UPD6708 datasheet(8/72 Pages) NEC

8 / 72 page

µPD6708

2.2.1 Bus mastership determination (arbitration)

When a unit connected to the IEBus controls another unit, it performs an operation to occupy the bus. This operation

is called arbitration.

Arbitration is to select one unit, and if several units begin to transmit data simultaneously, gives permission to occupy

the bus to that one unit.

So that one unit is granted the permission to occupy the bus as a result of the arbitration, the following priority conditions

are determined.

Remark

The units not given permission through arbitration are automatically allowed to get into retransfer mode (number

of retransfer times for the

µPD6708: 3).

(1)

Priority according to type of communication

Broadcast communication (between a single and multiple units) takes precedence over ordinary communication

(between single units).

(2)

Priority according to master address

If the communication devices are of the same type, the unit with the lowest master address has the highest priority.

Example

The master address comprises 12 bits, and unit 000H has the highest priority while unit FFFH has the lowest

priority.

2.2.2 Communication modes

The IEBus is provided with three communication modes with different transfer speeds. The transfer speed and maximum

number of transfer bytes in a single communication frame in each communication mode are shown in Table 2-1.

Table 2-1. Transfer Speed and Maximum Number of Transfer Bytes in Each Communication Mode

Communication Mode

Maximum Number of Transfer Bytes (bytes/frame)

Actual Transfer Speed Note 1 (Kbps)

fX = 12 MHz Note 2

fX = 12.58 MHz Note 2

Approx. 3.9

Approx. 4.1

Approx. 17

Approx. 18

128

Approx. 26

Approx. 27

Notes 1.

Actual transfer speed when the maximum number of bytes is transferred

Oscillation frequency when the

µPD6708 is used

Cautions

1. A communication mode is selected for each unit connected to the IEBus before communication is

performed. If the communication mode of the master unit is not the same as that of the unit with

which the master unit is to communicate (slave unit), communication cannot be performed correctly.

2. If the oscillation frequency of one unit is fx = 12 MHz and that of the other unit is fx = 12.58 MHz,

communication cannot be performed correctly even if the communication mode is the same. Make

sure that the oscillation frequencies of the two units to communicate are the same.

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