Datasheet

219
8011Q–AVR–02/2013
ATmega164P/324P/644P
SLA: Slave Address
In Figure 18-12 on page 221 to Figure 18-18 on page 230, 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 con-
tinue 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 soft-
ware action. For each status code, the required software action and details of the following serial
transfer are given in Table 18-2 on page 220 to Table 18-5 on page 229. Note that the prescaler
bits are masked to zero in these tables.
18.7.1 Master Transmitter Mode
In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver
(see Figure 18-11 on page 219). 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 18-11. Data Transfer in Master Transmitter Mode
A START condition is sent by writing the following value to TWCR:
TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one to trans-
mit a START condition and TWINT must be written to one to clear the TWINT Flag. The TWI will
then test the 2-wire Serial Bus and generate a START condition as soon as the bus becomes
free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and the
status code in TWSR will be 0x08 (see Table 18-2 on page 220). In order to enter MT mode,
SLA+W must be transmitted. This is done by writing SLA+W to TWDR. Thereafter the TWINT bit
should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing
the following value to TWCR:
When SLA+W have been transmitted and an acknowledgement bit has been received, TWINT is
set again and a number of status codes in TWSR are possible. Possible status codes in Master
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X10X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X00X10 X
Device 1
MASTER
TRANSMITTER
Device 2
SLAVE
RECEIVER
Device 3
Device n
SDA
SCL
........
R1 R2
V
CC