Kart/serial link

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Serial link protocol

This section defines the serial link protocol used to communicate between the Kart and the BLE module / PC / Android Smartphone.

General principle

The system is using a BLE module.

To avoid congesting the line, data are sent from the Kart to the User on specific events or if requested by the User.

Serial port

Communication is done with:

  • Resting state : HIGH
  • Data bits : 8
  • Parity : none
  • Stop bits : 1
  • Handshake : none
  • Baudrate : 115'200

Message Format

SoF (1 byte) Address (1 byte) Data (2 bytes) EoF (1 byte)

The address is decomposed as follows: 0bMMWRRRRR

  • MM : targeted module
    • 0b00 : DC Motor
    • 0b01 : Stepper Motor
    • 0b10 : Sensors
    • 0b11 : Control Registers
  • W : defines if the data is saved to ('1') or read from ('0') the FPGA
    • The FPGA will respond to a request with the exact same address when W = '0'
    • The FPGA will save incoming data in the targeted register when W = '1'
    • The FPGA will send data on predefined events with the W bit set to '0'
  • RRRRR : targeted register

Frame example

For the BLE module to light LED1 with it changing each 500 ms, the following frame is sent:

SoF (1 byte) Address (1 byte) Data High (1 byte) Data Low (1 byte) EoF (1 byte)
0xAA 0b10100001 0b10000001 0b11110100 0x74

If the BLE module wants to read the current battery voltage, supposing we have 4 LEDs, the frame is such as:

SoF (1 byte) Address (1 byte) Data High (1 byte) Data Low (1 byte) EoF (1 byte)
0xAA 0b10000101 Any Any CRC


DC Motor

Reading range : 0x00 to 0x1F

Writing range : 0x20 to 0x3F

Address Name Type Description Direction Event
0 Prescaler Uint16 DC PWM frequency, as fclk / (PWMsteps * prescaler) = 10M / (16 * prescaler) Smartphone -> Kart
1 Speed Int5 Desired speed, from -15 (0xFFF1) to 15 (0x000F) (negative = backward) Smartphone -> Kart

Stepper Motor

Reading range : 0x40 to 0x5F

Writing range : 0x60 to 0x7F

Address Name Type Description Direction Event
0 Prescaler Uint16 Stepper switching frequency, as 100k / prescaler Smartphone -> Kart
1 Target angle Uint16 Desired steering angle, in motor steps (0 = end switch) Smartphone -> Kart
2 Actual angle Uint16 Actual steering angle, in motor steps (0 = end switch) Kart -> Smartphone When a delta of at least STP_ANGLE_DELTA_DEG (3°) from the last registered value happens
3 Stepper HW Uint14 + Vector2 Bit 0: stepper end

Bit 1: position reached

Bits 15 - 2 : actual steering angle

Kart -> Smartphone Sent when stepper end is pressed (rising edge) or position reached (rising edge)


Reading range : 0x80 to 0x9F

Writing range : 0xA0 to 0xBF

Address Name Type Description Direction Event
0 LED1 Bit + Uint15 Bit 15: on / off/

Bits 14 -> 0: half-period in ms (if 0, led status = bit 15)

Smartphone -> Kart
7 LED8 Bit + Uint15 See LED1 Smartphone -> Kart
8 Voltage Uint16 Battery voltage, U = register * 250*e-6 * 7.8 [V] Kart -> Smartphone When a delta of at least SENS_BATT_DELTA_MV (50) from the last registered value happens
9 Current Uint16 Consumed current, I = register * 250*10*e-6 / (100 * 5*10*e-3) [A] Kart -> Smartphone When a delta of at least SENS_CURR_DELTA_MA (50) from the last registered value happens
A Range finder Uint16 Distance to sensor, register * 25.4 / (147*10*e-6 * (10M / 10)) [mm]

Register zeroed if less than 152 mm (sensor min distance) or greater than 1500 mm (arbitrary max distance), event not sent in such case

