ATA6613 ATMEL Corporation, ATA6613 Datasheet - Page 260

• Manufacturer Part Number: ATA6613
• Description: Ata6613
• Manufacturer: ATMEL Corporation

6.19.8.6
Figure 6-96. Combining Several TWI Modes to Access a Serial EEPROM
6.19.9
260
ATA6612/ATA6613
Multi-master Systems and Arbitration
S
S = START
Combining Several TWI Modes
Transmitted from master to slave
SLA+W
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:
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. Subsequently, 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 principle is violated in a multi master sys-
tem, 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.
If multiple masters are connected to the same bus, transmissions may be initiated simulta-
neously 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 transfer, 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.
1. The transfer must be initiated.
2. The EEPROM must be instructed what location should be read.
3. The reading must be performed.
4. The transfer must be finished.
A
Master Transmitter
ADDRESS
A
Rs = REPEATED START
Rs
Transmitted from slave to master
SLA+R
A
Master Receiver
DATA
P = STOP
A
9111C–AUTO–02/08
P