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MC74 Datasheet(PDF) 5 Page - ON Semiconductor |
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MC74 Datasheet(HTML) 5 Page - ON Semiconductor |
5 / 12 page MC74 http://onsemi.com 5 Figure 1. SMBus Protocols DATA S ADDRESS WR ACK 7 Bits COMMAND DATA P 8 Bits ACK ACK S ADDRESS WR ACK 7 Bits COMMAND RD NACK 8 Bits S ADDRESS 7 Bits ACK ACK 8 Bits P DATA RD NACK S ADDRESS 7 Bits ACK 8 Bits P Slave Address Command Byte: selects Data Byte: data goes which register you are writing to. into the register set by the command byte. Slave Address Command Byte: selects which register you are reading from. Slave Address: repeated due to change in data– flow direction. Data Byte: reads from the register set by the command byte. Data Byte: reads data from the register commanded by the last Read Byte. S = Start Condition P = Stop Condition Shaded = Slave Transmission Write Byte Format Read Byte Format Receive Byte Format 8 Bits Start Condition (START) The MC74 continuously monitors the SDA and SCL lines for a start condition (a HIGH to LOW transition of SDA while SCL is HIGH) and will not respond until this condition is met. Address Byte Immediately following the Start Condition, the host must transmit the address byte to the MC74. The states of A1 and A0 determine the 7–bit SMBus address for the MC74. The 7–bit address transmitted in the serial bit stream must match for the MC74 to respond with an Acknowledge (indicating the MC74 is on the bus and ready to accept data). The eighth bit in the Address Byte is a Read–Write Bit. This bit is a 1 for a read operation or 0 for a write operation. During the first phase of any transfer this bit will be set = 0 to indicate that the command byte is being written. Acknowledge (ACK) Acknowledge (ACK) provides a positive handshake between the host and the MC74. The host releases SDA after transmitting eight bits, then generates a ninth clock cycle to allow the MC74 to pull the SDA line LOW to acknowledge that it successfully received the previous eight bits of data or address. Data Byte After a successful ACK of the address byte, the host must transmit the data byte to be written or clock out the data to be read. (See the appropriate timing diagrams. ) ACK will be generated after a successful write of a data byte into the MC74. Stop Condition (STOP) Communications must be terminated by a stop condition (a LOW to HIGH transition of SDA while SCL is HIGH). The Stop Condition must be communicated by the transmitter to the MC74. NOTE: Refer to Timing Diagrams for serial bus timing. |
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