ATmega8535 Atmel Corporation, ATmega8535 Datasheet - Page 201

• Manufacturer Part Number: ATmega8535
• Manufacturer: Atmel Corporation

Specifications of ATmega8535

• Flash (kbytes): 8 Kbytes
• Pin Count: 44
• Max. Operating Frequency: 16 MHz
• Cpu: 8-bit AVR
• # Of Touch Channels: 16
• Hardware Qtouch Acquisition: No
• Max I/o Pins: 32
• Ext Interrupts: 3
• Usb Speed: No
• Usb Interface: No
• Spi: 1
• Twi (i2c): 1
• Uart: 1
• Graphic Lcd: No
• Video Decoder: No
• Camera Interface: No
• Adc Channels: 8
• Adc Resolution (bits): 10
• Adc Speed (ksps): 15
• Analog Comparators: 1
• Resistive Touch Screen: No
• Temp. Sensor: No
• Crypto Engine: No
• Sram (kbytes): 0.5
• Eeprom (bytes): 512
• Self Program Memory: YES
• Dram Memory: No
• Nand Interface: No
• Picopower: No
• Temp. Range (deg C): -40 to 85
• I/o Supply Class: 2.7 to 5.5
• Operating Voltage (vcc): 2.7 to 5.5
• Fpu: No
• Mpu / Mmu: no / no
• Timers: 3
• Output Compare Channels: 4
• Input Capture Channels: 1
• Pwm Channels: 4
• 32khz Rtc: Yes
• Calibrated Rc Oscillator: Yes

Combining Several TWI
Modes
Multi-master Systems
and Arbitration
2502K–AVR–10/06
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
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
SDA
SCL
S
S = START
TRANSMITTER
Device 1
Transmitted from master to slave
SLA+W
MASTER
TRANSMITTER
A
Device 2
MASTER
Master Transmitter
ADDRESS
Device 3
RECEIVER
SLAVE
A
Rs = REPEATED START
Rs
Transmitted from slave to master
........
SLA+R
Device n
ATmega8535(L)
V
CC
A
Master Receiver
R1
DATA
R2
P = STOP
A
P
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