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130

8008H–AVR–04/11

ATtiny48/88

15.3.1

TWI Terminology

The following definitions are frequently encountered in this section.

The PRTWI bit in “PRR – Power Reduction Register” on page 40 must be written to zero to

enable the 2-wire Serial Interface.

15.3.2

Electrical Interconnection

As depicted in Figure 15-1, 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 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 212. 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.

15.4

Data Transfer and Frame Format

15.4.1

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 15-2. Data Validity

Table 15-1.

TWI Terminology

Term

Description

Master

The device that initiates and terminates a transmission and generates the SCL clock.

Slave

The device addressed by a Master.

Transmitter

The device placing data on the bus.

Receiver

The device reading data from the bus.

SDA

SCL

Data Stable

Data Change

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