The Evaluation Kit runs with a demo application, the Explorer App. It establishes an USB or UART connection to the AFBR-S50 Explorer GUI that is running on a Windows PC. The Explorer App provides a serial interface that allows the GUI to transfer configuration and calibration parameters and receive measurement data.

The Explorer App hosts the AFBR-S50 Core Library and connects it to a a Serial Communication Interface (SCI) to allow accessing the sensor from the PC. The SCI implements an API that is equivalent to the AFBR-S50 API, but accessible via a serial peripheral interface such as USB or UART. The SCI is an communication protocol that can be ported to almost every serial communication interface. The Explorer App functions as the communication device, while an external host, e.g. the AFBR-S50 Explorer GUI running on a Windows machine, is implemented as a controller.

The following chapter gives an overview about the architecture and implementation of the SCI module in the Explorer App. After gaining a basic understanding of the implementation, it should be an easy task to adopt the Explorer App along with the interface to the user requirements and create the host interface that can connect to the provided SCI.

Fig. 5.1: An overview of the Explorer App Architecture. The Explorer App is hosting the AFBR-S50 API on the one hand and a serial communication interface on the other hand to connect to an external host via serial peripheral like USB or UART. The external host would be a PC with running the AFBR-S50 Explorer GUI in case of the evaluation kit.

Note: from SDK version v1.4.4 onwards, the Explorer App is able to connect to multiple devices at the same time. This can be either achieved by using multiple evaluation kits and establishing an unique USB/UART connection to each board or by using a multi-device adapter board and run multiple instances of the AFBR-S50 API on a single MCU.

Build And Run the Explorer App

The source files are located in Sources/ExplorerApp of the AFBR-S50 GitHub repository and the projects are located in Project/<IDE>/AFBR_S50_ExplorerApp_<MCU>, depending on your target. The following target/IDE combinations are provided:

MCU	IDE	Path	Comment
NXP MKL46z	MCUXpressoIDE	`Projects/MCUXpressoIDE/AFBR_S50_ExplorerApp_KL46z`	Runs on FRDM-KL46Z evaluation board by NXP
NXP MKL17z	MCUXpressoIDE	`Projects/MCUXpressoIDE/AFBR_S50_ExplorerApp_KL17z`
STM32 F401RE	STM32CubeIDE	`Projects/STM32CubeIDE/AFBR_S50_ExplorerApp_F401RE`	Runs on NUCLEO-F401RE evaluation board by STM
Renesas RA4M2	e² Studio	`Projects/e2studio/AFBR_S50_ExplorerApp_RA4M2`	Runs on AFBR-S50 Reference Board by MikroElektronika

See also Build And Run Projects for an overview of featured boards/MCUs/IDEs and how to import and build the project in the corresponding IDE. Just follow the steps and skip the steps that connect to a serial terminal. and connect via USB or UART with a SCI controller (e.g. using the AFBR-S50 Explorer GUI or the Python example) instead.

Serial Communication Interface

Introduction

The basic idea is to define simple commands (= 7-bit values) to access the AFBR-S50 API. These commands are either transactions with or without data phase. Usually, the commands with data phase are simple getter and setter commands, e.g. "get distance" or "set frame rate".

Each data package starts by the identifier byte (= command byte), followed by a predefined number of data bytes which may be zero. Finally, the package finishes with a CRC byte to verify the data integrity. These byte sequences are called data frames.

A new feature is the addressing mechanism which has been introduces with SDK v1.4.4. The addressing mechanism allows to connect to multiple devices at the same time. In order to distinguish between commands with and without addressing, the 8th bit of the command byte is used. If the 8th bit is set, the command will contain an addressing byte immediately after the command byte. Otherwise, the command has not addressing byte and it is addressed to the default device.

The data frames are transmitted in different manner dependent on the underlying hardware interface. However, at a higher level, the command or message layer does not depend on the hardware interface. The communication happens between various systems, whereby one of them needs to be the master. All other participants are slaves which are controlled by the master. Depending on the underlying hardware, there can be a single (e.g. UART) or multiple (e.g. SPI or I2C) slaves. In case of the latter, the slaves are addressed by the master via the corresponding hardware architecture.

Each command to a slave is acknowledged after successful execution. If any error occurs, a not-acknowledge is invoked by the slave. Only a single command can be sent to the slave at once, i.e. the slave has to (not-)acknowledge the command before the master can send another one. A timeout can be implemented in the master to check if the slave responds within a given time and is still alive or if it is stuck in some invalid state.

