Ultrasonic Sensor

This is yet another ultrasonic sensor project. What it makes interesting is that it uses a hardware timer in order to measure the time. Capturing is completely done in hardware. Upon a rising edge the timer is resetted, and upon a falling edge the timer is captured. All you need to do is to read the capture compare register and perform some calculation to convert the time into a distance.

Features

• Uses the HC-SR04 Ultrasonic sensor
• Uses hardware timer to measure the response time
• Writes values directly to the virtual com port
• Includes CubeMX project file
• Easy to use
• All logic included in main.c

How does it work?

Starting the tigger and waiting for an echo

The HC-SR04 starts in response to a falling edge which has to be HIGH for at least 10µs.

    for (;;) {
        HAL_GPIO_WritePin(TRIG_GPIO_Port, TRIG_Pin, GPIO_PIN_SET);
        osDelay(20);
        HAL_GPIO_WritePin(TRIG_GPIO_Port, TRIG_Pin, GPIO_PIN_RESET);

        while (_ui8Sync == 0);

        _ui8Sync = 0;
        // Send data over USB
        if(hUsbDeviceFS.dev_config) {
            uint8_t len = snprintf(_data, 24, "%.4f\n", _Distance);
            CDC_Transmit_FS(_data, len);
        }
        osDelay(20);
    }

Processing the capture interrupt

The capture callback converts the timer value to an actual distance.

// ticks per second calculated by (system clock / prescaler)
constexpr uint32_t _ui32TicksPerSecond = 1000000;
constexpr uint32_t _ui32SpeedinMeterPerSecond = 343;
constexpr float _fFactor = (float)_ui32SpeedinMeterPerSecond / _ui32TicksPerSecond;

volatile uint8_t _ui8Sync = 0;
volatile float _Distance = 0;
void HAL_TIM_IC_CaptureCallback(TIM_HandleTypeDef *htim) {
    // Get timer value
    uint32_t _ui32Time = htim->Instance->CCR1;

    // Calculate distance using physics and math
    _Distance = (_ui32Time/2.f)*_fFactor;

    // Set flag
    _ui8Sync = 1;
}

Brief timer description

PA8 is connected to TIMx_CH1. The edge detector is connected to TRGI using the TIF_ED signal (*1), which resets the counter upon a rising edge. A falling edge on TI1FP1 then captures the timer value (*2).

Wiring diagram

Pin	Function
PA8	Echo, connected to the capture compare unit
PA9	Trigger, triggers new measurement