ATmega328P datasheet(222/448 Pages) ATMEL

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8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

that slaves may prolong the SCL low period, thereby reducing the average TWI bus clock

period. The SCL frequency is generated according to the following equation:

• TWBR = Value of the TWI Bit Rate Register.

• PrescalerValue = Value of the prescaler, see Table 21-7 on page 243.

Note:

Pull-up resistor values should be selected according to the SCL frequency and the capacitive bus

line load. See Table 28-6 on page 321 for value of pull-up resistor.

21.5.3

Bus Interface Unit

This unit contains the Data and Address Shift Register (TWDR), a START/STOP Controller 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 Regis-

ter is not directly accessible by the application 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 continu-

ously 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.

21.5.4

Address Match Unit

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 compared 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 compare 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

(1).

Note:

1. This applies to all device revisions except ATmega88PA revision C or newer.

21.5.5

Control Unit

The Control unit monitors the TWI bus and generates responses corresponding to settings 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 Sta-

tus Register (TWSR) is updated with a status code identifying 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 avail-

SCL frequency

CPU Clock frequency

2(TWBR)

PrescalerValue

()

⋅

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