If the slave receiver does not acknowledge the master in the ninth bit, the slave mustleave SDA high. The master interprets the failed acknowledgement as an unsuccessfuldata transfer.If the master receiver does not acknowledge the slave transmitter after a data bytetransmission, the slave interprets it as an end to data transfer and releases the SDA line.In the case of a failed acknowledgement by either the slave or master, the data transfer isaborted and the master does one of two things:• Relinquishes the bus by generating a STOP signal.• Commences a new call by generating a repeated START signal.21.5.1.4 STOP signalThe master can terminate the communication by generating a STOP signal to free thebus. A STOP signal is defined as a low-to-high transition of SDA while SCL is asserted.21.5.1.5 Repeated START signalThe master may generate a START signal followed by a calling command withoutgenerating a STOP signal first. This action is called a repeated START. The master usesa repeated START to communicate with another slave or with the same slave in adifferent mode (transmit/receive mode) without releasing the bus.The master needs tosend a NACK signal before sending repeated-START in the buffering mode.21.5.1.6 Arbitration procedureThe I2C bus is a true multimaster bus that allows more than one master to be connectedon it.If two or more masters try to control the bus at the same time, a clock synchronizationprocedure determines the bus clock. The bus clock's low period is equal to the longestclock low period, and the high period is equal to the shortest one among the masters.The relative priority of the contending masters is determined by a data arbitrationprocedure. A bus master loses arbitration if it transmits logic level 1 while another mastertransmits logic level 0. The losing masters immediately switch to slave receive mode andChapter 21 Inter-Integrated Circuit (I2C)MC9S08SU16 Reference Manual, Rev. 5, 4/2017NXP Semiconductors 379
