Start LOW:SEXTT StopLOW:MEXTT ClkAckLOW:MEXTT ClkAckLOW:MEXTTSCLSDAFigure 21-4. Timeout measurement intervalsA master is allowed to abort the transaction in progress to any slave that violates theTLOW:SEXT or TTIMEOUT,MIN specifications. To abort the transaction, the master issues aSTOP condition at the conclusion of the byte transfer in progress. When a slave, the I2Cmodule must not cumulatively extend its clock cycles for a period greater thanTLOW:SEXT during any message from the initial START to the STOP. When CSMBCLKTIMEOUT SEXT occurs, SEXT rises and also triggers SLTF.NOTECSMBCLK TIMEOUT SEXT and CSMBCLK TIMEOUTMEXT are optional functions that are implemented in thesecond step.21.5.4.2 FAST ACK and NACKTo improve reliability and communication robustness, implementation of packet errorchecking (PEC) by SMBus devices is optional for SMBus devices but required fordevices participating in and only during the address resolution protocol (ARP) process.The PEC is a CRC-8 error checking byte, calculated on all the message bytes. The PEC isappended to the message by the device that supplied the last data byte. If the PEC ispresent but not correct, a NACK is issued by the receiver. Otherwise an ACK is issued.To calculate the CRC-8 by software, this module can hold the SCL line low afterreceiving the eighth SCL (8th bit) if this byte is a data byte. So software can determinewhether an ACK or NACK should be sent to the bus by setting or clearing the TXAK bitif the FACK (fast ACK/NACK enable) bit is enabled.SMBus requires a device always to acknowledge its own address, as a mechanism todetect the presence of a removable device (such as a battery or docking station) on thebus. In addition to indicating a slave device busy condition, SMBus uses the NACKmechanism to indicate the reception of an invalid command or invalid data. Because sucha condition may occur on the last byte of the transfer, SMBus devices are required toFunctional descriptionMC9S08SU16 Reference Manual, Rev. 5, 4/2017386 NXP Semiconductors
