Gait Analyser

Firmware for STM32 - Gait Analyser. Support serveral ICM20948 sensors. Communicate wirh IMU sensors and send measured data to serial port.

Basic properties:

• support up to 2 IMU sensors
• read acceleration and angle velocity (gyroscope)
• communication interface: USB (virtual com port)

Communication parameters:

• cummunication interface: UART
• speed: 921600 Baud

Operational modes:

• MANAGE - to set/get sesor or module parameters. The measuremet is not active in this mode.
• RUN - Module is measuring and sending data.

Used modules:

• ICM20948 - driver for sensors
• DataFrame - create data packets
• API - communication protocol

Hardware connection

SPI1:

• MISO - PA6 (A5)
• MOSI - PA7 (A6)
• SCK - PA1 (A1)
  • IMU sensors:
  • CS1 - PA4 (A3)
  • CS2 - PA5 (A4)
  • CS3 - N/A
  • INT1 - PB0 (D3)
  • INT2 - PB1 (D6)
  • INT3 - N/A
  • Radio Transmitter
  • CS - N/A

LED - PB3 (D13)

UART

• RX - PA15 - not wired to pin header
• TX - PA2 (A7)

Communication protocol - MANAGE mode

It is used binary communication protocol with special properites:

• no byte can have value 0x0
• only the last (terminating) byte is 0x0

Packet structure:

Request:

• Address: index of sensor. Allowed values:
  • for sensors: 1,2,3... up to real number od attached sensors (SENSOR_INDEX)
  • for MCU: MODULE_ADDR (0x70)
• Function: Requred functionality
  • this code is calculated as binary sum of: CMD_DIRECTION and CMD_CODE
• Data: optional part.
• CRC: CRC8 algorithm with CRC polymome (POLY=0x97) : x^8 + x^7 + x^4 + x^2 + x^1 +1
• TERM: termination character - 0x0

CMD_DIRECTION:

• SET: 0x00
• GET: 0x80

Response OK:

• Function* - Function code without READ prefix

Response ERR:

• Function* - Function code without READ prefix
• Address* - Adress | 0x80

Error Code:

• 0x10 - Not supported
• 0x11 - Bad parameters
• 0x12 - Not ready
• 0x13 - Sensor not exists

Supported functionality

1-st byte is fuction code. The code is coposec from these parts:

• Read/Write flag
  • READ: 0x80
  • WRITE: 0x00
• Functionality - see table bellow
• Target for command exucution - DESTINATION
  • DEST_MODULE 0x00 (IMU Sensor/all sensors)
  • DEST_ACC: 0x01 (Accelerometer)
  • DEST_GYRO: 0x02 (Gyroscope)
  • DEST_MAG: 0x03 (Magnetometer)

Supported fuction codes

Supported function code generation for selected sensor

Examples:

• Sleep function: is addressed to sensor only
  • Address: sensor 1,
  • fuction: CMD_SLEEP
  • packet content: 0x01 0x08 0x00
  • response: 0x01 0x08 0x01 0x00
• Echo fuction: is addressed to MCU
  • Address: 0x70
  • Function: READ | CMD_ECHO => 0x80 | 0x04 => 0x84
  • Data: ABC
  • packet content: 0x70 0x84 0x64 0x65 0x66 0x00
  • response: 0x70 0x84 0x64 0x65 0x66 0x00

Command details

CMD_ECHO 0x04

This command is always addressed to module, thus, destination address is MODULE_ADDR=0x70

CMD_STOP - 0x08

• SET - Stops the measure
• GET - Return state of measure Can be addresset to GYRO sensor, ACC sensor or to module. If it is addressed to module, all active sensors will be stopped.

CMD_START - 0x0C

• SET - Start the measure
• GET - Return state of measure Can be addresset to GYRO sensor, ACC sensor or to module. If it is addressed to module, all active sensors will be started.

CMD_SAMPLERATE - 0x10

• SET - Set the samplerate
• GET - Return samplerate from IMU

CMD_LOWPASSFIL - 0x14

• SET - Set the cut-off frequency of Low Pass Filter
• GET - Return index for Low Pass Filter (LPF_VALUE)

CMD_RANGE 0x18

• SET - Set the range for selected sensor
• GET - Return index for range settig (RANGE_VALUE)

Parameter's values

Command CMD_SAMPLERATE allowed values

Command CMD_LOWPASSFIL allowed values

Command CMD_RANGE allowed values

CMD_SENSOR_CNT 0x1C

Return numbers of connected sensors. This command is always addressed to module, thus, destination address is MODULE_ADDR=0x70

CMD_SLEEP 0x20

1. Put IMU sensor to sleep mode
2. Check, that IMU sensor is in sleep mode. This command can be eddessed to IMU sensors only (DEST_MODULE)

CMD_WAKEUP 0x24

1. Wakeup IMU sensor from sleep mode
2. Check, that IMU sensor is in ready/working mode. This command can be eddessed to IMU sensors only (DEST_MODULE)

CMD_CALIBRATE 0x28

1. Run calibration procedure on sensor or its part. Allowed parts: IMU sensor, accelerometer, gyroscope. Performed calibration is temporary. Result is used until next reset.
2. Get actual calibrated values. The values are loaded from sensor directly.

