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211
2593O–AVR–02/12
ATmega644
19.7
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 application. 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
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 soft-
ware action. For each status code, the required software action and details of the following serial
these tables.
6
wait3:
in
r16,TWCR
sbrs
r16,TWINT
rjmp
wait3
while
(!(TWCR & (1<<TWINT)))
;
Wait for TWINT Flag set. This
indicates that the DATA has been
transmitted, and ACK/NACK has
been received.
7
in
r16,TWSR
andi
r16, 0xF8
cpi
r16, MT_DATA_ACK
brne
ERROR
if
((TWSR & 0xF8) !=
MT_DATA_ACK)
ERROR();
Check value of TWI Status
Register. Mask prescaler bits. If
status different from
MT_DATA_ACK go to ERROR
ldi
r16, (1<<TWINT)|(1<<TWEN)|
(1<<TWSTO)
out
TWCR, r16
TWCR = (1<<TWINT)|(1<<TWEN)|
(1<<TWSTO);
Transmit STOP condition
Assembly Code Example
C Example
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