ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 238

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

22.7.6
22.8
238
Multi-master Systems and Arbitration
Atmel ATmega48PA/88PA/168PA [Preliminary]
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:
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. Subse-
quently, 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 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 22-19. Combining Several TWI Modes to Access a Serial EEPROM
If multiple masters are connected to the same bus, transmissions may be initiated simultane-
ously 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.
Figure 22-20. An Arbitration Example
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.
S
S = START
SDA
SCL
Transmitted from master to slave
SLA+W
TRANSMITTER
Device 1
MASTER
A
Master Transmitter
TRANSMITTER
ADDRESS
Device 2
MASTER
Device 3
RECEIVER
A
SLAVE
Rs = REPEATED START
Rs
Transmitted from slave to master
........
SLA+R
Device n
V
CC
A
Master Receiver
R1
DATA
R2
P = STOP
9223B–AVR–09/11
A
P