Hi,
Sorry for the delay in responding your question Erik sir. The microcontroller used in my project is C8051F120. Then, I’m unable to understand your question.’ derivative or deviate?’

void main(void)
{
SFRPAGE=LEGACY_PAGE;
WDTCN = 0xde; // disable watchdog timer
WDTCN = 0xad;

SYSCLK_Init(); // turn on the external oscillator

SFRPAGE = CONFIG_PAGE;
XBR0 = 0x01; // Route SMBus to GPIO pins through crossbar
XBR2 = 0x40; // Enable crossbar and weak pull-ups
P7MDOUT = 0x1f; // enable (P7^0,^1,^2,^3,^4)as push pull-LCD control lines
// P0MDOUT = 0x03;

// SFRPAGE = SMB0_PAGE;
// SMB0CN = 0x40; // Enable SMBus with acknowledge low (AA = 1)
// SMB0CR = -245; // SMBus clock rate = 100 kHz
// SMB0ADR = ADDRTC; // Set own slave address.

EA = 1; // Global interrupt enable
// EIE1 |= 0x02; // SMBus interrupt enable

sqw = 1; // set up for read, weak pull-up

while (1)
{
initialize();
disp_clk_regs(0x00); //To display seconds
writebyte(0x20,0x50);
readbyte(0x20);
}
}


void I2C_start()
{
sda = 1;
scl = 1; // Initiate start condition
sda = 0;
}

void I2C_stop()
{
sda = 0;
sda = 0;
sda = 0;
sda = 0; // Initiate stop condition
scl = 1;
scl = 1;
sda = 1;
}

void I2C_write(unsigned char d)
{
unsigned char i;

scl = 0;
for (i = 1; i <= 8; i++)
{
sda = (d >> 7);
scl = 1;
d = d << 1; // increase scl high time
scl = 0;
}
sda = 1; // Release the sda line
scl = 0;
scl = 1;
if(sda)
//Display 'ACK bit missing' in the lcd
scl = 0;
}

unsigned char I2C_read(unsigned char b)
{
unsigned char d, i;

sda = 1; // Let go of sda line
scl = 0;
for (i = 1; i <= 8; i++) // read the msb first
{
scl = 1;
d = d << 1;
d = d | (unsigned char)sda;
scl = 0;
}
sda = b; // Hold sda low for acknowledge

scl = 0;
scl = 1;
if(b == NACK)
sda = 1; // sda = 1 if next cycle is reset
scl = 0;

sda = 1; // Release the sda line
return d;
}

void readbyte(char Add)
{
I2C_start();
I2C_write(ADDRTC);
I2C_write(Add); // 'Add' = Address containing the data to be read
I2C_start();
I2C_write(ADDRTC | 1);
a=I2C_read(NACK);
//display the value in the 'a' variable
I2C_read(NACK);
I2C_stop();
}

void writebyte(char Add,char Data) /* -- write one byte of data to the specified address -- */
{
I2C_start();
I2C_write(ADDRTC);
I2C_write(Add); //'Add'= Address in which the data is to be written
I2C_write(Data); //'Data'=Data to be written in the specified address
I2C_stop();
}

Then, I want to know whether the SMBus peripheral is to be enabled for this RTC interfacing. Also, tell the changes to be done in the program.