ATmega16 datasheet(193/315 Pages) ATMEL

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ATmega16(L)

2466E–AVR–10/02

Combining Several TWI

Modes

In some cases, several TWI modes must be combined in order to complete the desired

action. Consider for example reading data from a serial EEPROM. Typically, such a

transfer involves the following steps:

The transfer must be initiated

The EEPROM must be instructed what location should be read

The reading must be performed

The transfer must be finished

Note that data is transmitted both from master to slave and vice versa. The master must

instruct the slave what location it wants to read, requiring the use of the MT mode. Sub-

sequently, data must be read from the slave, implying the use of the MR mode. Thus,

the transfer direction must be changed. The master must keep control of the bus during

all these steps, and the steps should be carried out as an atomical operation. If this prin-

ciple is violated in a multimaster system, another master can alter the data pointer in the

EEPROM between steps 2 and 3, and the master will read the wrong data location.

Such a change in transfer direction is accomplished by transmitting a REPEATED

START between the transmission of the address byte and reception of the data. After a

REPEATED START, the master keeps ownership of the bus. The following figure shows

the flow in this transfer.

Figure 94. Combining Several TWI Modes to Access a Serial EEPROM

Multi-master Systems

and Arbitration

If multiple masters are connected to the same bus, transmissions may be initiated simul-

taneously by one or more of them. The TWI standard ensures that such situations are

handled in such a way that one of the masters will be allowed to proceed with the trans-

fer, and that no data will be lost in the process. An example of an arbitration situation is

depicted below, where two masters are trying to transmit data to a slave receiver.

Figure 95. An Arbitration Example

Master Transmitter

Master Receiver

S = START

Rs = REPEATED START

P = STOP

Transmitted from Master to Slave

Transmitted from Slave to Master

SLA+W

ADDRESS

SLA+R

DATA

Device 1

MASTER

TRANSMITTER

Device 2

MASTER

TRANSMITTER

Device 3

SLAVE

RECEIVER

Device n

SDA

SCL

........

ATMEGA16 Datasheet(PDF) 193 Page - ATMEL Corporation

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