Architecture

The architecture of the SCI (see Fig. 5.2) consists of several layer. Each is communicating on a specific level with its corresponding counterpart.

The lowermost layer is the hardware layer which is the hardware abstraction layer for the underlying hardware, e.g. UART or I2C. The hardware layer transfers data bits over a physical link.

The second layer is the data link layer. It takes care about a reliable transmission of data frames, i.e. a bunch of associated bytes. Therefore it is responsible for putting together data by framing it with a start and a stop byte. It also applies the byte stuffing and finally adds an CRC value to allow the detection of invalid frames.

The third layer is the protocol or message layer. Its responsibility is to transfer messages or commands via the data link layer. In order to achieve a reliable connection, the handshaking is also implemented into this layer via acknowledgment messages.

The last and uppermost layer is the Application or API layer. It provides high level functionality to transfer application specific data.

Fig. 5.2: The SCI Layer Model.

To sum up:

Hardware Layer: transfer bits over physical medium
Data Link Layer: data framing, byte stuffing and CRC
Message Layer: command, messages and handshaking
Application Layer: host application specific data

See also: See also the OSI model, which was used as an reference: https://en.wikipedia.org/wiki/OSI_model

Hardware Layer

UART

The UART interface support only point-to-point communications. It has an independent line for sending as well as receiving data and the slave can transmit data at any time without special actions from the master are required. Therefore, this mode does not require an interrupt line to inform the master about new data ready or error conditions. The data is just transmitted immediately which means that the master must always listen to its Rx line.

The data framing is realized with byte stuffing. There are three special bytes, the start, stop and escape bytes, that are used to determine the boundaries of a data frame. In order to make the start and stop bytes unique and keep the full data range per byte, the corresponding data bytes are inverted and escaped with the escape byte.

In order to provide a handshaking mechanism, the slave acknowledges the successful reception of an data frame (and the successful invocation of the corresponding command) with a short ACK (= acknowledged) or NAK (= not_acknowledged) message within a define time.

Furthermore, due to the independent TX line, the special feature of log and error messages are supported by the UART protocol.

SPI

The SPI interface support multiple slave mode via the chip select (CS) lines. Data transfers can only be initiated by the master and thus an extra IRQ line is used to give the slave a chance to call the masters attention to it. This is however optional and the alternative method would be polling the status by the master.

The data framing is realized via the CS. After the command byte, the data is transferred either by on the MISO or MOSI for a read or write command respectively.

The handshaking is implemented via the IRQ line. If an error occurs, the IRQ is pulled to low. The master can now read the corresponding status in order to get the root cause of the IRQ. Also the new measurement ready event is determined via the IRQ. In addition, the acknowledgment of the successful reception and execution of a command could be implemented via the interrupt; the master would responsible for reading the acknowledge status.

I2C

The I2C interface supports multiple slave mode via the device address bytes. Data transfers can only be initiated by the master and thus an extra IRQ line is used to give the slave a chance to call the masters attention to it. This is however optional and the alternative method would be polling the status by the master.

The data framing is realized via the usual I2C protocol. Every frame is started with the I2C start condition followed by the devices write address and then master writes the command. In case of a write command, the data can follow immediately. In case of an read command, the devices read address is placed after an repeated start condition. The slave will put its data to the SDA line afterwards.

Besides the already build-in acknowledgment mechanism form the I2C protocol, the reception of an invalid data frame is advertised via the IRQ line. If an error occurs, the IRQ is pulled to low. The master can now read the corresponding status in order to get the root cause of the IRQ. Also the new measurement ready event is determined via the IRQ. In addition, the In addition, the acknowledgment of the successful reception and execution of a command could be implemented via the interrupt; the master would responsible for reading the acknowledge status.

Command Protocols

Master-to-Slave Transfer

Fig. 5.3: The master to slave communication. The left side shows the UART hardware and the right side the version for SPI/I2C hardware.

In case of UART, the master simply send data via its Tx line. After processing, the slave responds with an acknowledge or not-acknowledge signal on the masters Rx line.

In case of SPI or I2C, the slave can not send data without the master initiating the transfer. Thus, an additional IRQ is used to give the slave the chance to call the masters attention. In case of no IRQ available, the master must poll the slave on a regular basis. So after processing, the slave signals when he is ready to send the acknowledge or not-acknowledge signal and afterwards the master must initiate the transfer from slave to master.

Slave-to-Master Transfer

Fig. 5.4: The slave to master communication. The left side shows the UART hardware and the right side the version for SPI/I2C hardware.