Kart -> Smartphone When a delta of at least SENS_RANGEFNDR_MM (60) from the last registered value happens
B End switches Vector(16) Sensors current values, right justified (sensor 1 is bit 0) Kart -> Smartphone On any edge change of any sensor
C Hall1 Uint16 Hall pulses count, zeroed on overflow of the register Kart -> Smartphone Each 100 ms if value changed from last time
D Hall2 Uint16 See Hall1 Kart -> Smartphone Each 101 ms if value changed from last time

Control Registers

Reading range : 0xC0 to 0xDF

Writing range : 0xE0 to 0xFF

Address Name Type Description Direction Comment
0 Hardware Control Vector6 Bit 0: when '0', the Kart goes backwards when the motor turns foward

Bit 1: when '1', the Kart turns to the right as the stepper coils go from 1 to 4

Bit 2: when '1', the angles are measured clockwise

Bit 3: emulates the end switch contact for the stepper motor

Bit 4: restart the stepperMotor module, stops the DC motor while '1'

Smartphone -> Kart The end sensor always defines angle 0. Angles are always positive numbers in registers.

If bits 1 and 2 are different, the stepper motor phase sequence has to be inverted. If the Bluetooth connection is lost, the DC motor has to be stopped.

1 BT Status Vector1 Bit 0: when '0', the smartphone is disconnected NRF -> Kart The register is set by the NRF itself, since it is not possible to foresee the disconnection.

Kart init

To init the Kart, the following sequence should be sent to the Kart:

  • Write DC Motor | Prescaler to 31 (around 21 kHz PWM frequency)
  • Write Stepper Motor | Prescaler to 400 (250 Hz coil switching frequency)
  • Tell the smartphone is connected by writing Control Registers | BT status to 1
  • Write Control Registers | Hardware Control to restart the system (0b10xxx)
    • The stepper should turn until hitting the end switch, except if already on it
  • Read Stepper Motor | Stepper HW and check the last bit
    • If is '1', it means we are already zeroed
    • If not, wait for an event from this register to tell the reset is complete
  • Write Control Registers | Hardware Control to deassert the reset (0b00xxx)

The Kart is now ready to function !

Note that the Control Registers | restart bit is automatically reset back to 0 when the stepper_end input is activated.

Testing through USB

Kart Command Interpreter

To test the Kart from a PC directly, one can do as follows:

  • Remove the BLE from the motherboard (or, at least, do not connect to a smartphone)
  • Power the motherboard with a DC voltage regulator (+12V)
  • Wire the USB-C present on the daughterboard to your PC
    • Two new COM ports should be detected
  • Download and open the Kart Command Interpreter utility (available in the VHDL project -> CommandInterpreter)
  • In the top menu Serial -> Port, select the correct COM port (should be the biggest of the two new ports)
    • You can also change the baudrate, by default 115'200 (the correct one if it was not modified)

To test the connection, click the Read button. The Tx and Rx values should change, and a text added to the text area.

Simple operation

The simplest way to test both motors are the three button at the bottom.

  • Init Kart button
    • Will set the DC prescaler, stepper prescaler, and execute the restart sequence. Must be clicked first.
    • Answer the 4 prompts following your hardware configuration.
  • DC Motor button
    • Will set the DC speed to full for 2s, quickly 0, then reversed full speed for 2s
  • Stepper Motor button
    • Will set the stepper to 400 (30°) then 0

Custom operation

Each register can be read and/or written by hand following their data description.

For this, select the Module first, then which Register.


To read, simply click the Read button. Successful read will be shown in green (CRC is ok) and logged, with extra info (e.g. for the DC prescaler, will give the motor frequency).


To write, enter a value in the value box such as:

  • Direct integer (only DC speed should be negative)
  • 0bxxxx binary values
  • 0xxxxx hexadecimal values
  • Other values will throw an error

Then click on the Write button.


In case of problem, you can simply power-cycle the FPGA board to reset the registers.

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