Perform calibration

Get calibrated data

DATA - Calibration is the adjustment of the displacement for each axis of the accelerometer or gyroscope. Each value is of length 2B. When an accelerometer or gyroscope calibration is requested, the result is 3 values. When requesting calibration of all sensors from the selected IMU sensor, the result is 6 values. Data encoding: the data sent cannot contain zero bytes, so 2-to-3 encoding is used, where each 2B value is encoded into 3B as follows: Let V is 2-Bytes long value. We can represent it as a binary number: xxxxxxxx yyyyyyy, where x's are MSB and y's are LSB. These 2 bytes is divided into 3 bytes:

V - original value =>     2-to-3 coding
    MSB      LSB        MSB       MB     LSB
 xxxxxxxx yyyyyyyy => 11xxxxxx xx1111yy yyyyyy11

Format of DATA part

• data[2] - code of sensor:
  • 0x01 - Accelerometer
  • 0x02 - Gyroscope
• data[3] - data[5]: calibrated offset of x-axis
• data[6] - data[8]: calibrated offset of x-axis
• data[9] - data[11]: calibrated offset of x-axis

For calibration request from all sensors (in IMU sensor):

• data[12] - code of sensor
• data[13] - data[21] - calibrated offset of axis x, y, z

CMD_RESET 0x2C

Erase all settings from persistent memory of IMU sensor. It is allowed only DEST_MODULE as receiver of this code.

CMD_STORE 0x30

The values to store is i IMU sensor. There is no input values from API

• SET - Store IMU parameter to internal non-volatile memory
• GET - Return stored parameter from internal memory

PARAMETER:

• CMD_SAMPLERATE - Sample rate
• CMD_LOWPASSFILL - Low Pass Filter
• CMD_RANGE - Sensor rage
• CMD_CALIBRATE - calibration values (offsets)

Format for DATA in response

CMD_DATA 0x34

The raw data from IMU sensor.

• GET/MODULE - Return raw data from ACC and GYRO
• GET/ACC - Return raw data from ACC
• GET/GYRO - Return raw data from GYRO

TYPE:

• 0x10 - from accelerometer
• 0x20 - from gyroscope
• 0x30 - from magnetometer
• 0x40 - from termometer

value.x (.y, .z):

• this value represent 1 float number (4B)
• this 4 byte is encoded by 2-to-3 encoding algorithm described in part CMD_CALIBRATE

Communication protocol - RUN mode

After power-on module, the RUN mode is active. Format of output data frame:

• START_BYTE - Starting byte - 0xCC
• LENGTH - length of packet payload - without first 2 bytes.
  • LENGTH = n - 2 , where n is length of whole packet
• TIMESTAMP - number of ms from module start
• DATA - data from sensors
• CRC8 - CRC check of whole packet

Format of DATA part

The DATA part consists of multiple set of sensors. Each sensor module can contain several measured values. For example, the sensor ICM20948 contais accelerometer, gysoscope and magetometer.

Data is storen in Little Endianity.

SENSOR_IDENTIFICATION = SENSOR_ID + SENSOR_INDEX

• SENSOR_ID:
  • ACCELEROMETER - 0x10
  • GYROSCOPE - 0x20
  • MAGNETOMETER - 0x30
  • TEMPERATURE - 0x40
• SENSOR_INDEX - sensor sequence number
  • first sensor has number = 1

DATA_FORMAT = VALUES_COUNT + VALUE_FORMAT

• VALUES_COUNT - number of values for one measurement
  • allowed values: 1 - 15
• VALUE_FORMAT - format of single value. Actulally is not used. Format can be:
  • UINT8 - 0x1
  • UINT16 - 0x2
  • UINT32 - 0x3
  • INT8 - 0x4
  • INT16 - 0x5
  • INT32 - 0x6
  • FLOAT - 0x7

Example of data packet for 1 connected sensor ICM20948 with accelerometer and gyroscope: