UPD784938 Datasheet, PDF (468/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 20 IEBus CONTROLLER
20.1.2 Determination of bus mastership (arbitration)
An operation to occupy the bus is performed when a unit connected to the IEBus controls the other units. This operation
is called arbitration.
When two or more units simultaneously start transmission, arbitration is to grant one of the units the permission to occupy
the bus.
Because only one unit is granted the bus mastership as a result of arbitration, the priority condition of the bus is
predetermined as follows:
Caution The bus mastership is released if communication is aborted.
(1) Priority by communication type
Broadcasting communication (communication from one unit to plural units) takes precedence over normal communi-
cation (communication from one unit to another).
(2) Priority by master address
If the communication type is the same, communication with the lower master address takes precedence.
A master address consists of 12 bits, with unit 000H having the highest priority and unit FFFH having the lowest priority.
20.1.3 Communication mode
Although the IEBus has three communication modes each having a different transfer rate, the µPD784938 Subseries
supports only communication mode 1. The transfer rate and the maximum number of transmit bytes in one communication
frame in communication mode 1 are as shown in Table 20-1.
Table 20-1. Transfer Rate and Maximum Number of Transmit Bytes in Communication Mode 1
Communication Mode
1
Maximum Number of Transmit Bytes (bytes/frame)
32
Effective Transfer Rate (kbps)^Note
Approx. 17
Note The effective transfer rate when the maximum number of transmit bytes is transmitted.
Select the communication mode (mode 1) for each unit connected to the IEBus before starting communication. If the
communication mode of the master unit and that of the mating unit (slave unit) are not the same, communication is not
correctly executed.
20.1.4 Communication address
With the IEBus, each unit is assigned a specific 12-bit address. This communication address consists of the following
identification numbers:
High-order 4 bits: Group number (number to identify the group to which each unit belongs)
Low-order 8 bits: Unit number (number to identify each unit in a group)
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Preliminary User’s Manual U13987EJ1V0UM00