Hello! I have write a program to write and read to/from AT24c04 serial ROM (89C52 interface with AT24C04). I am using Keil C to program. But i can read any data which i have write to AT24c04. I don't know exacterly why. My purpose is very simple. I listent from USART and get data from PC key board. If the character which i get from pc is 'w' or 'W' then i do get data from PC and write to AT24c04 until i get the 0x0D. Now if i get the 'r' or 'R' character from PC i read data which i have just write to AT24c04 and send back to PC. But i can't read any. Here is my code #include <REG52.h> unsigned char data DEVICEADD; sbit SDA = P1^0; sbit SCL = P1^1; int data c; char data wrom; char data rrom; void outchar(char data chr) { while(!TI) { } TI = 0; SBUF = chr; } void outstr(char* str) { while(*str != 0) { outchar(*str++); } } char i2c_start(void) { /* if(!SDA || !SCL) return 0; // i2c_start error: bus not free */ // Both bus lines are high, SDA = 0; // so set data line low to generate a start condition. SCL = 0; /* SDA = 1; SCL = 1; SDA = 0; SCL = 0; */ return 1; } void i2c_stop(void) { /* SDA = 0; SCL = 1; SDA = 1; */ unsigned char input_var; SCL = 1; SDA = 0; SCL = 1; SDA = 1; input_var = SDA; } // return ack value: 0 for fale; 1 for success char i2c_write(char data byte) { int data i; for(i = 0; i < 8 ; i++) { SDA = ((byte & 0x80) ? 1 : 0); SCL = 1; // Ensure that the data is stable during the high period. byte <<= 1; SCL = 0; } // Get ack SDA = 1; SCL = 1; i = SDA; //i = 500; SCL = 0; /* while(i-- && SDA) { } */ return !i; // Get data at end of cycle. } char i2c_writebyte(int data address, char data byte) { if(!i2c_start()) { outstr("i2c_start error: bus not free.\n"); return 0; } if(!i2c_write(DEVICEADD)) { i2c_stop(); i2c_start(); if(!i2c_write(DEVICEADD)) // Interface failed to acknowledge device address { i2c_stop(); outstr("Interface failed to acknowledge device address.\n"); return 0; } } if(!i2c_write(address >>8)) // Interface failed to acknowledge byteH address { i2c_stop(); outstr("Interface failed to acknowledge byteH address.\n"); return 0; } if(!i2c_write(address)) // Interface failed to acknowledge byteL address { i2c_stop(); outstr("Interface failed to acknowledge byteL address.\n"); return 0; } if(!i2c_write(byte)) // Interface failed to acknowledge data { i2c_stop(); outstr("Interface failed to acknowledge data.\n"); return 0; } i2c_stop(); return 1; // success } char i2c_read(char data ACK) { char index, byte; index = SDA; // Put data pin into read mode byte = 0x00; for(index = 0; index < 8; index++) // Send 8 bits to the I2C Bus { byte <<= 1; SCL = 1; // Clock the data into the I2C Bus byte |= SDA; // Input the data from the I2C Bus SCL = 0; } // Write ack bit SDA = ACK; SCL = 1; // Ensure that the data is stable during the high period. SCL = 0; return byte; } char i2c_readbyte(int data address) { char data byte; if(!i2c_start()) { outstr("i2c_start error: bus not free.\n"); return 0; } if(!i2c_write(DEVICEADD)) { i2c_stop(); i2c_start(); if(!i2c_write(DEVICEADD)) // Interface failed to acknowledge device address { i2c_stop(); outstr("Interface failed to acknowledge device address.\n"); return 0; } } // Transmit address to read from. if(!i2c_write(address >> 8)) // Interface failed to acknowledge byteH address { i2c_stop(); outstr("Interface failed to acknowledge byteH address. \n"); return 0; } if(!i2c_write(address)) // Interface failed to acknowledge byteL address { i2c_stop(); outstr("Interface failed to acknowledge byteL address. \n"); return 0; } // Now terminate write operation and do a read. if(!i2c_start()) { outstr("i2c_start error: bus not free.\n"); return 0; } // Transmit address to read from. if(!i2c_write(DEVICEADD | 1)) // Slave failed to ack read access { i2c_stop(); outstr("Slave failed to ack read access.\n"); return 0; } // Read from address and ack = 1. byte = i2c_read(1); i2c_stop(); return byte; } char get_char() { while(!RI) { } RI = 0; return SBUF; } void WriteData() { char data ch; ch = get_char(); while(ch != 0x0D) { i2c_writebyte(c++,ch); outchar(ch); ch = get_char(); } wrom = 0; ES = 1; } void ReadData() { char data index; char data input; for(index = 0; index < c; index++) { input = i2c_readbyte(index); if(input != 0) outchar(input); } c = 0; rrom = 0; ES = 1; } void main(void) { // Init serial SCON = 0x52; // Set timer1 mode and reload value TMOD &= ~0xF0; // clear timer 1 mode bits TMOD |= 0x20; // put timer 1 into MODE 2 8 bit auto reload TH1 = 0xFD; TR1 = 1; // start timer 1 TI = 0; ES = 1; // Enable Serial interrupt EA = 1; // Enable all interrupts c = 0; rrom = 0; wrom = 0; DEVICEADD = 0xA0; // 10100000B while(1) { if(wrom == 1) { outchar('W'); WriteData(); } if(rrom == 1) { outchar('R'); ReadData(); } /* count = get_char(); outchar(count); */ } } void serial (void) interrupt 4 using 3 { char data inp; if(RI) // Received data interrupt { RI = 0; inp = (char)SBUF; if((inp == 'w') || (inp == 'W')) { ES = 0; rrom = 0; c = 0; TI = 1; wrom = 1; } else if((inp == 'r') || (inp == 'R')) { ES = 0; wrom = 0; TI = 1; rrom = 1; } } else if(TI) // Transmitted data interrupt { TI = 0; } } Could you help me ?