ATmega644PA datasheet(236/468 Pages) ATMEL

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236

8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

• Bit 0 – TWIE: TWI Interrupt Enable

When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be acti-

vated for as long as the TWINT Flag is high.

20.9.3

TWSR – TWI Status Register

• Bits 7:3 – TWS: TWI Status

These 5 bits reflect the status of the TWI logic and the 2-wire Serial Bus. The different status

codes are described ”Transmission Modes” on page 220. Note that the value read from TWSR

contains both the 5-bit status value and the 2-bit prescaler value. The application designer

should mask the prescaler bits to zero when checking the Status bits. This makes status check-

ing independent of prescaler setting. This approach is used in this datasheet, unless otherwise

noted.

• Bit 2 – Res: Reserved Bit

This bit is reserved and will always read as zero.

• Bits 1:0 – TWPS: TWI Prescaler Bits

These bits can be read and written, and control the bit rate prescaler.

To calculate bit rates, see ”Bit Rate Generator Unit” on page 215. The value of TWPS1..0 is

used in the equation.

20.9.4

TWDR – TWI Data Register

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 Regis-

ter cannot be initialized by the user before the first interrupt occurs. The data in TWDR remains

stable 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

Bit

76543210

(0xB9)

TWS7

TWS6

TWS5

TWS4

TWS3

–

TWPS1

TWPS0

TWSR

Read/Write

RRRRRR

R/W

Initial Value

11111000

Table 20-7.

TWI Bit Rate Prescaler

TWPS1

TWPS0

Prescaler Value

Bit

76543210

(0xBB)

TWD7

TWD6

TWD5

TWD4

TWD3

TWD2

TWD1

TWD0

TWDR

Read/Write

R/W

Initial Value

11111111

ATmega644PA Datasheet(PDF) 236 Page - ATMEL Corporation

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