ATmega128 datasheet(217/386 Pages) ATMEL

217 / 386 page

217

2467S–AVR–07/09

ATmega128

Master Receiver Mode

In the Master Receiver Mode, a number of data bytes are received from a slave transmitter (see

Figure 98). 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 98. 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 $08 (See Table 88). 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 $38, $40, or $48. The appropriate action to be taken for each of these status codes is

detailed in Table 97. 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

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

Device 1

MASTER

RECEIVER

Device 2

SLAVE

TRANSMITTER

Device 3

Device n

SDA

SCL

........

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