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18.8.2 Master Receiver Mode
In the Master Receiver Mode, a number of data bytes are received from a slave transmitter (see
Figure 18-13). In order to enter a Master mode, a START condition must be transmitted. The for-
mat 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 18-13. 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 18-3). 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 follow-
ing 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 18-12. 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:
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X10X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X00X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X01X10 X
Device 1
Device 2
Device 3
Device n
SDA
SCL
........
R1 R2
V
CC
MASTER
SLAVE
TRANSMITTER
RECEIVER
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