ATmega16 datasheet(176/357 Pages) ATMEL

176 / 357 page

176

2466S–AVR–05/09

ATmega16(L)

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.

The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from

all Masters will be wired-ANDed, yielding a combined clock with a high period equal to the one

from the Master with the shortest high period. The low period of the combined clock is equal to

the low period of the Master with the longest low period. Note that all Masters listen to the SCL

line, effectively starting to count their SCL high and low time-out periods when the combined

SCL line goes high or low, respectively.

Figure 82. SCL Synchronization between Multiple Masters

Arbitration is carried out by all Masters continuously monitoring the SDA line after outputting

data. If the value read from the SDA line does not match the value the Master had output, it has

lost the arbitration. Note that a Master can only lose arbitration when it outputs a high SDA value

while another Master outputs a low value. The losing Master should immediately go to Slave

mode, checking if it is being addressed by the winning Master. The SDA line should be left high,

but losing Masters are allowed to generate a clock signal until the end of the current data or

address packet. Arbitration will continue until only one Master remains, and this may take many

bits. If several Masters are trying to address the same Slave, arbitration will continue into the

data packet.

low

high

SCL from

Master A

SCL from

Master B

SCL bus

Line

low

high

Masters Start

Counting Low Period

Masters Start

Counting High Period

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