I2C Start and Stop Condition in Embedded C Explained

In I2C communication, the start condition signals the beginning of data transfer by pulling the data line low while the clock line is high. The stop condition signals the end by releasing the data line to high while the clock line is high. These conditions are essential to control the flow of data between devices in embedded C programs.

How It Works

Think of I2C communication like a conversation between two friends using a walkie-talkie. The start condition is like saying "Hello" to get the other person's attention before talking. It happens when the data line (SDA) changes from high to low while the clock line (SCL) stays high. This tells all devices on the bus that a message is about to begin.

The stop condition is like saying "Goodbye" to end the conversation. It occurs when the data line goes from low to high while the clock line remains high. This signals that the communication is finished and the bus is free for others to use.

These conditions help devices know when to listen and when to stop, ensuring smooth and organized data exchange on the shared I2C bus.

Example

This example shows how to generate I2C start and stop conditions by controlling the SDA and SCL lines using embedded C. It assumes you can set and clear these lines as GPIO pins.

embedded_c

#define SDA_HIGH()  (GPIO_SetPin(SDA_PIN))
#define SDA_LOW()   (GPIO_ClearPin(SDA_PIN))
#define SCL_HIGH()  (GPIO_SetPin(SCL_PIN))
#define SCL_LOW()   (GPIO_ClearPin(SCL_PIN))

void I2C_Start(void) {
    SDA_HIGH();
    SCL_HIGH();
    // Small delay to stabilize lines
    delay_us(5);
    SDA_LOW();  // Start condition: SDA goes low while SCL is high
    delay_us(5);
    SCL_LOW();  // Prepare for data transfer
}

void I2C_Stop(void) {
    SDA_LOW();
    SCL_HIGH();
    delay_us(5);
    SDA_HIGH(); // Stop condition: SDA goes high while SCL is high
    delay_us(5);
}

// Dummy delay function
void delay_us(int us) {
    volatile int count;
    while(us--) {
        count = 10;
        while(count--) {} // simple loop for delay
    }
}

When to Use

You use the I2C start and stop conditions whenever you want to begin or end communication with a device on the I2C bus. For example, when your microcontroller needs to read data from a sensor or write data to an EEPROM chip, it first sends a start condition to alert the devices, then sends the address and data, and finally sends a stop condition to finish.

These conditions are crucial in embedded systems where multiple devices share the same communication lines, preventing data collisions and ensuring each device knows when to listen or talk.

Key Points

• The start condition signals the beginning of I2C communication by pulling SDA low while SCL is high.
• The stop condition signals the end by releasing SDA to high while SCL is high.
• Both conditions help coordinate data transfer between multiple devices on the I2C bus.
• In embedded C, these are implemented by controlling GPIO pins connected to SDA and SCL lines.

Key Takeaways

I2C start and stop conditions control when communication begins and ends on the bus.

Start condition: SDA goes low while SCL is high; stop condition: SDA goes high while SCL is high.

These signals prevent data conflicts between multiple devices sharing the I2C lines.

In embedded C, start and stop are created by setting GPIO pins connected to SDA and SCL.

Always send a start before data and a stop after to properly frame I2C messages.