I2C scanner sketch in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this I2C scanner sketch?

Consider this Arduino I2C scanner sketch. What will it print to the Serial Monitor when no I2C devices are connected?

Arduino

#include <Wire.h>

void setup() {
  Wire.begin();
  Serial.begin(9600);
  while (!Serial);
  Serial.println("I2C Scanner");
}

void loop() {
  byte error, address;
  int nDevices = 0;

  Serial.println("Scanning...");

  for(address = 1; address < 127; address++ ) {
    Wire.beginTransmission(address);
    error = Wire.endTransmission();

    if (error == 0) {
      Serial.print("I2C device found at address 0x");
      if (address<16)
        Serial.print("0");
      Serial.print(address,HEX);
      Serial.println("  !");
      nDevices++;
    }
  }
  if (nDevices == 0)
    Serial.println("No I2C devices found\n");
  else
    Serial.println("done\n");

  delay(5000);
}

A

I2C Scanner
Scanning...
No I2C devices found

B

I2C Scanner
Scanning...
I2C device found at address 0x00  !
done

C

I2C Scanner
Scanning...
I2C device found at address 0x7F  !
done

D

I2C Scanner
Scanning...
Error: Bus busy

💡 Hint

Think about what happens when no devices respond to the I2C address scan.

The sketch scans all addresses from 1 to 126. If no device responds (error != 0), it prints "No I2C devices found". So with no devices connected, only that message appears after scanning.

❓ Predict Output

intermediate

What is the output if a device is connected at address 0x3C?

Using the same I2C scanner sketch, what will the Serial Monitor show if a device is connected at address 0x3C?

Arduino

#include <Wire.h>

void setup() {
  Wire.begin();
  Serial.begin(9600);
  while (!Serial);
  Serial.println("I2C Scanner");
}

void loop() {
  byte error, address;
  int nDevices = 0;

  Serial.println("Scanning...");

  for(address = 1; address < 127; address++ ) {
    Wire.beginTransmission(address);
    error = Wire.endTransmission();

    if (error == 0) {
      Serial.print("I2C device found at address 0x");
      if (address<16)
        Serial.print("0");
      Serial.print(address,HEX);
      Serial.println("  !");
      nDevices++;
    }
  }
  if (nDevices == 0)
    Serial.println("No I2C devices found\n");
  else
    Serial.println("done\n");

  delay(5000);
}

A

I2C Scanner
Scanning...
I2C device found at address 0x03  !
done

B

I2C Scanner
Scanning...
I2C device found at address 0x3C  !
done

C

I2C Scanner
Scanning...
No I2C devices found

D

I2C Scanner
Scanning...
I2C device found at address 0x7E  !
done

💡 Hint

Check the address range and how the address is printed in hexadecimal.

The device at 0x3C responds, so the scanner prints that address with leading zero if less than 0x10. 0x3C is 60 decimal, so no leading zero is added. The output shows the found device and then "done".

🧠 Conceptual

advanced

Why does the scanner skip address 0x00 and 0x7F?

In the I2C scanner sketch, the for loop scans addresses from 1 to 126. Why does it skip 0x00 and 0x7F?

A Because 0x00 and 0x7F are broadcast addresses that always respond with an error.

B Because 0x00 and 0x7F are invalid addresses that cause the Arduino to reset.

C Because 0x00 is the general call address and 0x7F is reserved for future use, so scanning them can cause errors or unexpected behavior.

D Because 0x00 and 0x7F are used for SPI communication, not I2C.

💡 Hint

Think about special reserved addresses in the I2C protocol.

Address 0x00 is the general call address used to address all devices simultaneously. Address 0x7F is reserved and not used for devices. Scanning these can cause issues or meaningless results.

🔧 Debug

advanced

What error does this modified I2C scanner code produce?

This code snippet is a modified I2C scanner. What error will it cause when compiled or run? #include void setup() { Wire.begin(); Serial.begin(9600); Serial.println("I2C Scanner"); } void loop() { byte error, address; int nDevices = 0; Serial.println("Scanning..."); for(address = 0; address <= 127; address++) { Wire.beginTransmission(address); error = Wire.endTransmission(); if (error == 0) { Serial.print("Device at 0x"); Serial.println(address, HEX); nDevices++; } } delay(5000); }

A Compilation error: 'address' variable overflow due to using byte type with 0 to 127 range

B Runtime error: Wire.endTransmission() returns unexpected error for address 0 and 127

C Compilation error: missing semicolon after Serial.println statement

D No error, but scanning address 0 and 127 may cause unexpected device responses

💡 Hint

Check the address range and the data type used for address variable.

The byte type can hold values 0 to 255, so no overflow occurs. The code compiles fine. Addresses 0 and 127 are special in I2C and may cause unexpected responses, but no runtime error occurs.

🚀 Application

expert

How to modify the I2C scanner to detect multiple devices and print their addresses in ascending order?

You want to modify the I2C scanner sketch to store all found device addresses in an array and then print them sorted ascending after scanning completes. Which code snippet correctly implements this?

A

byte devices[127]; int count = 0;
for(byte address=1; address<127; address++) {
  Wire.beginTransmission(address);
  if(Wire.endTransmission() == 0) {
    devices[count++] = address;
  }
}
for(int i=0; i<count-1; i++) {
  for(int j=i+1; j<count; j++) {
    if(devices[j] < devices[i]) {
      byte temp = devices[i]; devices[i] = devices[j]; devices[j] = temp;
    }
  }
}
for(int i=0; i<count; i++) {
  Serial.print("0x"); Serial.println(devices[i], HEX);
}

B

byte devices[127]; int count = 0;
int compare(const void *a, const void *b) {
  return (*(byte*)a - *(byte*)b);
}
for(byte address=1; address<127; address++) {
  Wire.beginTransmission(address);
  if(Wire.endTransmission() == 0) {
    devices[count++] = address;
  }
}
qsort(devices, count, sizeof(byte), compare);
for(int i=0; i<count; i++) {
  Serial.print("0x"); Serial.println(devices[i], HEX);
}

C

byte devices[127]; int count = 0;
for(byte address=1; address<127; address++) {
  Wire.beginTransmission(address);
  if(Wire.endTransmission() == 0) {
    devices[count++] = address;
  }
}
for(int i=0; i<count; i++) {
  Serial.print("0x"); Serial.println(devices[i], HEX);
}
// No sorting

D

byte devices[127]; int count = 0;
for(byte address=1; address<127; address++) {
  Wire.beginTransmission(address);
  if(Wire.endTransmission() == 0) {
    devices[count++] = address;
  }
}
std::sort(devices, devices + count);
for(int i=0; i<count; i++) {
  Serial.print("0x"); Serial.println(devices[i], HEX);
}

💡 Hint

Arduino C++ does not support std::sort by default. qsort requires a function declared before use.

Option A implements a manual bubble sort to order the addresses ascending, then prints them. Option A prints unsorted. Option A uses qsort but the compare function is declared after usage, causing compile error. Option A uses std::sort which is not available in Arduino environment by default.