Understanding the digital read pin on a microcontroller

In electronics one very often works with digital readings. Digital read pins are available on many microcontrollers and are one of the simpler pins to work with.

Table of contents

Introduction to the digital read pin on a microcontroller

Digital read pins are available on almost any microcontroller. Raspberry Pi B models have 22 digital read pins (which also serve as digital write pins). Digital read pink (also referred to as digital input pins) are used to determine whether an electronic signal is high or low. Depending on these two variables, further logic can be applied.

On many microcontrollers, many of the pins, including digital read pins doubles as other pins too. It is the programming software (e.g. Arduino IDE or Python) that defines the final function of the pin — in this case, digital read.

Analog pins can also be declared as digital pins, but are much slower.

Digital readings

A digital read pin is used to read values from a connected component that outputs voltages. Being digital, this input signal will either be interpreted as a (relative) HIGH or (relative) LOW (i.e. 1 or 0). This interpreted signal can then be used further for a simple yes/no, true/false or on/off logic.

A programmed microcontroller will run its code over and over again to detect input from (and/or send output to) the declared pins. Declared digital reading pins will read the state of the pin at the specific instant in time the code is triggered.

In order for a digital reading signal to be transmitted, the connected device also needs to be connected to the same (or a similar) power supply source as the microcontroller. Digital pins on a microcontroller use TTL (transistor-to-transistor logic) and will read voltages ranging from 0V to as high as the Vcc level of the microcontroller (usually 3.3 – 5 V DC).

Compared with analogue pins, which can also read the value of voltages, digital pins are much faster. Analogue readings can, however, tell more precisely what the (relative) voltage is on a pin.

Examples of digital output devices include buttons, sensor modules (e.g. HC-SR501 PIR Motion Sensors, Magnetic door sensors, etc.) and receiver modules (e.g. Laser Transmitter Receivers, etc.).

Arduino programming language

In the Arduino programming language, digital reading is programmed as follows:

// Constants to set the pin number:
const int PinNumber = 2; // the number of the digital read pin

void setup() {
  pinMode(PinNumber, INPUT); // initialize the digital read pin as an input
}

void loop() {
  State = digitalRead(PinNumber); // read the state of the digital read value

  if (State == HIGH) { // if the value is HIGH
    // do something
  } else { // if the value is LOW
    // do something else
  }
  delay(200); // wait for 200 ms
}

Raspberry Pi & Python

import RPi.GPIO as GPIO
import time

PinNumber = 17 # the number of the digital read pin
GPIO.setmode(GPIO.BCM)
GPIO.setup(PinNumber,GPIO.IN) # initialize the digital read pin as an input

while True:
  if (GPIO.input(PinNumber)):
    # do something
  else:
    # do something else
  time.sleep(0.2) # wait for 200 ms

Naming

Digital pins are usually named D1, D2, D3, etc.

Receiver modules

With certain connected receiver modules (e.g. 433MHz RF Transmitter Receivers), the model itself can convert their received data (e.g. strings) to a series of 1’s and 0’s, which can then be transmitted to the digital read pin as a set of HIGHs and LOWs. With the correct libraries, these combinations/sets of HIGH/LOW readings can then be converted back to the original string value.

The fact that the digital read pin can only read the state of the pin at a certain given moment means that an extra parameter, time, needs to be involved. Although this couldn’t be confirmed, I suspect that this is where an interrupt pin comes in.

The interrupt pin is able to detect a combination of inputs before declaring the signal complete. Different length pulses of HIGH/LOW are used to indicate a 1 or a 0. A long HIGH and short LOW could be a 1, and a short HIGH and long LOW could be a 0.

Conclusion

Almost any microcontroller has one or more digital read pins which are used to determine whether a signal is high or low. Depending on these two variables, further logic can be applied.