Class 9 – I2C Concept & RTC using DS1307 – BuddyKit

Hi Buddies!

Today on our BuddyKit, let’s explore one of the essential communication protocols – the I2C Protocol.

Microcontrollers support many protocols like I2C, SPI, and Serial. Among these, I2C (Inter-Integrated Circuit) is widely used for communication between devices on the same board or across boards.

I2C works with one master and multiple slave devices. Each slave device has a unique address, and communication starts when the master sends a data packet with the slave address.

Basics of I2C

I2C has two main lines:

SDA – Serial Data Line
SCL – Serial Clock Line

The master controls communication, initiates data transfer, and makes decisions. We’ll be writing the code from the master side.

I2C has 3 Main Stages:

Start Condition
Data Transfer (Write/Read)
Stop Condition

Start Condition:

Start condition occurs when SDA goes from HIGH to LOW while SCL is HIGH.

// Confirm both SDA and SCL are HIGH
SDA = 1;
SCL = 1;

// Then create a falling edge on SDA
SDA = 0; 
SCL = 0;

// Confirm both SDA and SCL are HIGH

SDA = 1;

SCL = 1;

// Then create a falling edge on SDA

SDA = 0;

SCL = 0;

Stop Condition:

Stop condition occurs when SDA goes from LOW to HIGH while SCL is HIGH.

SDA = 0;
SCL = 1;
SDA = 1;

SDA = 0;

SCL = 1;

SDA = 1;

Data Transfer:

During a write, master sets SDA as output, sends each bit with clock pulses. After 8 bits, 9^th bit is ACK (acknowledge).

During a read, master sets SDA as input and reads data on each clock pulse.

// Read ACK after writing
SDA = 1;
SCL = 1;
ack = SDA;
SCL = 0;

// Send ACK after reading
SDA = ack;
SCL = 1;
SCL = 0;

// Read ACK after writing

SDA = 1;

SCL = 1;

ack = SDA;

SCL = 0;

// Send ACK after reading

SDA = ack;

SCL = 1;

SCL = 0;

I2C Write Operation

u8 mask = 0x80;
while (mask) {
    SDA = (dat &amp; mask) ? 1 : 0;
    SCL = 1; SCL = 0;
    mask &gt;&gt;= 1;
}

u8 mask = 0x80;

while (mask) {

SDA = (dat & mask) ? 1 : 0;

SCL = 1; SCL = 0;

mask >>= 1;

}

I2C Read Operation

SDA = 1;
for (i = 0; i &lt; 8; i++) {
    dat &lt;&lt;= 1;
    SCL = 1;
    if (SDA) dat++;
    SCL = 0;
}

SDA = 1;

for (i = 0; i < 8; i++) {

dat <<= 1;

SCL = 1;

if (SDA) dat++;

SCL = 0;

}

Interfacing DS1307 RTC with I2C

Now let’s apply the I2C concept with DS1307 – a Real Time Clock (RTC) module.

This is an I2C slave device with these addresses:

0xD0 – for Write
0xD1 – for Read

RTC stores data in specific memory locations:

0x00 – Seconds
0x01 – Minutes
0x02 – Hours
0x03 – Day of Week
0x04 – Date
0x05 – Month
0x06 – Year

Note: The RTC uses 1970 as the base year. So, if you see 55 in RTC memory, it means 2025 (1970 + 55).

All values are stored in BCD format.

RTC Write Code:

void RTC_Write(void) {
    I2C_Start();
    I2C_Write(0xD0);     // Slave Write
    I2C_Write(0x00);     // Start address
    I2C_Write(HEX2BCD(seconds) &amp; 0x3F);
    I2C_Write(HEX2BCD(minutes));
    I2C_Write(HEX2BCD(hours));
    I2C_Write(HEX2BCD(dayOfWeek));
    I2C_Write(HEX2BCD(date));
    I2C_Write(HEX2BCD(month));
    I2C_Write(HEX2BCD(year + 30));
    I2C_Stop();
}

void RTC_Write(void) {

I2C_Start();

I2C_Write(0xD0); // Slave Write

I2C_Write(0x00); // Start address

I2C_Write(HEX2BCD(seconds) & 0x3F);

I2C_Write(HEX2BCD(minutes));

I2C_Write(HEX2BCD(hours));

I2C_Write(HEX2BCD(dayOfWeek));

