Freescale-semiconductor DSP56366 User Manual download pdf (Page 141)

Characteristics Of The I

C Bus

DSP56366 24-Bit Digital Signal Processor User Manual, Rev. 4

Freescale Semiconductor 7-17

7.6.1 Overview

The I

C bus protocol must conform to the following rules:

• Data transfer may be initiated only when the bus is not busy.

• During data transfer, the data line must remain stable whenever the clock line is high. Changes in

the data line when the clock line is high are interpreted as control signals (see Figure 7-7).

Figure 7-7 I

C Bit Transfer

Accordingly, the I

C bus protocol defines the following events:

• Bus not busy—Both data and clock lines remain high.

• Start data transfer—The start event is defined as a change in the state of the data line, from high

to low, while the clock is high (see Figure 7-8).

• Stop data transfer—The stop event is defined as a change in the state of the data line, from low

to high, while the clock is high (see Figure 7-8).

• Data valid—The state of the data line represents valid data when, after a start event, the data line

is stable for the duration of the high period of the clock signal. The data on the line may be changed

during the low period of the clock signal. There is one clock pulse per bit of data.

Figure 7-8 I

C Start and Stop Events

Each 8-bit word is followed by one acknowledge bit. This acknowledge bit is a high level put on the bus

by the transmitter when the master device generates an extra acknowledge-related clock pulse. A slave

receiver that is addressed must generate an acknowledge after each byte is received. Also, a master

receiver must generate an acknowledge after the reception of each byte that has been clocked out of the

slave transmitter. The acknowledging device must pull down the SDA line during the acknowledge clock

pulse so that the SDA line is stable low during the high period of the acknowledge-related clock pulse (see

Figure 7-9).

SDA

SCL

Data Line

Stable:

Data Valid

Change

of Data

Allowed

AA0422

S P

Start Event Stop Event

SDA

SCL

AA0423

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