ATMEGA640 datasheet(237/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

24.2.2

Electrical Interconnection

As depicted in Figure 24-1 on page 236, both bus lines are connected to the positive supply voltage through pull-up

resistors. The bus drivers of all TWI-compliant devices are open-drain or open-collector. This implements a wired-

AND function which is essential to the operation of the interface. A low level on a TWI bus line is generated when

one or more TWI devices output a zero. A high level is output when all TWI devices trim-state their outputs, allow-

ing the pull-up resistors to pull the line 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 400pF and

the 7-bit slave address space. A detailed specification of the electrical characteristics of the TWI is given in “SPI

Timing Characteristics” on page 363. Two different sets of specifications are presented there, one relevant for bus

speeds below 100kHz, and one valid for bus speeds up to 400kHz.

24.3 Data Transfer and Frame Format

24.3.1

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 24-2. Data Validity

24.3.2

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 with-

out relinquishing 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 condi-

tions are signalled by changing the level of the SDA line when the SCL line is high.

SDA

SCL

Data Stable

Data Change

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