I2C_Write(HEX2BCD(date));

I2C_Write(HEX2BCD(month));

I2C_Write(HEX2BCD(year + 30));

I2C_Stop();

}

RTC Read Code:

u8 RTC_Read(void) {
    I2C_Start();
    I2C_Write(0xD0);     // Slave Write
    I2C_Write(0x00);     // Start address
    I2C_Start();
    I2C_Write(0xD1);     // Slave Read

    seconds    = BCD2HEX(I2C_Read(0)) &amp; 0x3F;
    minutes    = BCD2HEX(I2C_Read(0));
    hours      = BCD2HEX(I2C_Read(0));
    dayOfWeek  = BCD2HEX(I2C_Read(0));
    date       = BCD2HEX(I2C_Read(0));
    month      = BCD2HEX(I2C_Read(0));
    year       = BCD2HEX(I2C_Read(1)) - 30;

    I2C_Stop();
    return 0;
}

u8 RTC_Read(void) {

I2C_Start();

I2C_Write(0xD0); // Slave Write

I2C_Write(0x00); // Start address

I2C_Start();

I2C_Write(0xD1); // Slave Read

seconds = BCD2HEX(I2C_Read(0)) & 0x3F;

minutes = BCD2HEX(I2C_Read(0));

hours = BCD2HEX(I2C_Read(0));

dayOfWeek = BCD2HEX(I2C_Read(0));

date = BCD2HEX(I2C_Read(0));

month = BCD2HEX(I2C_Read(0));

year = BCD2HEX(I2C_Read(1)) - 30;

I2C_Stop();

return 0;

}

Circuit Diagram

RTC Clock Display Demo

Using the above code and our BuddyKit RTC & LCD libraries, we can display a real-time clock on LCD.

#include "../library/I2C/I2C.h"
#include "../library/LCD4/LCD4.h"
#include "../library/DS1307/DS1307.h"
#include "../library/Delay/Delay.h"

void main() {
    u8 buff[17];

    I2C_Init();    // Initialize I2C
    LCD_Init();    // Initialize LCD
    RTC_Init();    // Initialize RTC

    LCD_LeftToRight();   // Default: Left-to-right text
    LCD_NoAutoscroll();  // Disable autoscroll (manual scrolling only)

    LCD_Puts("RTC CLOCK DEMO");

    while (1) { // Infinite loop
        RTC_Read();          // Read current time from RTC
        RTC_Time2Str(buff);  // Convert time to string format
        LCD_ClearRow(0);     // Clear first row of LCD
        LCD_Puts(buff);      // Display time on LCD

        Delay1s();           // Wait 1 second before updating again
    }
}

#include "../library/I2C/I2C.h"

#include "../library/LCD4/LCD4.h"

#include "../library/DS1307/DS1307.h"

#include "../library/Delay/Delay.h"

void main() {

u8 buff[17];

I2C_Init(); // Initialize I2C

LCD_Init(); // Initialize LCD

RTC_Init(); // Initialize RTC

LCD_LeftToRight(); // Default: Left-to-right text

LCD_NoAutoscroll(); // Disable autoscroll (manual scrolling only)

LCD_Puts("RTC CLOCK DEMO");

while (1) { // Infinite loop

RTC_Read(); // Read current time from RTC

RTC_Time2Str(buff); // Convert time to string format

LCD_ClearRow(0); // Clear first row of LCD

LCD_Puts(buff); // Display time on LCD

Delay1s(); // Wait 1 second before updating again

}

Final Note

We hope this I2C protocol explanation and RTC example helped you understand how real-world sensors and timekeeping modules communicate with microcontrollers.

There are many more I2C-based devices like gyroscopes, ADCs, temperature & humidity sensors, accelerometers, and more. In our upcoming videos, we’ll cover many of them using BuddyKit.

Stay tuned for our next topic. See you soon!

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Class 9 - I2C Concept & RTC using DS1307

June 06, 2025

Basics of I2C

I2C has 3 Main Stages:

Start Condition:

Stop Condition:

Data Transfer:

I2C Write Operation

I2C Read Operation

Interfacing DS1307 RTC with I2C

RTC Write Code:

RTC Read Code:

Circuit Diagram

RTC Clock Display Demo

Final Note

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Class 9 – I2C Concept & RTC using DS1307