192
ATmega64A [DATASHEET]
8160D–AVR–02/2013
The TWINT flag is set in the following situations:
After the TWI has transmitted a START/REPEATED START condition.
After the TWI has transmitted SLA+R/W.
After the TWI has transmitted an address byte.
After the TWI has lost arbitration.
After the TWI has been addressed by own slave address or general call.
After the TWI has received a data byte.
After a STOP or REPEATED START has been received while still addressed as a Slave.
When a bus error has occurred due to an illegal START or STOP condition.
22.3
Two-wire Serial Interface Bus Definition
The Two-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The TWI protocol
allows the systems designer to interconnect up to 128 different devices using only two bi-directional bus lines, one
for clock (SCL) and one for data (SDA). The only external hardware needed to implement the bus is a single pull-
up resistor for each of the TWI bus lines. All devices connected to the bus have individual addresses, and mecha-
nisms for resolving bus contention are inherent in the TWI protocol.
Figure 22-2. TWI Bus Interconnection
22.3.1
TWI Terminology
The following definitions are frequently encountered in this section.
22.3.2
Electrical Interconnection
As depicted in
Figure 22-2, 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 func-
tion 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
Device 1
Device 2
Device 3
Device n
SDA
SCL
........
R1
R2
V
CC
Table 22-1.
TWI Terminology
Term
Description
Master
The device that initiates and terminates a transmission. The Master also 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.