參數(shù)資料
型號: AT90USB1286
廠商: Atmel Corp.
英文描述: 8-bit Microcontroller with 64/128K Bytes of ISP Flash and USB Controller
中文描述: 8位64/128K的ISP字節(jié)閃存和控制器的USB微控制器
文件頁數(shù): 34/132頁
文件大?。?/td> 10886K
代理商: AT90USB1286
201
ATmega8535(L)
2502K–AVR–10/06
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:
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.
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
S
SLA+W
A
ADDRESS
A
Rs
SLA+R
A
DATA
A
P
Device 1
MASTER
TRANSMITTER
Device 2
MASTER
TRANSMITTER
Device 3
SLAVE
RECEIVER
Device n
SDA
SCL
........
R1
R2
V
CC
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