should be higher than the external signal frequency.
The external synchronization can be enabled or disabled by using the function vl53l8cx_set_external_sync_pin_enable(). Ranging can be started as usual by using the function vl53l8cx_start_ranging(). When a user wants to stop the sensor, it is recommended to toggle the SYNC pin to unpause the VL53L8CX firmware.
A topical flow for using the external synchronization pin is shown below in Section 4.15 .

Figure 14. External synchronization flow

Ranging results

5.1 Available data
An extensive list of target and environment data may be output during ranging activities. The following table describes the parameters available to the user.
Table 3. Available output using VL53L8CX sensor

Element

| Nb bytes (RAM)| Unit|

Description

---|---|---|---
Ambient per SPAD| 256| Kcps/SPAD| Ambient rate measurement performed on the SPAD array, with no active photon emission, to measure the ambient signal rate due to noise.
Number of targets detected|

64

| None| Number of detected targets in the current zone. This value should be the first one to check to know a measurement validity.
Number of SPADs enabled| 256| None| Number of SPADs enabled for the current measurement. A far or low reflective target will activate more SPADs.

Signal per SPAD

| 256 x nb targets programmed|

Kcps/SPAD

| Quantity of photons measured during the VCSEL

pulse.

Range sigma

| 128 x nb targets programmed|

Millimeter

| Sigma estimator for the noise in the reported target distance.

Distance

| 128 x nb targets programmed| Millimeter| Target distance
Target status| 64 x nb targets programmed| None| Measurements validity. See Section 5.5 Results interpretation for more information.
Reflectance| 64 x number targets programmed| Percent| Estimated target reflectance in percent
Motion indicator| 140| None| Structure containing the motion indicator results. The field ‘motion’ contains the motion intensity.

Note:
For several elements (signal per spad, sigma, …) access to data is different if user has programmed more than 1 target per zone (see Section 4.10 Multiple targets per zone).  See example codes for more information.

5.2 Customize output selection
By default, all VL53L8CX outputs are enabled. If needed, the user can disable some sensor output.
Disabling measurements is not available on the driver; it must be performed in the ‘platform.h’ file. The user can declare the following macros to disable outputs:

define VL53L8CX _DISABLE_AMBIENT_PER_SPAD

define VL53L8CX _DISABLE_NB_SPADS_ENABLED

define VL53L8CX _DISABLE_NB_TARGET_DETECTED

define VL53L8CX _DISABLE_SIGNAL_PER_SPAD

define VL53L8CX _DISABLE_RANGE_SIGMA_MM

define VL53L8CX _DISABLE_DISTANCE_MM

define VL53L8CX _DISABLE_TARGET_STATUS

define VL53L8CX _DISABLE_REFLECTANCE_PERCENT

define VL53L8CX _DISABLE_MOTION_INDICATOR

Consequently, the fields are not be declared in the results structure, and the data is not transferred to the host.
The RAM size and I2C/SPI size are reduced.
To ensure data consistency, ST recommends to always keep ‘number of target detected’ and ‘target status’ enabled. It allows filtering the measurements depending of the  target status (refer to Section 5.5 Results interpretation).

5.3 Getting ranging results
During the ranging session, there are two ways to know if new ranging data is available:

• Polling mode: Continuously uses function vl53l8cx_check_data_ready(). It detects a new stream count returned by the sensor.
• Interrupt mode: Waits for an interrupt raised on pin A1 (INT). The interrupt is automatically cleared after ~100 μs.
  When new data is ready, the results can be read using function vl53l8cx_get_ranging_data(). It returns an updated structure containing all selected output. As the device  is asynchronous, there is no interrupt to clear to continue the ranging session.
  This feature is available for both continuous and autonomous ranging modes.

5.4 Using raw firmware format
After transferring ranging data through I2C/SPI, there is a conversion between the firmware format and the host format. This operation is typically performed to have a  ranging distance in millimeters as a default output of the sensor. If the user wants to use the firmware format, the following macro must be defined in the platform file:
VL53L8CX#define VL53L8CX _USE_RAW_FORMAT
5.5 Results interpretation
The data returned by the VL53L8CX can be filtered in order to take into account the target status. The status indicates the measurement validity. The full status list is described in the following table.

Table 4. List of available target status

targets.
255| No target detected (only if number of target detected is enabled)

To have consistent data, the user needs to filter invalid target status. To give a confidence rating, a target with status 5 is considered as 100 % valid. A status of 6 or 9 can be  considered with a confidence value of 50 %. All other statuses are below 50 % confidence level.

5.6 Driver errors
When an error occurs using VL53L8CX sensor, the driver returns a specific error. The following table lists the possible errors.

Table 5. List of errors available using the driver

frequency too high, …)
255| Major error. Usually a timeout error, due to an I2C/SPI error.
other| Combination of multiple errors described above

Note:
More error codes can be implemented by the host using the platform files.

Table 6. Document revision history

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