Datasheet

222
8011Q–AVR–02/2013
ATmega164P/324P/644P
18.7.2 Master Receiver Mode
In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter
(Slave see Figure 18-13 on page 222). 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 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 2-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 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 hard-
ware, and the status code in TWSR will be 0x08 (See Table 18-2 on page 220). 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 0x38, 0x40, or 0x48. The appropriate action to be taken for each of these status codes
is detailed in Table 18-3 on page 223. 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
MASTER
RECEIVER
Device 2
SLAVE
TRANSMITTER
Device 3
Device n
SDA
SCL
........
R1 R2
V
CC