AT90USB646 datasheet(230/456 Pages) ATMEL

230 / 456 page

230

7593L–AVR–09/12

AT90USB64/128

Figure 21-14. 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 21-3 on page 227). 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 231. 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 0x10) the 2-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 with-

out 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

........

AT90USB646 Datasheet(PDF) 230 Page - ATMEL Corporation

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