AT90USB646 datasheet(227/434 Pages) ATMEL

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7593A–AVR–02/06

AT90USB64/128

caler bits to zero when checking the Status bits. This makes status checking 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 223. The value of TWPS1..0 is

used in the equation.

20.6.4

TWI Data Register – TWDR

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

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 2-wire Serial Bus.

20.6.5

TWI (Slave) Address Register – TWAR

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.

Table 20-2.

TWI Bit Rate Prescaler

TWPS1

TWPS0

Prescaler Value

001

014

1016

1164

Bit

765

4321

TWD7

TWD6

TWD5

TWD4

TWD3

TWD2

TWD1

TWD0

TWDR

Read/Write

R/W

Initial Value

111

1111

Bit

765

4321

TWA6

TWA5

TWA4

TWA3

TWA2

TWA1

TWA0

TWGCE

TWAR

Read/Write

R/W

Initial Value

111

1111

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