INPUT DEVICES

During this week, we will look at input devices, that is to say, the devices or components that send information to the microcontroller. It can be understood more simply if we think of it as a system that tells what is happening on the outside. For me, one of the fundamental things that must always be considered is understanding the theory and then applying it. Both are activities that allow complementing information for future work.

This week it is also expected to understand the data input mechanism to the microcontroller.

There are different types of input devices, as it is any element that detects something and converts it into a signal, whether analog or digital.

Input devices are the data source while the microcontroller is the one that processes the data. Some input devices are:

Mechanical devices

Such as buttons, switches, toggles, etc. These detect physical contact and their signal type is digital.

Analog sensors

They are temperature sensors like the LM35, light sensors, potentiometers, etc. These measure continuous physical magnitudes and their signals are analog.

Digital sensors

Such as ultrasonic sensors, motion sensors, infrared sensors. They allow detecting events or generating pulses and are digital signals.

Smart sensors

Sensors like VL53L0X, MPU6050, etc. They measure complex variables. And their signal types are digital I2C, SPI.

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Regarding microcontroller modules, which are those that process or interpret signals, we have:

Regarding interfaces or communication methods, that is, those that transport data, we have:

Based on the above, for this week our main task will be to use a sensor on the manufactured board and obtain values or measurements generated by this sensor. To do this, it is important to consider various aspects so that the process turns out as efficiently and accurately as possible. For this, I focused on the following aspects.

For this, I was given a list of different types of inputs that I could use. It is important to know them and know how they work to subsequently select the type of sensor that will be worked with.

PCB Board Made

First of all, I considered the board I manufactured, you can find my process at this link. It has a XIAO RP2350 microcontroller, which works with 3.3V. Likewise, based on the PCB schematic I made in the Altium design program, I guided myself to know which pins I should connect the sensor to. To do this, recalling the pins that my PCB contains:

From this guide, it was easier for me to know how to take advantage of the pins I have on my PCB.

Pinout	Pin type
A0-GPIO26-D0	Digital/Analog/GPIO
A1-GPIO27-D1	Digital/Analog/GPIO
A2-GPIO28-D2	Digital/Analog/GPIO
SPI0_CSn-GPIO5-D3	Digital/SPI/GPIO
SDA1-GPIO6-D4	IIC/GPIO/Digital
SCL1-GPIO7-D5	IIC/GPIO/Digital
TX0-GPIO0-D6	UART/GPIO/Digital
5V	Power
GND	GND
3V3	Power
D10-GPIO3-SPI0_MOSI	Digital/SPI/GPIO
D9-GPIO4-SPI0_MISO	Digital/SPI/GPIO
D8-GPIO2-SPI0_SCK	Digital/SPI/GPIO
D7-GPIO1-RX0	Digital/GPIO/UART

For me, it was extremely important to consider the type of pins that exist. It is necessary to highlight that the same pin can have several functions, but they are chosen based on the code, some have already been described, but others are:

Sensor Used

To choose the sensor I based it on what I want to do for my project. Based on that, I chose the HC-SR04 Ultrasonic Sensor. I think using this sensor will be very useful for my project, since I need to detect objects at certain distances so that my robot can evade them.

I considered that my XIAO RP2350 microcontroller works at 3.3V, however, the ultrasonic sensor works at 5V. This must be considered, since there could be a risk of burning the microcontroller if a higher voltage than the one it can operate with is used. In my case, I used a more current sensor that works with 3.3V.

How the sensor Works

It is a sensor that measures distances using high-frequency sound waves. This basically works when the microcontroller sends a pulse (TRIG), the sensor emits ultrasound, the wave bounces off an object and returns to the sensor. From then on, the ECHO pin goes HIGH for a certain time, and through the time, the distance is obtained.

This type of sensor has two metallic cylinders, one is the transmitter (T) that works like a speaker, and a signal is sent through the Trig pin. While the other cylinder is the receiver, which works like a microphone. When the sound hits an object and bounces back, this cylinder hears it and sends a signal through the Echo pin.

The microcontroller counts the time that passed from when the sound went out until it returned. Based on this, in its programming, the constant speed at which sound travels through the air (343 meters per second) is considered.

The 3.3V version has a different chip, this one is designed to work with low voltages. This allows it to connect directly to my microcontroller without the need to build any circuit with resistors to lower the voltage. Finally, it can measure objects from 2 cm to 400 cm.

The pins I used on my board were the following: GND, 3.3V, D2 for Trig, and D1 for Echo. I specifically used D1 and D2, since these pins work as digital pins. I used female-female DuPont cables to connect the sensor and my PCB.

Programming in ARDUINO IDE

To program I used ARDUINO IDE, I installed it directly (you can download it here). It is not necessary to download any library, since it can be programmed manually. Based on how the sensor works and what was learned in week 4, I followed the following logic:

The code for the ultrasonic sensor is as follows:

// Definimos los pines de la XIAO RP2350
// Usamos los pines etiquetados como A2 y A3 en tu placa
const int trigPin = D2; // Conectado al pin Trig del sensor
const int echoPin = D1; // Conectado al pin Echo del sensor

void setup() {
  // Iniciamos la comunicación con la computadora a 115200 baudios
  Serial.begin(115200);
  
  // Damos un par de segundos para que te dé tiempo de abrir el Monitor Serie
  delay(2000);
  Serial.println("Iniciando prueba del sensor ultrasónico HC-SR04...");
  
  // Configuramos el comportamiento de los pines
  pinMode(trigPin, OUTPUT); // El Trig emite el sonido (salida)
  pinMode(echoPin, INPUT);  // El Echo escucha el rebote (entrada)
}

void loop() {
  long duracion;
  float distancia;
  
  // 1. Limpiamos el pin Trig por seguridad poniéndolo en 0V
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  
  // 2. Disparamos un pulso ultrasónico rápido (10 microsegundos a 3.3V)
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  
  // 3. Leemos cuánto tiempo tardó el pin Echo en recibir el rebote (en microsegundos)
  duracion = pulseIn(echoPin, HIGH);
  
  // 4. Calculamos la distancia en centímetros
  // (La velocidad del sonido es 0.0343 cm por microsegundo. Dividimos entre 2 porque el sonido va y regresa)
  distancia = (duracion * 0.0343) / 2.0;
  
  // 5. Imprimimos el resultado en el Monitor Serie
  Serial.print("Distancia: ");
  Serial.print(distancia);
  Serial.println(" cm");
  
  // Esperamos medio segundo antes de tomar la siguiente medida
  delay(500);
}
                

Obtained results

Finally, here a video can be shown where the measurements calculated by the program and the functioning of the sensor are observed.