ATmega128 datasheet(201/386 Pages) ATMEL

201 / 386 page

201

2467S–AVR–07/09

ATmega128

Figure 90. Data Packet Format

Combining Address

and Data Packets Into

a Transmission

A transmission basically consists of a START condition, a SLA+R/W, one or more data packets

and a STOP condition. An empty message, consisting of a START followed by a STOP condi-

tion, is illegal. Note that the Wired-ANDing of the SCL line can be used to implement

handshaking between the master and the slave. The slave can extend the SCL low period by

pulling the SCL line low. This is useful if the clock speed set up by the master is too fast for the

slave, or the slave needs extra time for processing between the data transmissions. The slave

extending the SCL low period will not affect the SCL high period, which is determined by the

master. As a consequence, the slave can reduce the TWI data transfer speed by prolonging the

SCL duty cycle.

Figure 91 shows a typical data transmission. Note that several data bytes can be transmitted

between the SLA+R/W and the STOP condition, depending on the software protocol imple-

mented by the application software.

Figure 91. Typical Data Transmission

Multi-master Bus

Systems,

Arbitration and

Synchronization

The TWI protocol allows bus systems with several masters. Special concerns have been taken

in order to ensure that transmissions will proceed as normal, even if two or more masters initiate

a transmission at the same time. Two problems arise in multi-master systems:

•

An algorithm must be implemented allowing only one of the masters to complete the

transmission. All other masters should cease transmission when they discover that they

have lost the selection process. This selection process is called arbitration. When a

contending master discovers that it has lost the arbitration process, it should immediately

switch to slave mode to check whether it is being addressed by the winning master. The fact

that multiple masters have started transmission at the same time should not be detectable to

the slaves, i.e., the data being transferred on the bus must not be corrupted.

•

Different masters may use different SCL frequencies. A scheme must be devised to

synchronize the serial clocks from all masters, in order to let the transmission proceed in a

lockstep fashion. This will facilitate the arbitration process.

Data MSB

Data LSB

ACK

Aggregate

SDA

SDA from

Transmitter

SDA from

Receiver

SCL from

Master

SLA+R/W

Data Byte

STOP, REPEATED

START or Next

Data Byte

Data MSB

Data LSB

ACK

SDA

SCL

START

789

Addr MSB

Addr LSB

R/W

ACK

SLA+R/W

STOP

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