Z32F0641MCU Datasheet, PDF (150/205 Pages) Zilog, Inc. – High Performance low-power Cortex-M3 core

Z32F0641 Product Specification
I2C Interface
Arbitration
A master may start a transfer only if the bus is free. Two or more masters may generate a START condition
within the minimum hold time of the START condition which results in a defined START condition to the bus.
Arbitration takes place on the SDA line, while the SCL line is at the “H” level, in such a way that the master
which transmits “H” level, while another master is transmitting “L” level will switch off its DATA output stage
because the level on the bus doesn’t correspond to its own level.
Arbitration can continue for many bits. Its first stage is comparison of the address bits. If the masters are each
trying to address the same device, arbitration continues with comparison of the data-bits if they are master-
transmitter or acknowledge-bits if they are master-receiver. Because address and data information on the I²C-
bus is determined by the winning master, 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. If a master also incorporates a slave function and it loses arbitration during the addressing stage,
it’s possible that the winning master is trying to address it. The losing master must therefore switch over
immediately to its slave mode.
Figure 14.11 shows the arbitration procedure for two masters. Of course, more may be involved (depending
on how many masters are connected to the bus). As soon as there is a difference between the internal data
level of the master generating Device1 Dataout and the actual level on the SDA line, its data output is
switched off, which means that a “H” output level is then connected to the bus. This will not affect the data
transfer initiated by the winning master.
Arbitration
adapted
Device1
DataOut
Process
not _Device
1
Arbitration
Device2
DataOut
SDA on
BUS
loses _{Device1 outputs High}
SCL on
BUS
S
Figure 14.11 Arbitration Procedure Between Two Masters
PS034404-0417
PRELIMINARY
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