Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 226
21.7.2 Master Receiver mode
In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter (Slave see Figure
21-13 on page 226). 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 21-13. 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 21-3 on
page 224). 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 21-4 on page 227.
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:
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
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X10X10 X
Device 1
MASTER
RECEIVER
Device 2
SLAVE
TRANSMITTER
Device 3
Device n
SDA
SCL
........
R1 R2
V
CC