Connecting multiple I2C devices in Arduino - Step-by-Step Execution

Concept Flow - Connecting multiple I2C devices

Start I2C Bus

↓

Scan for Devices

↓

Identify Device Addresses

↓

Connect Devices with Unique Addresses

↓

Communicate with Each Device

↓

Handle Data Separately

↓

End Communication

This flow shows how the Arduino starts the I2C bus, finds devices, connects to each with unique addresses, and communicates with them one by one.

Execution Sample

Arduino

Wire.begin();
Wire.beginTransmission(0x3C);
Wire.write(0x00);
Wire.endTransmission();
Wire.beginTransmission(0x68);
Wire.write(0x00);
Wire.endTransmission();

This code initializes I2C, then sends data to two devices with addresses 0x3C and 0x68 separately.

Execution Table

Step	Action	Device Address	Data Sent	Result
1	Initialize I2C bus	-	-	Bus ready
2	Begin transmission	0x3C	-	Started communication with device 0x3C
3	Write data	0x3C	0x00	Data queued for device 0x3C
4	End transmission	0x3C	-	Data sent to device 0x3C
5	Begin transmission	0x68	-	Started communication with device 0x68
6	Write data	0x68	0x00	Data queued for device 0x68
7	End transmission	0x68	-	Data sent to device 0x68

💡 All devices communicated with separately using their unique addresses.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
currentDevice	none	0x3C	0x3C	none	0x68	0x68	none
dataBuffer	empty	empty	0x00	empty	empty	0x00	empty

Key Moments - 2 Insights

Why do we need to use different addresses for each I2C device?

Each device on the I2C bus must have a unique address so the Arduino knows which device to talk to, as shown in steps 2 and 5 where different addresses 0x3C and 0x68 are used.

Can we send data to multiple devices at the same time on I2C?

No, the Arduino communicates with one device at a time by starting and ending transmission separately for each device, as seen in steps 2-4 and 5-7.

Visual Quiz - 3 Questions

Look at the execution table, what device address is used at step 5?

A 0x3C

B 0x68

C 0x00

D No device

💡 Hint

Check the 'Device Address' column at step 5 in the execution_table.

At which step is data actually sent to the device 0x3C?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look for 'End transmission' action for device 0x3C in the execution_table.

If both devices had the same address, what would happen in the communication?

A Both devices receive data correctly

B Arduino stops communication

C Only one device responds, causing conflicts

D Data is sent to a new address automatically

💡 Hint

Recall the key moment about unique addresses and how Arduino communicates with one device at a time.

Concept Snapshot

Connecting multiple I2C devices:
- Each device must have a unique address.
- Use Wire.begin() to start I2C.
- Communicate by beginTransmission(address), write(data), endTransmission().
- Talk to devices one at a time.
- Addresses avoid conflicts on the bus.

Full Transcript

This lesson shows how to connect multiple I2C devices to an Arduino. The Arduino starts the I2C bus with Wire.begin(). It then communicates with each device separately by using the device's unique address. For example, it sends data to device 0x3C first, then to device 0x68. Each communication starts with beginTransmission(address), then data is written, and finally endTransmission() sends the data. Devices must have different addresses so the Arduino knows who to talk to. The execution table shows each step clearly. This method avoids conflicts and ensures each device gets the right data.

Practice

1. What is the main reason multiple I2C devices can share the same two wires on an Arduino?

easy

A. They use different clock speeds to avoid conflicts

B. Each device uses a separate data wire

C. Each device has a unique address to identify it on the bus

D. They communicate one at a time using different power pins

Solution

1. Step 1: Understand I2C wiring basics

   I2C devices share two wires: SDA (data) and SCL (clock).

2. Step 2: Identify how devices are distinguished

   Devices use unique addresses so the master knows which device to talk to.

3. Final Answer:

   Each device has a unique address to identify it on the bus -> Option C

4. Quick Check:

   Unique addresses allow multiple devices on same wires [OK]

Hint: Remember: unique addresses let devices share wires [OK]

Common Mistakes:

• Thinking devices use separate wires
• Assuming different clock speeds separate devices
• Believing devices use different power pins to communicate

2. Which Arduino code snippet correctly initializes the I2C bus for multiple devices?

easy

A. Wire.init();

B. Wire.begin();

C. Wire.begin(0x3C);

D. Wire.start();

Solution

1. Step 1: Recall the correct function to start I2C

   The Arduino Wire library uses Wire.begin() to start the I2C bus as master.

