ATMEGA2561V ATMEL [ATMEL Corporation], ATMEGA2561V Datasheet - Page 252

• Manufacturer Part Number: ATMEGA2561V
• Description: 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Bus Interface Unit
Address Match Unit
Control Unit
252
ATmega640/1280/1281/2560/2561
•
•
Note:
This unit contains the Data and Address Shift Register (TWDR), a START/STOP Con-
troller and Arbitration detection hardware. The TWDR contains the address or data
bytes to be transmitted, or the address or data bytes received. In addition to the 8-bit
TWDR, the Bus Interface Unit also contains a register containing the (N)ACK bit to be
transmitted or received. This (N)ACK Register is not directly accessible by the applica-
tion software. However, when receiving, it can be set or cleared by manipulating the
TWI Control Register (TWCR). When in Transmitter mode, the value of the received
(N)ACK bit can be determined by the value in the TWSR.
The START/STOP Controller is responsible for generation and detection of START,
REPEATED START, and STOP conditions. The START/STOP controller is able to
detect START and STOP conditions even when the AVR MCU is in one of the sleep
modes, enabling the MCU to wake up if addressed by a Master.
If the TWI has initiated a transmission as Master, the Arbitration Detection hardware
continuously monitors the transmission trying to determine if arbitration is in process. If
the TWI has lost an arbitration, the Control Unit is informed. Correct action can then be
taken and appropriate status codes generated.
The Address Match unit checks if received address bytes match the seven-bit address
in the TWI Address Register (TWAR). If the TWI General Call Recognition Enable
(TWGCE) bit in the TWAR is written to one, all incoming address bits will also be com-
pared against the General Call address. Upon an address match, the Control Unit is
informed, allowing correct action to be taken. The TWI may or may not acknowledge its
address, depending on settings in the TWCR. The Address Match unit is able to com-
pare addresses even when the AVR MCU is in sleep mode, enabling the MCU to wake
up if addressed by a Master. If another interrupt (e.g., INT0) occurs during TWI Power-
down address match and wakes up the CPU, the TWI aborts operation and return to it’s
idle state. If this cause any problems, ensure that TWI Address Match is the only
enabled interrupt when entering Power-down.
The Control unit monitors the TWI bus and generates responses corresponding to set-
tings in the TWI Control Register (TWCR). When an event requiring the attention of the
application occurs on the TWI bus, the TWI Interrupt Flag (TWINT) is asserted. In the
next clock cycle, the TWI Status Register (TWSR) is updated with a status code identify-
ing the event. The TWSR only contains relevant status information when the TWI
Interrupt Flag is asserted. At all other times, the TWSR contains a special status code
indicating that no relevant status information is available. As long as the TWINT Flag is
set, the SCL line is held low. This allows the application software to complete its tasks
before allowing the TWI transmission to continue.
The TWINT Flag is set in the following situations:
•
•
TWBR = Value of the TWI Bit Rate Register.
TWPS = Value of the prescaler bits in the TWI Status Register.
After the TWI has transmitted a START/REPEATED START condition.
After the TWI has transmitted SLA+R/W.
Pull-up resistor values should be selected according to the SCL frequency and the
capacitive bus line load. See “2-wire Serial Interface Characteristics” on page 379 for
value of pull-up resistor.
SCL frequency
=
---------------------------------------------------------- -
16
CPU Clock frequency
+
2(TWBR) 4
⋅
TWPS
2549K–AVR–01/07