In case of UART, the slave simply sends data via its Tx line. After receiving on the master, the sent data is dismissed. If an error occurs, the master is responsible to react accordingly, e.g. re-initiating the transfer by re-sending the previous command.

In case of SPI or I2C, the slave can not send data without the master initiating the transfer. Thus, an additional IRQ is used to give the slave the chance to call the masters attention. In case of no IRQ available, the master must poll the slave on a regular basis. The slave signals when he wants to send data via the IRQ and afterwards the master must initiate the transfer from slave to master.

Command Byte Format

Every command message is identified by the first byte in a data frame. This byte is an unique number that is mapped to a specified parameter/value/command.

The command identifier consists of 7-bit. The MSB is used to determine the difference between an message with (MSB=1) or without (MSB=0) addressing byte.

Fig. 5.5: The command byte format.

Reserved Command Bytes

Byte	Comment
0x02	ASCII: Start of text
0x03	ASCII: End of text
0x1B	ASCII: Escape
0x21	ASCII: ! - reserved for later use
0x23	ASCII: # - reserved for later use
0x24	ASCII: $ - reserved for later use
0x3F	ASCII: ? - reserved for later use

Addressing Mechanism

Since version 1.4.4 of the SDK, a new addressing mechanism has been introduced. The new addressing mechanism is based on the command byte format. The MSB of the command byte is used to determine if the message is addressed to a specific device or not. If the MSB is set to 1, the message is addressed to a specific device. In this case, the next byte in the data frame is the address byte. The address byte is the byte that immediately follows the command byte. Some commands do not require an address byte. In this case, the address byte is omitted and the command byte is followed by the data bytes. In this case, the MSB of the command byte is set to 0.

Sending a message without device address is considered equivalent to sending the message with device address 0. If the device address is omitted, the message is sent to the default device, usually the first device in the device list.

Thus, there are two kind of frames:

Extended Mode: MSB of command is set; the address byte follows the command byte.
Basic Mode: MSB of command is NOT set; no address byte is present.

Command Types

There are several command types defined:

Command (cmd): A data frame with a command byte that determines a simple command message that will invoke an action on the slave side. The commands are send from the master to the slave. The slave executes a corresponding function. Usually there is no data phase but in some cases there might be some (optional) function parameter.
Setter (set): Command byte followed by a given sequence of data bytes representing the data that needs to be transferred from the master to the slave.
Getter (get): A request from the master to read data from the slave. The actual data read phase depends a bit on the underlying hardware. While for SPI and I2C, the data is read directly in the context of the message, the response is sent as an autonomous message from the slave via its UART Tx line. Note that a get message is actually a command message that invokes the data transfer from the slave to the master in case of UART. Note that for some get messages there might be a short data phase for additional specification of the data to be read, e.g. an index number.
Automatic / Autonomous Push (Auto / Push): A message or data transfer that is initiated by the slave. Depending on the hardware, the slave requests a data transfer by the data-ready pin or simply invokes the transfer in an autonomous way. The first is the case for I2C and SPI modes while the latter is the case for UART mode. This type of message is utilized by the slave to send log messages or establish an data stream of new measurement data to the master without the requirement of polling the line.

Thus a single command byte can have up to three different intentions.

cmd: Executing actions.
get: The usual getter for read-only data.
set / get The setter/getter combination e.g. for configuration parameters that can be applied to the slave and also read back.
auto / push: Data that is only send from the slave as needed without the request form the master, e.g. log messages or data streaming.

Data Frame Format

Fig. 5.6: The basic data frame (without addressing byte) format for different hardware.

Fig. 5.7: The extended data frame (with addressing byte) format for different hardware.

Each message or command that is sent over underlying hardware is put into a data frame. A data frame is a sequence of bytes with variable length that contains arbitrary data. Depending on the hardware, the boundaries of the data frames are given in an unique way.

The first byte within a data frame is the command byte that uniquely determines the purpose and thus the format of the data frame. The last byte within the data frame is the security byte that contains a CRC value that guarantees the data integrity of the receive frame. Between the command and CRC bytes, there is an optional data phase of arbitrary length. The format and interpretation of the data is determined by the command byte in the higher layers of the communication stack and does not have any influence for the data framing. Note that the addressing byte is equivalent to the data bytes and does not require any special treatment here.

Byte Stuffing Algorithm

In case of SPI and I2C, the data framing is incorporated into the hardware protocol itself, while for the UART interface, a software solution is implemented. The SPI data frame is given by the chip select signal. Each data byte that comes while the chip select line stays at low are combined to a single frame until the chip select is released. The I2C protocol implements an embedded start and stop signal that gives the boundaries of a single data frame.

