前段時間使用ATMEGA32的TWI，總結如下：

unsigned char WriteRtc( unsigned char SlaAddr, unsigned char RegAddr, unsigned char *SlaData, unsigned char Length )
{
unsigned char i;
TWCR = ( 1TWINT ) | ( 1TWSTA ) | ( 1TWEN );
while( !( TWCR ( 1TWINT ) ) ); //wait for TWINT set , caused by start signal


TWDR = SlaAddr; //TWDR must be written when TWINT set
TWCR = ( 1TWINT ) | ( 1TWEN );
while( !( TWCR ( 1TWINT ) ) ); //wait for TWINT set , caused by transmit SlaAddr signal

TWDR = RegAddr; //write register address
TWCR = ( 1TWINT ) | ( 1TWEN ); //clear TWINT
while( !( TWCR ( 1TWINT ) ) ); //wait for TWINT set , caused by RegAddr signal

for( i = 0; i Length; i++ )
{
TWDR = *( SlaData+i );
TWCR = ( 1TWINT ) | ( 1TWEN );
while( !( TWCR ( 1TWINT ) ) );
}

TWCR = ( 1TWINT ) | ( 1TWEN ) | ( 1TWSTO ); // I2c stop

return(1);
}


//read RTC

unsigned char ReadRtc( unsigned char SlaAddr, unsigned char RegAddr, unsigned char *pRead, unsigned char Length )
{

unsigned char i;

TWCR = ( 1TWINT ) | ( 1TWSTA ) | ( 1TWEN ); //I2c start
while( !( TWCR ( 1TWINT ) ) ); //wait TWINT


TWDR = SlaAddr-1; //send slave address
TWCR = ( 1TWINT ) | ( 1TWEN );
while( !( TWCR ( 1TWINT ) ) );


TWDR = RegAddr; //send register of device
TWCR = ( 1TWINT ) | ( 1TWEN );
while( !( TWCR ( 1TWINT ) ) );

TWCR = ( 1TWINT ) | ( 1TWEN ) | ( 1TWSTO ); //I2c stop


DDRC = 0XFD; //set SDA as input
PORTC |= 0X02; //set pull-up resistor

TWCR = ( 1TWINT ) | ( 1TWSTA ) | ( 1TWEN ); //start I2c
while( !( TWCR ( 1TWINT ) ) ) ;


TWDR = SlaAddr;
TWCR = ( 1TWINT ) | ( 1TWEN ) ;
while( !( TWCR ( 1TWINT ) ) );

for( i = 0; i Length - 1; i++ )
{
TWCR = ( 1TWINT ) | ( 1TWEN ) | ( 1TWEA ); //send ack
while( !( TWCR ( 1TWINT ) ) );
*( pRead+i ) = TWDR;
}

TWCR = ( 1TWINT ) | ( 1TWEN ); //send nack
while( !( TWCR ( 1TWINT ) ) );
*( pRead+i ) = TWDR;

TWCR = ( 1TWINT ) | ( 1TWEN ) | ( 1TWSTO ); //I2c stop

DDRC |= 0X03;
return(1);
}

剛開始用的時候，老是讀出錯誤的數據，最后用Beagle-GUI.抓取I2C，結果發現讀I2C時，送出叢機地址和寄存器地址后就退出，原來是從機未發出ACK響應主機，因此關于ATMEGA32的TWI有幾點要注意：

1. 首先，關于TWI寫順序是： I2cStart ----> WriteSlaAddr -----> WriteRegAddr ----> WriteData ----> I2cStop;

2. 其次， 關于TWI讀N字節的順序是：I2cStart ----> WriteSlaAddr ----> WriteRegAddr ----> I2cStop ----> I2cStart ---> WriteSlaAddr ----> sendAck----> receive data( (N-1) bytes ) ----->SendNACK ---->receive data ( the last byte) -----> I2cStop;

3. 最后提醒：讀TWI的時候，讀時的ACK是不一樣的，前（N-1）個字節應該發出ACK信號響應，但是最后一個字節應該發送ACK信號。

以上的代碼未加入容錯機制，需要改善