7.3.1 Communication Protocol

Each communication of the SMAART wire protocol consists of 8-bit words, transferred least significant bit (LSB) first. Each 8-bit word begins with a Start bit that is logic low, and ends with a Stop bit that is logic high. By using a Start bit and Stop bit for each 8-bit word, the TMP104 can calibrate each word and maintain synchronous communication throughout the process. The host commences the communication by sending a Start bit followed by the calibration byte (55h), allowing the TMP104 to sync to the baud rate of the host, followed by the Stop bit. Then, another Start bit is sent, followed by the command register byte and a Stop bit. Finally, a third Start bit is sent followed by the data byte, where master sends data if the instruction is a write command, or the TMP104 breaks the chain and sends data if the instruction is a read command. The process finishes with a Stop bit. The sequence is shown in Table 2 and Figure 7.

Figure 7. Generic Communication BitStream

The TMP104 has two dedicated pins for communication: TX and RX. Usually, these two pins are connected internally and the signal on the RX propagates to the TX; that is, the TMP104 works in a transparent mode. The TMP104 breaks this buffer configuration only when it must send data on the bus or during address assignment and alert procedures.

The TMP104 supports unique address assignment and alert interrupt procedures. There are general-call read and write commands that allow simultaneous reads or writes to all devices in the daisy-chain. The interface has built-in time-outs (typically 28 ms) that return the interface to a known state if communication is disrupted.