ATmega16 datasheet(184/315 Pages) ATMEL

184 / 315 page

184

ATmega16(L)

2466E–AVR–10/02

Figure 88. 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 73).

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 sta-

tus 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 74. 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 con-

dition 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 enables the master to switch between slaves, Master Transmitter mode and

Master Receiver mode without losing control over the bus.

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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