The UART version implements the data framing in software via byte stuffing, since there is no mechanism to detect the start of a new data package build into the hardware interface. The ideas is to reserve some unique byte values to serve as special control signal, e.g. start and stop bytes. Now, all bytes within a start and a stop byte are interpreted as data bytes for a single data frame. In order to not loose the full range of 256 valid values per data byte, an additional escape signal is introduced. Whenever a byte value equal to the value of start, stop or escape byte appears as a data byte within the current frame, an escape byte is added prior to the byte in question in order to signal that the following value is not an control signal but an ordinary data value. In order to increase security further, escaped data bytes are inverted to make the three control values unique.

Here are the control byte definitions:

Name	Byte	Comment
Start Flag	0x02	ASCII: Start of text
Stop Flag	0x03	ASCII: End of text
Escape Flag	0x1B	ASCII: Escape

See also: https://eli.thegreenplace.net/2009/08/12/framing-in-serial-communications

Byte Stuffing Algorithm for Sending Data

The algorithm to send a data frame with byte stuffing:

Create a new data buffer with start byte included at the first position.
Append the data in the buffer (command and data) and add escape bytes on the fly, invert escaped bytes.
Calculate the CRC on that buffer, but ignore all escaped bytes, i.e resolve byte stuffing
Add the CRC value (and maybe escape signal) and the Stop byte.
Send the buffer.

Byte Stuffing Algorithm for Receiving Data

The algorithm to receive data with byte stuffing:

After detecting a start byte, start receiving all data until the subsequent stop byte.
Write the received bytes into a buffer, removing escape bytes and invert escaped bytes on the fly
Calculate and verify the CRC value.
Evaluate the data buffer by invoking the corresponding function belonging to the given command byte.

Error checking: 8-bit CRC

In order to guarantee the data integrity of the received data frame, a CRC value is added to each data frame.

CRC8_SAE_J1850_ZERO definitions:

Name	Byte
CRC Generator Byte	0x1D
CRC Start Value	0x00

Refer to the following link to verify your implementation of the CRC8_SAE_J1850_ZERO algorithm:

See also: http://www.sunshine2k.de/coding/javascript/crc/crc_js.html

Command Definitions

Overview

Go to: Command Overview

Details

Go to: Command Details

Python Example on Using the SCI Interface

Here is an example that sets some configuration parameters (i.e. data output = 1D data only; measurement frame rate = 5 Hz) and starts timer based measurements. The range is extracted from the received data structure and printed to the console.

The example is very basic though. The sent data frames are manually created (i.e. data encoding, byte stuffing and CRC are hard coded). It ignores the received Acknowledge/NotAcknowledge commands and cannot handle unexpected data from the device.

The file runs with Python (3.6) and requires the Python serial module (pySerial).

Please find the file in [INSTALL_DIR]\Device\Examples\sci_python_example.py (default is C:\Program Files (x86)\Broadcom\AFBR-S50 SDK\Device\Examples\sci_python_example.py).

# #############################################################################
# ###     Example for using the AFBR-S50 API with UART interface            ###
# #############################################################################
#
# Prepare your evaluation kit (w/ NXP MKL46z MCU) by flashing the UART binary
# to the device. Connect the OpenSDA USB port (NOT the one labeled with KL46Z)
# to your computer. Go to the Device/Binary folder install directory of you SDK
# (default: C:\Program Files (x86)\Broadcom\AFBR-S50 SDK\Device\Binary) and copy
# the AFBR.S50.ExplorerApp.vX.X.X_KL46z_UART.bin (not the *_USB.*!!) file to
# the OpenSDA USB drive.
#
# After flashing, the device is ready to receive commands via the OpenSDA serial
# port. Go to your device manager to find out which COM port is assigned to the
# device. Type it to the "port" variable below, before starting the script.
#
# Use Python 3 to run the script. The script requires the pySerial module which
# might need to be installed.
# To install, run: "pip install pyserial"
# See: https://pyserial.readthedocs.io/en/latest/index.html
#
#
# The script sends configuration commands to set the data output mode to 1D data
# only and the frame rate to 5 Hz. After setting the configuration, the
# measurements are started and the data is extracted from the received data
# frames and printed to the console.
#
#
# Note: The CRC values are calculated manually and added before the frames are
# sent. You can use the online calculator from the following link w/
# CRC8_SAE_J1850_ZERO to obtain the CRC values for a frame:
# http://www.sunshine2k.de/coding/javascript/crc/crc_js.html
#
# #############################################################################
 
