User Manual

220
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
22.7 Transmission Modes
The TWI can operate in one of four major modes. These are named Master Transmitter (MT), Master Receiver
(MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these modes can be used in the same applica-
tion. As an example, the TWI can use MT mode to write data into a TWI EEPROM, MR mode to read the data back
from the EEPROM. If other masters are present in the system, some of these might transmit data to the TWI, and
then SR mode would be used. It is the application software that decides which modes are legal.
The following sections describe each of these modes. Possible status codes are described along with figures
detailing data transmission in each of the modes. These figures contain the following abbreviations:
S: START condition
Rs: REPEATED START condition
R: Read bit (high level at SDA)
W: Write bit (low level at SDA)
A: Acknowledge bit (low level at SDA)
A
: Not acknowledge bit (high level at SDA)
Data: 8-bit data byte
P: STOP condition
SLA: Slave Address
In Figure 22-12 to Figure 22-18, circles are used to indicate that the TWINT Flag is set. The numbers in the circles
show the status code held in TWSR, with the prescaler bits masked to zero. At these points, actions must be taken
by the application to continue or complete the TWI transfer. The TWI transfer is suspended until the TWINT Flag is
cleared by software.
When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate software action. For
each status code, the required software action and details of the following serial transfer are given in Table 22-2 to
Table 22-5. Note that the prescaler bits are masked to zero in these tables.
22.7.1 Master Transmitter Mode
In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver (see Figure 22-11). In
order to enter a Master mode, a START condition must be transmitted. The format of the following address packet
determines whether Master Transmitter or Master Receiver mode is to be entered. If SLA+W is transmitted, MT
mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes mentioned in this section
assume that the prescaler bits are zero or are masked to zero.
Figure 22-11. Data Transfer in Master Transmitter Mode
Device 1
MASTER
TRANSMITTER
Device 2
SLAVE
RECEIVER
Device 3
Device n
SDA
SCL
........
R1 R2
V
CC