ATmega32(L)
Figure 78. START, REPEATED START, and STOP Conditions
SDA
SCL
START
STOP START
REPEATED START
STOP
Address Packet Format
All address packets transmitted on the TWI bus are nine bits long, consisting of seven
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 master’s 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 79. Address Packet Format
Addr MSB
Addr LSB
R/W
ACK
SDA
SCL
1
2
7
8
9
START
Data Packet Format
All data packets transmitted on the TWI bus are nine bits long, consisting of one data
byte and an acknowledge bit. During a data transfer, the master generates the clock and
the START and STOP conditions, while the receiver is responsible for acknowledging
the reception. An Acknowledge (ACK) is signalled by the receiver pulling the SDA line
low during the ninth SCL cycle. If the receiver leaves the SDA line high, a NACK is sig-
nalled. When the receiver has received the last byte, or for some reason cannot receive
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2503J–AVR–10/06
ATMEL [ ATMEL ]
