如何通过MAX2990 I2C接口向标准EEPROM (24C04)连接

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　　引言
　　本文介绍了如何通过MAX2990电力线通信调制解调器的I2C接口与外部EEPROM 24C04连接，并给出了相应的固件例程。I2C总线受控于MAX2990 (主机)，24C04 EEPROM为从机器件。以下框图给出了本文示例的硬件配置。
　　




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　　固件说明
　　I2C接口初始化
　　一旦使能I2C模块，SCL和SDA必须配置成漏极开路状态，以确保I2C总线通信正常。由于I2C是GPIO端口的一个替代功能，固件必须确保SCL和SDA输入在初始化期间禁止上拉(通过对端口控制器的输出位写零实现)。
示例中，时钟频率为250kHz。首先需要配置MAX2990的I2C接口：

PO1_bit.bit2 = 0;                 // Disables the GPIO function of the
PO1_bit.bit3 = 0;                 // I2C pins

I2CCN_bit.I2CEN = 0;         // Makes sure that I2C is disabled
                        // to allow the changing of the I2C settings

I2CCN_bit.I2CMST = 1;                 // Sets the I2C engine to master mode
I2CCN_bit.I2CEA = 0;                 // 7-bit address mode
I2CCK_bit.I2CCKL = 0x40;         // 2μs CLK-low, to define I2C frequency
I2CCK_bit.I2CCKH = 0x40;         // 2μs CLK-high, to define I2C frequency

I2CTO = 200;                 // I2C_TIMEOUT
I2CST = 0x400;                 // Resets I2C status register

I2CCN_bit.I2CEN = 1;                 // Enables the I2C engine

写模式
写入24C04 EEPROM时，必须通过I2C接口写入以下字节：
[ol]
   

EEPROM的I2C总线地址(这里为0xA0)
   

EEPROM存储器的地址
   

数据字节(地址将自动递增)
[/ol]
示例中试图写入以下字节，从0x00地址开始，向EEPROM写入：0x12、0x34、0x56、0x78和0x90。

i2c_init_write();         // Sets the MAX2990 I2C Engine into write mode
i2c_write(0x50);         // 24C04 write (adr = 0b1010 000 0) = 0xA0
                // The MAX2990 I2C engine shifts the I2C address by
                        // 1 bit, because it will generate the R/W bit
                        // automatically

i2c_write(0x00);         // word address location
i2c_write(0x12);         // data1
i2c_write(0x34);         // data2
i2c_write(0x56);         // data3
i2c_write(0x78);         // data4
i2c_write(0x90);         // data5
I2C_STOP;                 // Sends I2C stop-condition




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读模式
读取我们写入EEPROM的数据时，为24C04留出足够的写时间非常关键。通常在“停止条件”后留出几个毫秒的时间，请参考数据资料，确认您的时间设置符合IC的要求。

i2c_init_write();         // Sets the MAX2990 I2C engine into write mode
i2c_write(0x50);         // 24C04 write (adr = 0b1010 000 0) = 0xA0
                // The MAX2990 I2C engine shifts the I2C address by
                        // 1 bit, because it will generate the R/W bit
                        // automatically

i2c_write(0x00);         // word address location

i2c_init_read();         // Sets the MAX2990 I2C engine into read mode

i2c_write(0x50);         // 24C04 read (adr = 0b1010 000 1) = 0xA1
                // The MAX2990 I2C engine shifts the I2C address by
                        // 1 bit, because it will generate the R/W bit
                        // automatically

unsigned char data[5];         // Array to store the received data
i2c_read(data[0]); // Reads 1 byte from I2C and writes it to the array
i2c_read(data[1]); // Reads 1 byte from I2C and writes it to the array
i2c_read(data[2]); // Reads 1 byte from I2C and writes it to the array
i2c_read(data[3]); // Reads 1 byte from I2C and writes it to the array
i2c_read(data[4]); // Reads 1 byte from I2C and writes it to the array
I2C_STOP;                 // Sends I2C stop-condition
                          
                       
                          
                               

现在，我们可以验证一下用于EEPROM读、写操作的功能。

i2c_init_write(void)
i2c_init_read(void)
i2c_write(UINT8 data)
i2c_read(UINT8 *data)

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void i2c_init_write(void)
{
I2CCN_bit.I2CMODE = 0; // I2C transmit mode
I2CCN_bit.I2CACK = 1; // Creates I2C NACK so that slave can create ACK
I2C_START;                 // Generates I2C START condition
while( I2CCN_bit.I2CSTART == 1 );         // Waits until the START condition
                                // was put to the I2C bus
I2CST_bit.I2CSRI = 0;                 // Resets the I2C interrupt flag
}

int i2c_init_read(void)
{
I2CCN_bit.I2CMODE = 1;         // I2C read-mode
I2CCN_bit.I2CACK = 0;         // Creates I2C ACK after receive
I2C_START;                 // Generates I2C START condition

while( I2CCN_bit.I2CSTART == 1 );         // Waits until the START condition
I2CST_bit.I2CSRI = 0;                 // Resets the I2C interrupt flag
}

void i2c_write(UINT8 data)
{
I2CbUF = data;                         // Puts the data on the I2C bus
while( I2CST_bit.I2CTXI == 0 );         // Waits for transfer complete
I2CST_bit.I2CTXI = 0;                 // Resets the I2C transmit complete
                                // interrupt flag
}

void i2c_read(UINT8 *data)
{
I2CbUF = 0xff;         // Puts "all ones" on the I2C bus so that slave can pull
                // the bus down to generate zeros

while( !I2CST_bit.I2CRXI );                 // Waits for receive complete
I2CST_bit.I2CRXI=0;                 // Resets the I2C receive complete
                                        // interrupt flag

*data = I2CbUF;                         // Writes the data to the pointer
}