Datasheet

183
ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
22.2.2 Electrical Interconnection
As depicted in Figure 22-1, 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 tri-state their outputs, allowing 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 “Two-wire Serial Interface
Characteristics” on page 275. Two different sets of specifications are presented there, one relevant for bus speeds below
100kHz, and one valid for bus speeds up to 400kHz.
22.3 Data Transfer and Frame Format
22.3.1 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 22-2. Data Validity
22.3.2 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 without 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 conditions are signalled by changing the level of the SDA line when the SCL
line is high.
Figure 22-3. START, REPEATED START and STOP conditions
SDA
SCL
Data Stable
Data Change
Data Stable
SDA
SCL
START START REPEATED STARTSTOP STOP