203
ATmega64A [DATASHEET]
8160D–AVR–02/2013
Figure 22-13. 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 hardware, and the status code in TWSR will be 0x08 (see
Table 22-2). 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 acknowledgment 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 0x38, 0x40, or 0x48. The
appropriate action to be taken for each of these status codes is detailed in
Table 22-5. 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 0x10) 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
Table 22-3.
Status Codes for Master Receiver Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Status of the Two-wire Serial
Bus and Two-wire Serial Inter-
face Hardware
Application Software Response
Next Action Taken by TWI Hardware
To/from TWDR
To TWCR
STA
STO
TWINT
TWEA
0x08
A START condition has been
transmitted
Load SLA+R
0
1
X
SLA+R will be transmitted
ACK or NOT ACK will be received