7.5.1 Transfer of Data with the SMBus Interface

The System Management Bus (SMBus) is a two-wire serial interface through which a master can communicate with various system components. Slave devices are identified by a unique device address. The two-wire serial interface consists of SDC and SDA signals. SDC is a clock output from the master to all of the slave devices on the bus. SDA is a bidirectional data signal between the master and slave devices. The DS280BR810 SMBus, SDC, and SDA signals are open drain and require external pull-up resistors.

Start and Stop Conditions:

The master generates Start and Stop conditions at the beginning and end of each transaction:

• Start: High to LOW transition (falling edge) of SDA while SDC is HIGH.
• Stop: Low to HIGH transition (rising edge) of SDA while SDC is HIGH.

The master generates 9 clock pulses for each byte transfer. The 9th clock pulse constitutes the acknowledge (ACK) cycle. The transmitter releases SDA to allow the receiver to send the ACK signal. An ACK is when the device pulls SDA LOW, while a NACK (no acknowledge) is recorded if the line remains HIGH.

Writing data from a master to a slave consists of three parts:

• The master begins with a start condition followed by the slave device address with the R/W bit cleared.
• The master sends the 8-bit register address that will be written.
• The master sends the data byte to write for the selected register address. The register address pointer will then increment, so the master can send the data byte for the subsequent register without re-addressing the device, if desired. The final data byte to write should be followed by a stop condition.

SMBus read operations consist of four parts:

• The master initiates the read cycle with start condition followed by slave device address with the R/W bit cleared.
• The master sends the 8-bit register address that will be read.
• After acknowledgment from the slave, the master initiates a re-start condition.
• The slave device address is resent followed with R/W bit set.
• After acknowledgment from the slave, the data is read back from the slave to the master. The last ACK is HIGH if there are no more bytes to read.