2. Step 2: Check other options for correctness

   Wire.begin(0x3C) sets device as slave with address 0x3C, not for master init. Wire.init() and Wire.start() do not exist in Wire library.

3. Final Answer:

   Wire.begin(); -> Option B

4. Quick Check:

   Wire.begin() starts I2C master bus [OK]

Hint: Use Wire.begin() once to start I2C bus [OK]

Common Mistakes:

• Using Wire.begin(address) to start master
• Calling non-existent Wire.init() or Wire.start()
• Not calling Wire.begin() at all

3. Given this Arduino code snippet, what will be the output on the Serial Monitor if device at address 0x40 returns 25 and device at 0x41 returns 30?

Wire.begin();
Wire.beginTransmission(0x40);
Wire.write(0x00);
Wire.endTransmission();
Wire.requestFrom(0x40, 1);
int temp1 = Wire.read();
Wire.beginTransmission(0x41);
Wire.write(0x00);
Wire.endTransmission();
Wire.requestFrom(0x41, 1);
int temp2 = Wire.read();
Serial.print(temp1);
Serial.print(", ");
Serial.println(temp2);

medium

A. Compilation error

B. 30, 25

C. 0, 0

D. 25, 30

Solution

1. Step 1: Understand the communication sequence

   The code requests one byte from device 0x40, reads it as temp1 (25), then requests one byte from 0x41, reads it as temp2 (30).

2. Step 2: Check the Serial output commands

   Serial.print(temp1) prints 25, then ", ", then Serial.println(temp2) prints 30 with newline.

3. Final Answer:

   25, 30 -> Option D

4. Quick Check:

   Read values match device addresses in order [OK]

Hint: Match device address to read value order [OK]

Common Mistakes:

• Mixing up device addresses and values
• Assuming output is reversed
• Thinking code causes compile error

4. This Arduino code tries to read from two I2C devices but always reads zero. What is the likely error?

Wire.begin();
Wire.beginTransmission(0x50);
Wire.write(0x01);
Wire.endTransmission();
Wire.requestFrom(0x50, 1);
int val1 = Wire.read();
Wire.beginTransmission(0x50);
Wire.write(0x02);
Wire.endTransmission();
Wire.requestFrom(0x51, 1);
int val2 = Wire.read();
Serial.println(val1 + val2);

medium

A. Requesting from 0x51 instead of 0x50 causes no data

B. Missing Wire.begin() call

C. Using Wire.write() before Wire.requestFrom() is wrong

D. Serial.println() cannot print integers

Solution

1. Step 1: Analyze device addresses used

   The code writes to 0x50 twice but requests data from 0x50 first, then 0x51 second. If 0x51 device is not connected, no data is returned.

2. Step 2: Understand impact on read values

   Reading from 0x51 returns no data, so Wire.read() returns 0, causing sum to be zero or incorrect.

3. Final Answer:

   Requesting from 0x51 instead of 0x50 causes no data -> Option A

4. Quick Check:

   Address mismatch causes no data read [OK]

Hint: Check device addresses match in all calls [OK]

Common Mistakes:

• Confusing write and requestFrom addresses
• Forgetting to call Wire.begin() once
• Thinking Wire.write() before requestFrom() is invalid

5. You want to connect three I2C temperature sensors with addresses 0x48, 0x49, and 0x4A to an Arduino. Which of the following is the best approach to read all sensors reliably?

hard

A. Call Wire.begin() once, then read each sensor by its unique address with Wire.requestFrom()

B. Call Wire.begin() before reading each sensor separately

C. Use separate SDA and SCL wires for each sensor to avoid conflicts

D. Assign the same address to all sensors and read them one by one

Solution

1. Step 1: Understand I2C bus initialization

   Wire.begin() must be called once to initialize the bus as master before any communication.

2. Step 2: Read sensors by unique addresses on shared bus

   Each sensor has a unique address; read each by calling Wire.requestFrom() with that address on the same SDA and SCL lines.

3. Step 3: Evaluate other options

   Calling Wire.begin() multiple times is unnecessary. Using separate wires defeats I2C design. Assigning same address causes conflicts.

4. Final Answer:

   Call Wire.begin() once, then read each sensor by its unique address with Wire.requestFrom() -> Option A

5. Quick Check:

   One Wire.begin(), unique addresses, shared wires [OK]

Hint: Initialize once, read each device by unique address [OK]

Common Mistakes:

• Calling Wire.begin() multiple times
• Trying separate wires for each I2C device
• Using same address for multiple devices