Microcontroller Unit Lab 6

Ultra-Sonic Range Finder

Using USB Serial Communications Port Output

Summary

Use a microcontoller to trigger sound pulses and use the echo time to
calculate distance then output via USB Serial Communication.

Pt. 1  USB Serial Communications Setup & Test

Pt. 2  Add a button - print a message on the usb com port terminal when it is pressed.

Pt. 3  Connect the ultrasonic module. The button should trigger a measurement. Time the echo pulse using CPU sleep, system clock and loops.

Pt. 4  Adjust the distance calculation using measured distances. Create a function for later re-use

Pt. 5  Create a function for later re-use. For instance, getUltrasonicDist() which returns a float or double.

Required Equipment and Supplies

- HCSR04 ultrasonic range finder - datasheet

- Rasperry Pi Pico with Headers - pinout diagram

- Momentary Switch (Push Button) [x1]

- USB Micro Cable

- Breadboard

- Cables and 22ga wire as needed

Wiring

- The HCSR04 does not use much powers, you may power everything from the USB cable.

- Use the Vsys pin to supply your breadboard power rail from the Pi Pico.

- The HCSR04 has only four wires to connect. Gnd, ECHO TRIG and Vcc.

- The ECHO is an ouput from the sensor should connect to a GP Input on the microcontroller.
  optional: An oscilloscope probe can be connected to see the signal directly if you wish, as shown below.

- The TRIG is an input to the sensor should connect to a GP Output on the microcontroller.

Control Signal

The HCSR04 works by echo location. It sends sound pulses then uses a logic signal to communicate the time of flight of sound pulses.

- A logic pulse with a width of at least 10 us is given to the TRIG pin

- A short delay occurs while the sensor activates and sends 8 high frequency clicks. The sensor sends no signals during this.

- The sensor then pulls the ECHO pin to logic high. ECHO remains high until the sensor hears the echo or a time-out occurs.

- The duration of the high signal on ECHO indicates how long sound was travelling. A longer delay means farther distance travelled.

- The distance is traversed twice because sound must leave the sensor and travel back to it after reflecting off a surface.

Pt. 1 - Setting Up Serial Debugging

  -  Add the following lines of code to the CMakeLists.txt file of any project where you wish to use USB stdio messages.

      #USB com enable
      pico_enable_stdio_usb(${program_name} 1)
      pico_enable_stdio_uart(${program_name} 0)

  -  Call the following function in main() some time before your primary loop. After this point printf() and puts() will send text to the a USB serial communcations port.

      stdio_init_all();

  -   To test the USB com port, call printf() with a 'hello world' message followed by a delay using sleep_ms() with an appropriate time - maybe 500 ms.

  -   Compile the code using cmake and make then flash the pi pico as usual.

  -   To view the text output from the pico, open Serial Port Terminal (GTKterm). Under Configuration - Port select /dev/ttyACM0. If that port is not visible, the pico may not be communicating. You should be able to see your message printing at a half second interval or so.

Example Code For Pt. 5

...

/// Macros required by ultrasonic funciton below
#define USONIC_TRIG 19
#define USONIC_ECHO 20
#define BUTTON_MEASURE 16
#define SPEED_OF_SOUND_CM_PER_US 0.0334f

...

void ultasonic_pin_setup(){
	gpio_init(USONIC_TRIG);
	gpio_set_dir(USONIC_TRIG, GPIO_OUT);

	gpio_init(USONIC_ECHO);
	gpio_set_dir(USONIC_ECHO, GPIO_IN);
}

/// Returns distance in cm

float ultrasonic_get_dist(){
		//1. send the trigger pulse
		gpio_put(USONIC_TRIG, 1);
		sleep_us(20);
		gpio_put(USONIC_TRIG, 0);

		//.2 measure the echo
		//TODO: allow for timeout
		float timeout_us = 0;
		while(gpio_get(USONIC_ECHO) == 0) {
			sleep_us(20);
			timeout_us += 20;
			if(timeout_us > 40000) return -1; //consider 40ms timeout
		}  //polling loop, CPU blocking
		// Echo line is now high
		float echo_time_us = 0;
		while(gpio_get(USONIC_ECHO) == 1) {
				sleep_us(20);
				echo_time_us += 20;
				if(echo_time_us > 40000) break;
		} //polling loop, CPU blocking
		//Echo line now low again
		//CONVERT
		float dist_cm = (SPEED_OF_SOUND_CM_PER_US * echo_time_us)/2.0f;
		// divide by 2 because an echo travels the distance twice.
		return dist_cm;
}