![](http://datasheet.mmic.net.cn/30000/MR80C32-20-D_datasheet_2377239/MR80C32-20-D_190.png)
190
8154B–AVR–07/09
ATmega16A
Figure 20-13. Data Transfer in Master Receiver Mode
A START condition is sent by writing the following value to TWCR:
TWEN must be written to one to enable the Two-wire Serial Interface, TWSTA must be written to
one to transmit a START condition and TWINT must be set 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 hard-
ware, and the status code in TWSR will be $08 (See
Table 20-2). In order to enter MR mode,
SLA+R must be transmitted. This is done by writing SLA+R 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+R 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
mode are $38, $40, or $48. The appropriate action to be taken for each of these status codes is
detailed in
Table 20-3. Received data can be read from the TWDR Register when the TWINT
Flag is set high by hardware. This scheme is repeated until the last byte has been received.
After the last byte has been received, the MR should inform the ST by sending a NACK after the
last received data byte. The transfer is ended by generating a STOP condition or a repeated
START condition. A STOP condition is generated by writing the following value to TWCR:
A REPEATED START condition is generated by writing the following value to TWCR:
After a repeated START condition (state $10) the Two-wire Serial Interface can access the
same Slave again, or a new Slave without transmitting a STOP condition. Repeated START
enables the Master to switch between Slaves, Master Transmitter mode and Master Receiver
mode without losing control over the bus.
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X1
0
X1
0
X
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X0
0
X1
0
X
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X0
1
X1
0
X
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X1
0
X1
0
X
Device 1
MASTER
RECEIVER
Device 2
SLAVE
TRANSMITTER
Device 3
Device n
SDA
SCL
........
R1
R2
V
CC