ATMEGA64-16MJ Atmel, ATMEGA64-16MJ Datasheet - Page 202

• Manufacturer Part Number: ATMEGA64-16MJ
• Description: IC MCU AVR 64K 5V 16MHZ 64-QFN
• Manufacturer: Atmel
• Series: AVR® ATmegar

Specifications of ATMEGA64-16MJ

• Core Processor: AVR
• Core Size: 8-Bit
• Speed: 16MHz
• Connectivity: I²C, SPI, UART/USART
• Peripherals: Brown-out Detect/Reset, POR, PWM, WDT
• Number Of I /o: 53
• Program Memory Size: 64KB (32K x 16)
• Program Memory Type: FLASH
• Eeprom Size: 2K x 8
• Ram Size: 4K x 8
• Voltage - Supply (vcc/vdd): 4.5 V ~ 5.5 V
• Data Converters: A/D 8x10b
• Oscillator Type: Internal
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 64-MLF®, 64-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Multi-master Bus
Systems,
Arbitration and
Synchronization
2490Q–AVR–06/10
The TWI protocol allows bus systems with several masters. Special concerns have been taken
in order to ensure that transmissions will proceed as normal, even if two or more masters initiate
a transmission at the same time. Two problems arise in multi-master systems:
•
•
The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from
all masters will be wired-ANDed, yielding a combined clock with a high period equal to the one
from the master with the shortest high period. The low period of the combined clock is equal to
the low period of the master with the longest low period. Note that all masters listen to the SCL
line, effectively starting to count their SCL high and low Time-out periods when the combined
SCL line goes high or low, respectively.
Figure 92. SCL Synchronization between Multiple Masters
Arbitration is carried out by all masters continuously monitoring the SDA line after outputting
data. If the value read from the SDA line does not match the value the master had output, it has
lost the arbitration. Note that a master can only lose arbitration when it outputs a high SDA value
while another master outputs a low value. The losing master should immediately go to Slave
mode, checking if it is being addressed by the winning master. The SDA line should be left high,
but losing masters are allowed to generate a clock signal until the end of the current data or
address packet. Arbitration will continue until only one master remains, and this may take many
bits. If several masters are trying to address the same slave, arbitration will continue into the
data packet.
An algorithm must be implemented allowing only one of the masters to complete the
transmission. All other masters should cease transmission when they discover that they
have lost the selection process. This selection process is called arbitration. When a
contending master discovers that it has lost the arbitration process, it should immediately
switch to Slave mode to check whether it is being addressed by the winning master. The fact
that multiple masters have started transmission at the same time should not be detectable to
the slaves (that is, the data being transferred on the bus must not be corrupted).
Different masters may use different SCL frequencies. A scheme must be devised to
synchronize the serial clocks from all masters, in order to let the transmission proceed in a
lockstep fashion. This will facilitate the arbitration process.
SCL from
SCL from
Master A
Master B
SCL bus
Line
TA
Counting Low Period
low
Masters Start
TB
low
TA
Counting High Period
high
Masters Start
TB
ATmega64(L)
high
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