ATMEGA128-16AI Atmel, ATMEGA128-16AI Datasheet - Page 207

• Manufacturer Part Number: ATMEGA128-16AI
• Description: IC AVR MCU 128K 16MHZ 64-TQFP
• Manufacturer: Atmel
• Series: AVR® ATmegar

Specifications of ATMEGA128-16AI

• Core Processor: AVR
• Core Size: 8-Bit
• Speed: 16MHz
• Connectivity: EBI/EMI, I²C, SPI, UART/USART
• Peripherals: Brown-out Detect/Reset, POR, PWM, WDT
• Number Of I /o: 53
• Program Memory Size: 128KB (64K x 16)
• Program Memory Type: FLASH
• Eeprom Size: 4K 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-TQFP, 64-VQFP
• For Use With: ATSTK501 - ADAPTER KIT FOR 64PIN AVR MCU
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: Q1167170A

TWI Data Register –
TWDR
TWI (Slave) Address
Register – TWAR
Using the TWI
2467V–AVR–02/11
In Transmit mode, TWDR contains the next byte to be transmitted. In receive mode, the TWDR
contains the last byte received. It is writable while the TWI is not in the process of shifting a byte.
This occurs when the TWI interrupt flag (TWINT) is set by hardware. Note that the Data Register
cannot be initialized by the user before the first interrupt occurs. The data in TWDR remains sta-
ble as long as TWINT is set. While data is shifted out, data on the bus is simultaneously shifted
in. TWDR always contains the last byte present on the bus, except after a wake up from a sleep
mode by the TWI interrupt. In this case, the contents of TWDR is undefined. In the case of a lost
bus arbitration, no data is lost in the transition from Master to Slave. Handling of the ACK bit is
controlled automatically by the TWI logic, the CPU cannot access the ACK bit directly.
• Bits 7..0 – TWD: TWI Data Register
These eight bits constitute the next data byte to be transmitted, or the latest data byte received
on the Two-wire Serial Bus.
The TWAR should be loaded with the 7-bit slave address (in the seven most significant bits of
TWAR) to which the TWI will respond when programmed as a slave transmitter or receiver, and
not needed in the master modes. In multimaster systems, TWAR must be set in masters which
can be addressed as slaves by other masters.
The LSB of TWAR is used to enable recognition of the general call address ($00). There is an
associated address comparator that looks for the slave address (or general call address if
enabled) in the received serial address. If a match is found, an interrupt request is generated.
• Bits 7..1 – TWA: TWI (Slave) Address Register
These seven bits constitute the slave address of the TWI unit.
• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a General Call given over the Two-wire Serial Bus.
The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like
reception of a byte or transmission of a START condition. Because the TWI is interrupt-based,
the application software is free to carry on other operations during a TWI byte transfer. Note that
the TWI Interrupt Enable (TWIE) bit in TWCR together with the Global Interrupt Enable bit in
SREG allow the application to decide whether or not assertion of the TWINT flag should gener-
ate an interrupt request. If the TWIE bit is cleared, the application must poll the TWINT flag in
order to detect actions on the TWI bus.
When the TWINT flag is asserted, the TWI has finished an operation and awaits application
response. In this case, the TWI Status Register (TWSR) contains a value indicating the current
state of the TWI bus. The application software can then decide how the TWI should behave in
the next TWI bus cycle by manipulating the TWCR and TWDR Registers.
Figure 95
example, a master wishes to transmit a single data byte to a slave. This description is quite
Bit
Read/Write
Initial Value
Bit
Read/Write
Initial Value
is a simple example of how the application can interface to the TWI hardware. In this
TWD7
TWA6
R/W
R/W
7
1
7
1
TWD6
TWA5
R/W
R/W
6
1
6
1
TWD5
TWA4
R/W
R/W
5
1
5
1
TWD4
TWA3
R/W
R/W
4
1
4
1
TWD3
TWA2
R/W
R/W
3
1
3
1
TWD2
TWA1
R/W
R/W
2
1
2
1
TWD1
TWA0
R/W
R/W
1
1
1
1
TWGCE
TWD0
R/W
R/W
ATmega128
0
1
0
0
TWDR
TWAR
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