ATMEGA640V_0701 Datasheet, PDF(247/449 Page) ATMEL Corporation – 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash

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ATMEGA640V_0701 Datasheet, PDF (247/449 Pages) ATMEL Corporation – 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash

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ATmega640/1280/1281/2560/2561

Figure 92. START, REPEATED START and STOP conditions

SDA

Address Packet Format

SCL

START

STOP START

REPEATED START STOP

All address packets transmitted on the TWI bus are 9 bits long, consisting of 7 address

bits, one READ/WRITE control bit and an acknowledge bit. If the READ/WRITE bit is

set, a read operation is to be performed, otherwise a write operation should be per-

formed. When a Slave recognizes that it is being addressed, it should acknowledge by

pulling SDA low in the ninth SCL (ACK) cycle. If the addressed Slave is busy, or for

some other reason can not service the Masterâs request, the SDA line should be left

high in the ACK clock cycle. The Master can then transmit a STOP condition, or a

REPEATED START condition to initiate a new transmission. An address packet consist-

ing of a slave address and a READ or a WRITE bit is called SLA+R or SLA+W,

respectively.

The MSB of the address byte is transmitted first. Slave addresses can freely be allo-

cated by the designer, but the address 0000 000 is reserved for a general call.

When a general call is issued, all slaves should respond by pulling the SDA line low in

the ACK cycle. A general call is used when a Master wishes to transmit the same mes-

sage to several slaves in the system. When the general call address followed by a Write

bit is transmitted on the bus, all slaves set up to acknowledge the general call will pull

the SDA line low in the ack cycle. The following data packets will then be received by all

the slaves that acknowledged the general call. Note that transmitting the general call

address followed by a Read bit is meaningless, as this would cause contention if several

slaves started transmitting different data.

All addresses of the format 1111 xxx should be reserved for future purposes.

Figure 93. Address Packet Format

Addr MSB

Addr LSB R/W

ACK

SDA

SCL

START

1

2

7

8

9

2549KâAVRâ01/07

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