Z8 Encore!® Motor Control Flash MCUs
Product Specification
170
Master Arbitration
If a master loses arbitration during the address byte, it releases the SDA line, switches to
SLAVE mode and monitors the address to determine if it is selected as a slave. If a master
loses arbitration during the transmission of a data byte, it releases the SDA line and waits
for the next STOPor STARTcondition.
The master detects a loss of arbitration when a 1 is transmitted but a 0 is received from the
bus in the same bit-time. This loss occurs if more than one master is simultaneously
accessing the bus. Loss of arbitration can occur during the address phase (two or more
masters accessing different slaves) or during the data phase, when the masters are attempt-
ing to write different data to the same slave.
When a master loses arbitration, the software is informed by means of the Arbitration Lost
interrupt. The software can repeat the same transaction at a later time.
A special case can occur when a slave transaction starts just before the software attempts
to start a new master transaction by setting the STARTbit. In this case, the state machine
enters its slave states before the STARTbit is set, and as a result, the I2C controller will not
arbitrate. If a slave address match occurs and the I2C controller receives/transmits data, the
STARTbit is cleared and an Arbitration Lost interrupt is asserted. The software can mini-
mize the chance of this instance occurring by checking the BUSYbit in the I2CSTATE
Register before initiating a master transaction. If a slave address match does not occur, the
Arbitration Lost interrupt will not occur, and the STARTbit will not be cleared. The I2C
controller will initiate the master transaction after the I2C bus is no longer busy.
Master Address-Only Transactions
It is sometimes preferable to perform an address-only transaction to determine if a particu-
lar slave device is able to respond. This transaction can be performed by monitoring the
ACKVbit in the I2CSTATE Register after the address has been written to the I2CDATA
Register and the STARTbit has been set. After the ACKVbit is set, the ACKbit in the
I2CSTATE Register determines if the slave is able to communicate. The STOPbit must be
set in the I2CCTL Register to terminate the transaction without transferring data. For a 10-
bit slave address, if the first address byte is acknowledged, the second address byte should
also be sent to determine if the preferred slave is responding.
Another approach is to set both the STOPand STARTbits (for sending a 7-bit address).
After both bits have cleared (7-bit address has been sent and transaction is complete), the
ACKbit can be read to determine if the slave has acknowledged. For a 10-bit slave, set the
STOPbit after the second TDRE interrupt (which indicates that the second address byte is
being sent).
Master Transaction Diagrams
In the following transaction diagrams, the shaded regions indicate the data that is trans-
ferred from the master to the slave, and the unshaded regions indicate the data that is trans-
ferred from the slave to the master. The transaction field labels are defined as follows:
I2C Master/Slave Controller
P R E L I M I N A R Y
PS024604-1005
ZILOG [ ZILOG, INC. ]
