ATmega32HVB datasheet(172/273 Pages) ATMEL

172 / 273 page

172

8042A–AVR–07/09

ATmega16HVB/32HVB

27.7.2

Master Receiver Mode

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

Figure 27-13). 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 27-13. 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 27-2 on page 170). 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 27-12 on page 171. 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

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

Device 1

MASTER

RECEIVER

Device 2

SLAVE

TRANSMITTER

Device 3

Device n

SDA

SCL

........

BUS

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