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.

General principle

The system using a BLE module, the throughput is limited by design to a payload of 27 bytes each connection interval (from 7.5 ms to 4 s).

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
  • Bit order : LSB first

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 '1'
  • RRRRR : targeted register


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 (10°) from the last registered value happens
3 Stepper HW Vector2 Bit 0 : stepper end

Bit 1 : position reached

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


SmartPhone to Kart

Address Datatype Description
0 UINT16 DC motor PWM period
1 INT5 DC motor speed: -15 to 15 (negative numbers = backwards)
2 UINT16 Stepper motor period (speed proportional to 1/period)
3 UINT16 Desired steering angle (in motor steps, 0 = end switch)
4 UINT5 End switch address
5 UINT5 Hardware control
7 UINT2 Sequence control register
8 UINT16 Sequence operations register
15 UINT16 Serial link update interval [ms]

Hardware control register

The hardware control register allows to adapt the controls to the hardware of the car and to simulate events such as a stepper end switch.

Bit Meaning Description
0 Kart goes as DC motor When '0', the kart goes backwards when the motor turns forwards
1 Kart turns right (clockwise) When '1', kart turns to the right as stepper motor phases go from 1 to 4
2 End sensor is on left side If '1', the angles are measured clockwise
3 Stepper end Emulates an end switch contact for the direction motor
4 Restart Puts following slaves back in startup mode: stepper motor
The DC motor is stopped as long as the signal is active
5 Bluetooth connected Indicates that the Bluetooth connection is established

The end sensor always defines position 0. Angles are always positive.

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.

Sequence control register

The sequence control register is used to load a new sequence from start of memory as well as to start and stop a sequence.

Bit Meaning Description
0 Reset sequence sets the sequence RAM write and read addresses back to 0
1 Run / stop starts or stops a sequence

Sequence operations register

The sequence memory is made out of 16-bit controls and the sequence can range up to 210 operations. From the communication point of view, the sequence memory is seen as a FIFO : the sequence operations are pushed one after the other into the sequence register, starting at the first one of the list after the reset sequence bit has been set in the control register

The sequence operations are split into 2 parts: a 4-bit command and optional parameters. A sequence register has been defined in order to build loops: the goto operation is only carried out if the register is not zero. The register is initialised with all bits to '1'.

command parameter comment
Bits 15 ÷ 12 11 ÷ 0
0 nop
1 set speed speed set the propulsion motor speed
2 set angle angle set the direction motor angle
3 drive LEDs pattern specify a given pattern for turning the LEDs on or off
4 set LEDs mask set bits from '1's of mask
5 clear LEDs mask clear bits from '1's of mask
6 blink LEDs period, mask not implemented yet
8 run distance hall ticks run until the Hall counter has augmented by at least a given tick number
9 run for milliseconds run for a specified time span
A run until event not implemented yet
D modify register operation, operand bits 11÷10 : 00 = set, 10 = add, 11 = sub / bits 9÷0 : operand
E goto nz address jumps to the specified sequence step as long as the register is not zero
F end the sequence controller in the FPGA stops and the running status bit is cleared, announcing that the sequence is terminated

Kart to SmartPhone

Address Datatype Description
0 UINT16 Hall speed count 1
1 UINT16 Hall speed count 2
2 UINT2 Steering status: bit 0 stepper end, bit 1 position reached
3 UINT16 Actual Steering Angle
4 UINT1 Stepper End from I/O board, bit 0
5 UINT16 ADC Value of the Battery Voltage Level
6 UINT16 Distance
7 UINT16 Sequence status
8 UINT16 Proximity 4
9 UINT16 Proximity 3
10 UINT16 Proximity 2
11 UINT16 Proximity 1
12 UINT16 Ambient Light 4
13 UINT16 Ambient Light 3
14 UINT16 Ambient Light 2
15 UINT16 Ambient Light 1

Sequence status register

The sequence status register tells what operation is currently active.

Bit Meaning Description
15 ÷ 12 Current command indicates which command is currently being processed
0 Running indicates that a sequence is currently running

Kart I2C bus

The data on the kart's I2C bus reflects the one on the RS232 link. On the I2C, both data and addresses are 8-bit.

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