Техническая Спецификация для Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI
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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
22.3.2 START and STOP Conditions
The Master initiates and terminates a data transmission. The transmission is initiated when the Master issues a
START condition on the bus, and it is terminated when the Master issues a STOP condition. Between a START
and a STOP condition, the bus is considered busy, and no other master should try to seize control of the bus. A
special case occurs when a new START condition is issued between a START and STOP condition. This is
referred to as a REPEATED START condition, and is used when the Master wishes to initiate a new transfer
without relinquishing control of the bus. After a REPEATED START, the bus is considered busy until the next
STOP. This is identical to the START behavior, and therefore START is used to describe both START and
REPEATED START for the remainder of this datasheet, unless otherwise noted. As depicted below, START
and STOP conditions are signalled by changing the level of the SDA line when the SCL line is high.
START condition on the bus, and it is terminated when the Master issues a STOP condition. Between a START
and a STOP condition, the bus is considered busy, and no other master should try to seize control of the bus. A
special case occurs when a new START condition is issued between a START and STOP condition. This is
referred to as a REPEATED START condition, and is used when the Master wishes to initiate a new transfer
without relinquishing control of the bus. After a REPEATED START, the bus is considered busy until the next
STOP. This is identical to the START behavior, and therefore START is used to describe both START and
REPEATED START for the remainder of this datasheet, unless otherwise noted. As depicted below, START
and STOP conditions are signalled by changing the level of the SDA line when the SCL line is high.
Figure 22-3.
START, REPEATED START and STOP conditions
22.3.3 Address Packet Format
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 performed. 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 consisting of a slave address and a READ or a WRITE bit is called SLA+R or SLA+W,
respectively.
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 performed. 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 consisting 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 allocated by the designer, but
the address 0000 000 is reserved for a general call.
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 message 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.
call is used when a Master wishes to transmit the same message 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.
SDA
SCL
START
STOP
REPEATED START
STOP
START