UPD784938 Datasheet, PDF (470/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 20 IEBus CONTROLLER
(2) Broadcasting bit
This bit indicates whether the master selects one slave (individual communication) or plural slaves (broadcasting
communication) as the other party of communication.
When the broadcasting request bit is 0, it indicates broadcasting communication; when it is 1, individual communication
is indicated. Broadcasting communication is classified into two types: group-unit communication and all-unit
communication. These communication types are identified by the value of the slave address (for the slave address,
refer to (4) Slave address field).
Because two or more slave units exist in the case of broadcasting communication, the acknowledge bit in each field
subsequent to the master address field is not returned.
If two or more units start transmitting a communication frame at the same time, broadcasting communication takes
precedence over individual communication, and wins in arbitration.
If one station occupies the bus as the master, the value set to the broadcasting request bit (ALLRQ) of the bus control
register (BCR) is output.
(3) Master address field
The master address field is output by the master to inform a slave of the master’s address.
The configuration of the master address field is as shown in Figure 20-2.
If two or more units start transmitting the broadcasting bit at the same time, the master address field makes a judgment
of arbitration.
The master address field compares the data it outputs with the data on the bus each time it has output one bit. If the
master address output by the master address field is found to be different from the data on the bus as a result of
comparison, it is assumed that the master has lost in arbitration. As a result, the master stops transmission and enters
the reception status.
Because the IEBus is configured of wired AND, the unit having the minimum master address of the units participating
in arbitration (arbitration masters) wins in arbitration.
After a 12-bit master address has been output, only one unit remains in the transmission status as one master unit.
Next, this master unit outputs a parity bit, determines the master address of other unit, and starts outputting a slave
address field.
If one unit occupies the bus as the master, the address set by the unit address register (UAR) is output.
Figure 20-2. Master Address Field
MSB
Master address field
Master address (12 bits)
Parity
LSB
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