ATmega16 datasheet(187/357 Pages) ATMEL

187 / 357 page

187

2466S–AVR–05/09

ATmega16(L)

A START condition is sent by writing the following value to TWCR:

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

transmit a START condition and TWINT must be written to one 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 hard-

ware, and the status code in TWSR will be $08 (See Table 74). 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 following 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 $18, $20, or $38. The appropriate action to be taken for each of these status codes is

detailed in Table 74.

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 $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 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

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

Value

Table 74. Status Codes for Master Transmitter Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the Two-wire Serial

Bus and Two-wire Serial Inter-

face Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR

To TWCR

STA

STO

TWINT

TWEA

$08

A START condition has been

transmitted

Load SLA+W

SLA+W will be transmitted;

ACK or NOT ACK will be received

$10

A repeated START condition

has been transmitted

Load SLA+W or

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

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