I2C Transaction
The data transmission includes the following steps:
The master initiates communication by generating a START (S) Condition;
The master sends the first byte that includes a Slave Address and Data Direction;
The slave generates the acknowledgement (ACK) signal. If the master receives no acknowledgement signal, it generates the STOP (P) condition to terminate the transmission.
The transmitter (master or slave) writes a byte of data to the bus and the receiver (slave or master) reads this byte of data from the bus.
After each byte of data, the receiver sends the acknowledgement (ACK) signal and the transmission continues. If the receiver sends no acknowledgement signal, the transmitter stops writing data to the I2C bus.
To terminate transmission, the master generates the STOP (P) Condition. To change transmission parameters, the master generates the Repeated START (Sr) Condition.
START, STOP and Repeated START Conditions
All transactions begin with a START (S) condition and finish with a STOP (P) condition.
START (S) Condition
To generate a START condition, the master changes the SDA line from one to zero while the SCL line is HIGH (marked in red on the following diagram). The I2C bus is considered busy after the START condition. To prepare the bus for transmission of the first bit, the master outputs zero on the SCL line (marked in green).
STOP (P) Condition
To generate a STOP condition, the master changes the SDA line from zero to one while the SCL line is HIGH (marked in red). The I2C bus is considered free after the STOP condition. To prepare for the STOP condition, the master sets the SDA line to zero during the LOW phase of the SCL line (marked in green).
Repeated START (Sr) Condition
Instead of the STOP condition, the master can generate a repeated START (Sr) condition. Like a START condition, to generate a repeated START condition, the master changes the SDA line from one to zero while the SCL line is HIGH (marked in red). In this case, the I2C bus remains busy. To prepare for the repeated START condition, the master sets the SDA line to one during the LOW phase of the SCL line (marked in green).
The START (S) and repeated START (Sr) conditions are functionally identical. The repeated start conditions is used in the following situations:
To continue transmission with the same slave device in the opposite direction. After the repeated START condition, the master sends the same slave device address followed by another direction bit.
To start transmission to or from another slave device. After the repeated START condition, the master sends another slave address.
To provide a READ operation from internal address. See READ Operation for details.
DLN adapters use the repeated START condition to read from the internal address (the DlnI2cMasterRead() function) and to write to and then read from the same slave device (the DlnI2cMasterTransfer() function). If a DLN adapter needs to communicate with different slaves, it finishes one transmission (with the STOP condition) and starts another transmission.
Slave Address and Data Direction
Every byte on the SDA line must be eight bits long. The first byte after START contains seven bits of the slave device address and one bit that defines the direction of the transmission.
As any other data, the address is transmitted sequentially starting with the Most Significant Bit (MSB) and ending with the Least Significant Bit (LSB).
The direction bit has the following values:
0 – Write: the master transmits data to the slave;
1 – Read: the master receives data from the slave.
Reserved I2C Slave Addresses
There are 16 reserved I2C addresses. The following table shows the purposes of these addresses:
| I2C slave address | Direction bit (R/W) | Description |
0000 000 | 0 | General call address |
0000 000 | 1 | START byte |
0000 001 | X | CBUS address |
0000 010 | X | Reserved for different bus format |
0000 011 | X | Reserved for future purposes |
0000 1XX | X | Hs-mode master code |
1111 1XX | 1 | Device ID |
1111 0XX | X | 10-bit slave addressing |
The general call address is for addressing all devices connected to the I2C bus. If a device does not need the provided data, it can ignore this address (it does not issue the acknowledgement). If a device requires data from a general call address, it acknowledges this address and behaves as a slave-receiver. If one or more slaves acknowledge the general call address, the master does not know how many devices did it and does not see not-acknowledged slaves.
If you use a DLN-series adapter as I2C slave, you can configure it to support general call addressing or to ignore it.