Datasheet

237
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
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 20-12 on page 240 to Figure 20-18 on page 248, 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 20-1
on page 238 to Table 20-4 on page 248. Note that the prescaler bits are masked to zero in these tables.
20.7.1 Master Transmitter Mode
In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver (see Figure 20-11).
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 20-11. 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 transmit 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 20-1 on
page 238). In order to enter MT mode, SLA+W must be transmitted. This is done by writing SLA+W to TWDR.
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1 X10X10 X
Device 1
MASTER
TRANSMITTER
Device 2
SLAVE
RECEIVER
Device 3
Device n
SDA
SCL
........
R1 R2
V
CC