Data Sheet

User manual Rev. 6 — 4 April 2014 12 of 64

NXP Semiconductors

UM10204

C-bus specification and user manual

transmissions are identical. The first time a master tries to send a HIGH, but detects that

the SDA level is LOW, the master knows that it has lost the arbitration and turns off its

SDA output driver. The other master goes on to complete its transaction.

No information is lost during the arbitration process. A master that loses the arbitration

can generate clock pulses until the end of the byte in which it loses the arbitration and

must restart its transaction when the bus is free.

If a master also incorporates a slave function and it loses arbitration during the addressing

stage, it is possible that the winning master is trying to address it. The losing master must

therefore switch over immediately to its slave mode.

Figure 8

shows the arbitration procedure for two masters. More may be involved

depending on how many masters are connected to the bus. The moment there is a

difference between the internal data level of the master generating DATA1 and the actual

level on the SDA line, the DATA1 output is switched off. This does not affect the data

transfer initiated by the winning master.

Since control of the I

C-bus is decided solely on the address and data sent by competing

masters, there is no central master, nor any order of priority on the bus.

There is an undefined condition if the arbitration procedure is still in progress at the

moment when one master sends a repeated START or a STOP condition while the other

master is still sending data. In other words, the following combinations result in an

undefined condition:

• Master 1 sends a repeated START condition and master 2 sends a data bit.

• Master 1 sends a STOP condition and master 2 sends a data bit.

• Master 1 sends a repeated START condition and master 2 sends a STOP condition.

Fig 8. Arbitration procedure of two masters

msc609

DATA

SDA

SCL

master 1 loses arbitration

DATA 1 SDA