import time
import serial
 
# input parameters
port = "COM4"
baudrate = 115200
 
 
class AFBR_S50:
    """
    Serial Communication Interface for the AFBR-S50 Device.
    Connects to the device via a UART interface.
    """
 
    
    start_byte = b"\x02"
    
    stop_byte = b"\x03"
    
    esc_byte = b"\x1B"
    
    cmd_ack = 0x0A
    
    cmd_nak = 0x0B
    
    ser = None
 
    def __init__(self, port, baudrate):
        """!
        Initializes the class and opens a serial port w/
        "115200,8,N,1" serial settings and no timeout.
 
        @param port (str): The port number string, e.g. "COM1"
        @param baudrate (int): The baud rate in bauds per second, e.g. 115200
        """
        print("AFBR-S50: Open Serial Port " + port)
        self.ser = serial.Serial(port, baudrate)
        self.ser.timeout = 1.0  # seconds
        print("AFBR-S50: Serial Port is open " + port + ": " + str(self.ser.is_open))
 
        # discard old data
        if self.ser.inWaiting() > 0:
            self.ser.read(self.ser.inWaiting())
 
    def __del__(self):
        """!
        Deletes the class and closes the opened serial port.
        """
        self.ser.close()
 
    def write(self, tx: bytes):
        """!
        Sends a SCI message and waits for an optional answer and
        the mandatory acknowledge.
 
        If any answer is received, it is returned as bytearray.
 
        @param tx (bytes): The data message (incl. excape bytes) as byte array to be sent.
        @return Returns the received answer (ACK or NAK) as byte array. None if no answer was received.
        """
        print("Sending: " + tx.hex())
        self.ser.write(tx)
        return self.__wait_for_ack(tx[1])  # read acknowledge
 
    def __wait_for_ack(self, txcmd):
        """!
        Waits for an acknowledge signal for the specified command.
        If an answer is received before the acknowledge is received,
        the answer is returned as a bytearray.
        If no acknowledge or any other command is received, an
        exception is raised.
        @param txcmd (byte): The TX command byte to await an acknowledge for.
        @return Returns the received answer (ACK or NAK) as byte array. None if no answer was received.
        """
        answer = None
 
        while True:
            # Read until next stop byte and remove escape bytes
            rx = bytearray(self.ser.read_until(self.stop_byte))
            if len(rx) == 0:
                raise Exception("No data was read from the RX line.")
 
            if rx[0] != self.start_byte[0] or rx[-1] != self.stop_byte[0]:
                raise Exception("Invalid data frame received (start or stop byte missing).")
 
            rx = self.__remove_byte_stuffing(rx)
 
            # Extract command byte (first after start byte)
            rxcmd = rx[1]
 
            if rxcmd == txcmd:  # response received
                answer = rx
 
            # acknowledge signal received
            elif rxcmd == self.cmd_ack:
                ackcmd = rx[2]
 
                # acknowledge for the current command
                if ackcmd == txcmd:
                    return answer
 
                # acknowledge for any other command
                else:
                    raise Exception("Invalid ACK received")
 
            # not-acknowledge signal received
            elif rxcmd == self.cmd_nak:
                nakcmd = rx[2]
 
                # not-acknowledge for current command
                if nakcmd == txcmd:
                    raise Exception("NAK received")
 
                # not-acknowledge for any other command
                else:
                    raise Exception("Invalid NAK received")
 
    def __remove_byte_stuffing(self, rx: bytearray):
        """!
        Removes escape bytes from the incoming message if any
        @param rx (bytearray): The data message as byte array with escape bytes.
        """
        rxi = rx.split(self.esc_byte)
        rx = b""
        for i in range(1, len(rxi)):
            # invert byte after escape byte (also inverts start byte, but we don't care..)
            rxi[i][0] ^= 0xFF
        return rx.join(rxi)
 
    def __extract_1d_data(self, rx: bytearray):
        """!
        Extracts the 1D data values from the 1D data message.
        @param rx (bytearray): The 1D data message as byte array without escape bytes.
        @return Returns the read data as dictionary.
        """
        d = dict()
 
        # Extract Status:
        s = (rx[3] << 8) + rx[4]
        d["status"] = s if s < 0x8000 else s - 0x10000  # convert to signed 16-bit int
 
