Understanding Smart Sensors - Nomads.usp

Communications for Smart Sensors 143protocols are used, the need to transition between various protocols can occur,especially in complex systems. MCU protocols, the transition between protocols,and modular protocol solutions are the final topics on communication inthis chapter.6.9.1 MCU ProtocolsThe serial peripheral interface (SPI) is a high-speed protocol that is used forsynchronously transferring data between master and slave units in an onboardserial network. As shown in Figure 6.13, the SPI module is a defined buildingblock that can be used for transmitting digital information in a system througha software-defined protocol. This module is already contained in a number ofMCU families. The queued SPI (QSPI) is an intelligent, synchronous serialinterface with a 16-entry, full-duplex queue. It can continuously scan up to 16independent peripherals and maintain a queue of the most recently acquiredinformation without CPU intervention.The serial communications interface (SCI) transmits using only two pinson MCUs with an SCI peripheral. The SCI protocol was used to develop theMI-Bus (discussed in Section 6.8.4). That option is discussed in more detail inSection 6.9.2.The inter-IC, or I 2 C, is a two-wire half-duplex serial interface with thedata transmitted/received by the most significant bit first. The two wires are aserial data line (SDL) and a serial clock line (SCL). The protocol consists of astart condition, a slave address, n bytes of data, and a stop condition. Each byteis followed by an acknowledge bit. The I 2 C peripheral can be interfaced bymeans of a synchronous serial I/O port (SIOP).6.9.2 Transition Between ProtocolsBecause several communication protocols exist, the transition between protocolsfrequently is required. A gateway node provides the transition betweenMaster MCUMaster in/slave outMISOMaster in/slave outSlave peripheral8-bit shift regMaster out/slave inMOSISCKSSMaster out/slave in8-bit shift regSerial clockGeneral purpose out (PD5)Serial clockSlave select (PD5)Figure 6.13 The SPI.