avr-twi

Really nonblocking TWI/I2C master driver for Atmel AVR

API

void twi_init()

Initializes the driver. Should be called once before calling any other TWI functions.

TWI_STATUS twi_write(uint8_t address, uint8_t* data, uint8_t length)

Writes data to the given address.

• address - TWI slave address
• data - pointer to data buffer
• length - numer of bytes to write from the given data buffer

Returns TWI_STATUS

• TWI_BUSY - still sending
• TWI_OK - writing was ok
• TWI_NOK - writing was not ok

TWI_STATUS twi_write1(uint8_t address, uint8_t data)

Writes one byte to the given address.

TWI_STATUS twi_read(uint8_t address, uint8_t *length, uint8_t **data)

Reads data from the given address.

• address - TWI slave address
• length - pointer to number of bytes to read
• data - address of pointer which points to the first data byte

Returns TWI_STATUS, see above - only for reading

On TWI_OK

• length - contains received count of data
• data - points to first data byte

void twi_resetstate()

Resets the internal state. Normally a write is repeated until its TWI_OK, after that we expect a read - which is repeated until its TWI_OK. If this is not wanted, you can reset the internal state.

switch (operation) {
   case 1:
     if (twi_write(address, &data, 2)==TWI_OK) {
        twi_resetstate();
        operation++;
     }
    case 2:
     if (twi_write(address, &data, 2)==TWI_OK) {
        operation++;
     }
    case 3:
     if (twi_read(address, &length, &data)==TWI_OK) {
        operation++;
     }
 }

Definitions

• F_CPU - you should define this before including this library
• TWI_FREQ - defaults to 100kHz if left undefined
• TWI_BUFFER_LENGTH - defaults to 32 if left undefined

Example

This example reads temperature and humidity from an SHT45 sensor. First we send the command 0xFD to the address 0x44, than we read 6 bytes from the sensor. The sensor replies with NACK if the conversion is not done. The read is performed until we get an ACK. After reading 6 Bytes temperature and humidity gets calculated.

 if (readtemp==1) {
    twi_write(0x44,0xFD);
    uint8_t length=6;
    uint8_t *buffer=NULL;
    if (twi_read(0x44,&length,&buffer)==TWI_OK) {
       if (length==6) {
            uint16_t t=buffer[0]*256+buffer[1];
            uint16_t h=buffer[3]*256+buffer[4];
            reg.temperature=-45.0f+175.0f*((float) t/65535.0f);
            reg.humidity=-6.0f+125.0f*((float) h/65535.0f);
            if (reg.humidity<0) reg.humidity=0;
            if (reg.humidity>100) reg.humidity=100;
       }
       readtemp++;
    }
 }