ATmega128 datasheet(199/386 Pages) ATMEL

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2467S–AVR–07/09

ATmega128

high. Note that all AVR devices connected to the TWI bus must be powered in order to allow any

bus operation.

The number of devices that can be connected to the bus is only limited by the bus capacitance

limit of 400 pF and the 7-bit slave address space. A detailed specification of the electrical char-

acteristics of the TWI is given in “Two-wire Serial Interface Characteristics” on page 322. Two

different sets of specifications are presented there, one relevant for bus speeds below 100 kHz,

and one valid for bus speeds up to 400 kHz.

Data Transfer and

Frame Format

Transferring Bits

Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level

of the data line must be stable when the clock line is high. The only exception to this rule is for

generating start and stop conditions.

Figure 87. Data Validity

START and STOP

Conditions

The master initiates and terminates a data transmission. The transmission is initiated when the

master issues a START condition on the bus, and it is terminated when the master issues a

STOP condition. Between a START and a STOP condition, the bus is considered busy, and no

other master should try to seize control of the bus. A special case occurs when a new START

condition is issued between a START and STOP condition. This is referred to as a REPEATED

START condition, and is used when the master wishes to initiate a new transfer without relin-

quishing control of the bus. After a REPEATED START, the bus is considered busy until the next

STOP. This is identical to the START behavior, and therefore START is used to describe both

START and REPEATED START for the remainder of this datasheet, unless otherwise noted. As

depicted below, START and STOP conditions are signalled by changing the level of the SDA

line when the SCL line is high.

Figure 88. START, REPEATED START and STOP Conditions

SDA

SCL

Data Stable

Data Change

SDA

SCL

START

STOP

REPEATED START

STOP START

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