HC705E5GRSSLASHD datasheet(76/148 Pages) NXP

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Motorola Bus (M Bus) Interface

General Release Specification

MC68HC705E5 — Rev. 1.0

Motorola Bus (M Bus) Interface

11.5.4 Repeated Start Signal

As shown in Figure 11-1, a repeated start signal is used to generate a

start signal without first generating a stop signal to terminate the

communication. This is used by the master to communicate with another

slave or with the same slave in a different mode (transmit/receive mode)

without releasing the bus.

11.5.5 Stop Signal

The master can terminate the communication by generating a stop

signal to free the bus. However, the master may generate a start signal

followed by a calling command without first generating a stop signal.

This is called repeat start. A stop signal is defined as a low-to-high

transition of SDA while SCL is at logical high. (See Figure 11-1.)

11.5.6 Arbitration Procedure

This interface circuit is a true multimaster system which allows more

than one master to be connected to it. If two or more masters try to

control the bus at the same time, a clock synchronization procedure

determines the bus clock, for which the low period is equal to the longest

clock low period and the high is equal to the shortest one among the

masters. A data arbitration procedure determines the priority. The

masters will lose arbitration if they transmit a logic 1 while another

transmits logic 0. The losing masters will immediately switch over to

slave receive mode and stop its data and clock outputs. In this case, the

transition from master to slave mode will not generate a stop condition;

however, a software bit will be set by hardware to indicate loss of

arbitration.

Freescale Semiconductor, Inc.

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