Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 223
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 21-12 on page 225 to Figure 21-18 on page 233, 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 21-3
on page 224 to Table 21-6 on page 232. Note that the prescaler bits are masked to zero in these tables.
21.7.1 Master Transmitter mode
In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver (see Figure 21-11
on page 223). 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 21-11. 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 two-wire Serial Interface, TWSTA must be written to one to transmit a START
condition and TWINT must be written to one to clear the TWINT Flag. The TWI will then test the two-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 21-3 on
page 224). 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:
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