AT90USB646 datasheet(227/456 Pages) ATMEL

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227

7593L–AVR–09/12

AT90USB64/128

TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one to trans-

mit a START condition and TWINT must be written to one 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 hardware, and the

status code in TWSR will be 0x08 (see Table 21-3). In order to enter MT mode, SLA+W must be

transmitted. This is done by writing SLA+W 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+W 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 0x18, 0x20, or 0x38. The appropriate action to be taken for each of these status codes

is detailed in Table 21-3.

When SLA+W has been successfully transmitted, a data packet should be transmitted. This is

done by writing the data byte to TWDR. TWDR must only be written when TWINT is high. If not,

the access will be discarded, and the Write Collision bit (TWWC) will be set in the TWCR Regis-

ter. After updating TWDR, 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:

This scheme is repeated until the last byte has been sent and the transfer is ended by generat-

ing 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 of 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

Table 21-3.

Status codes for Master Transmitter mode.

Status code

(TWSR)

prescaler bits

are 0

Status of the 2-wire serial bus and

2-wire serial interface hardware

Application software response

Next Action Taken by TWI Hardware

To/from TWDR

To TWCR

STA

STO

TWINT

TWEA

0x08

A START condition has been

transmitted

Load SLA+W

SLA+W will be transmitted;

ACK or NOT ACK will be received

0x10

A repeated START condition has

been transmitted

Load SLA+W

Load SLA+R

SLA+W will be transmitted;

ACK or NOT ACK will be received

SLA+R will be transmitted;

Logic will switch to Master Receiver mode

0x18

SLA+W has been transmitted;

ACK has been received

Load data byte

No TWDR action or

No TWDR action

Data byte will be transmitted and ACK or NOT ACK

will be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START condition will

be transmitted and TWSTO Flag will be reset

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