        # Extract Time Stamp
        t_sec = (rx[5] << 24) + (rx[6] << 16) + (rx[7] << 8) + rx[8]
        t_usec = (rx[9] << 8) + rx[10]
        d["timestamp"] = t_sec + t_usec * 16.0 / 1.0e6
 
        # Extract Range:
        r = (rx[15] << 16) + (rx[16] << 8) + rx[17]
        d["range"] = r / 16384.0  # convert from Q9.14
 
        # Extract Amplitude:
        a = (rx[18] << 8) + rx[19]
        d["amplitude"] = a / 16.0  # convert from UQ12.4
 
        # Extract Signal Quality:
        q = rx[20]
        d["signal quality"] = q
 
        return d
 
    def read_data(self):
        """!
        Reads the serial port and decodes the SCI data messages.
        Currently only 1D data messages are supported.
        If no data is pending to be read, the function immediately
        return with None. If other data than measurement data was read,
        the function returns with None.
        Otherwise it returns a dictionary with the extracted data values.
        @return Returns the read data as dictionary. None if no data has been read.
        """
        if self.ser.inWaiting() == 0:
            return None
 
        # Read until next stop byte and remove escape bytes
        rx = bytearray(self.ser.read_until(self.stop_byte))
        if len(rx) == 0:
            raise Exception("No data was read from the RX line.")
 
        if rx[0] != self.start_byte[0] or rx[-1] != self.stop_byte[0]:
            raise Exception("Invalid data frame received (start or stop byte missing).")
 
        rx = self.__remove_byte_stuffing(rx)
 
        # extract command byte (first after start byte)
        cmd = rx[1]
 
        if cmd == 0x06:  # Log Message
            print("Device Log: " + str(rx[8:-2]))
 
        elif cmd == 0xB6:  # 1D Data Set
            return self.__extract_1d_data(rx)
 
        else:  # Unknown or not handled here
            print("Received Unknown Data Frame: " + rx.hex())
 
 
if __name__ == "__main__":
 
    try:
        # Create a new instance and open a serial port connection to the device.
        s50 = AFBR_S50(port, baudrate)
 
        # Setting data output mode to 1D data only
        # The message is composed of:
        # [START][CMD][PARAM][CRC][STOP]
        # where:
        # [START] = 0x02; start byte
        # [CMD]   = 0x41; command byte: data streaming mode
        # [PARAM] = 0x07; parameter of command: 1d data streaming
        # [CRC]   = 0xF5; checksum: pre-calculated in online calculator
        # [STOP]  = 0x03; stop byte
        print("setting data output mode to 1d data only")
        s50.write(bytes.fromhex("02 41 07 F5 03"))
 
        # Setting frame time to 200000 µsec = 0x00030D40 µsec
        # The message is composed of:
        # [START][CMD][PARAM(0)]...[PARAM(N)][CRC][STOP]
        # where:
        # [START]    = 0x02; start byte
        # [CMD]      = 0x43; command byte: measurement frame time
        # [PARAM(x)] = 0x001BFC0D40; 4-bit parameter of command: 0x00 03 0D 40 w/ escape bytes
        # [CRC]      = 0x85; checksum: pre-calculated in online calculator
        # [STOP]     = 0x03; stop byte
        #
        # NOTE: the 0x03 byte must be escaped and inverted (i.e. use 0x1BFC instead of 0x03)
        #       The CRC is calculated on the original data, i.e. 0x43 00 03 0D 40 => 0x85
        print("setting frame rate to 5 Hz (i.e. frame time to 0.2 sec)")
        s50.write(bytes.fromhex("02 43 00 1B FC 0D 40 85 03"))
 
        # Starting measurements
        # [CMD] = 0x11; command byte: start timer based measurements
        print("starting measurements in timer based auto mode")
        s50.write(bytes.fromhex("02 11 D0 03"))
 
        # Read measurement data
        print("read measurement data")
        while True:
            d = s50.read_data()
            if d != None:
                print(
                    f"{d['timestamp']:10.6f} sec | "
                    + f"range: {d['range']:6.3f} m | "
                    + f"amplitude: {d['amplitude']:8.3f} | "
                    + f"signal quality: {d['signal quality']:3d} | "
                    + f"status: {d['status']:5d} |"
                )
 
            else:
                # do other stuff
                time.sleep(0.1)
 
    except KeyboardInterrupt:
        # Try to stop measurements
        # [CMD] = 0x12; command byte: stop timer based measurements
        print("stop measurements")
        s50.write(bytes.fromhex("02 12 F7 03"))
 