ATMEGA8L-8AUR Atmel, ATMEGA8L-8AUR Datasheet - Page 184
ATMEGA8L-8AUR
Manufacturer Part Number
ATMEGA8L-8AUR
Description
MCU AVR 8KB FLASH 8MHZ 32TQFP
Manufacturer
Atmel
Series
AVR® ATmegar
Datasheet
1.ATMEGA8L-8MU.pdf
(302 pages)
Specifications of ATMEGA8L-8AUR
Core Processor
AVR
Core Size
8-Bit
Speed
8MHz
Connectivity
I²C, SPI, UART/USART
Peripherals
Brown-out Detect/Reset, POR, PWM, WDT
Number Of I /o
23
Program Memory Size
8KB (4K x 16)
Program Memory Type
FLASH
Eeprom Size
512 x 8
Ram Size
1K x 8
Voltage - Supply (vcc/vdd)
2.7 V ~ 5.5 V
Data Converters
A/D 8x10b
Oscillator Type
Internal
Operating Temperature
-40°C ~ 85°C
Package / Case
*
Package
32TQFP
Device Core
AVR
Family Name
ATmega
Maximum Speed
8 MHz
Operating Supply Voltage
3.3|5 V
Data Bus Width
8 Bit
Number Of Programmable I/os
23
Interface Type
SPI/TWI/USART
On-chip Adc
8-chx10-bit
Number Of Timers
3
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
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Quantity
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Combining Several
TWI Modes
Multi-master
Systems and
Arbitration
184
ATmega8(L)
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. 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 multimaster 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.
Figure 86. 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 87. An Arbitration Example
Several different scenarios may arise during arbitration, as described below:
•
Two or more masters are performing identical communication with the same Slave. In this
case, neither the Slave nor any of the masters will know about the bus contention
S
S = START
SDA
SCL
Transmitted from master to slave
SLA+W
TRANSMITTER
Device 1
MASTER
A
Master Transmitter
ADDRESS
TRANSMITTER
Device 2
MASTER
Device 3
A
RECEIVER
SLAVE
Rs = REPEATED START
Rs
Transmitted from slave to master
........
SLA+R
Device n
V
CC
A
R1
Master Receiver
DATA
R2
P = STOP
2486Z–AVR–02/11
A
P
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