ATmega128 datasheet(207/386 Pages) ATMEL

207 / 386 page

207

2467S–AVR–07/09

ATmega128

• Bit 4 – TWSTO: TWI STOP Condition Bit

Writing the TWSTO bit to one in Master mode will generate a STOP condition on the Two-wire

Serial Bus. When the STOP condition is executed on the bus, the TWSTO bit is cleared auto-

matically. In slave mode, setting the TWSTO bit can be used to recover from an error condition.

This will not generate a STOP condition, but the TWI returns to a well-defined unaddressed

Slave mode and releases the SCL and SDA lines to a high impedance state.

• Bit 3 – TWWC: TWI Write Collision Flag

The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is

low. This flag is cleared by writing the TWDR Register when TWINT is high.

• Bit 2 – TWEN: TWI Enable Bit

The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to

one, the TWI takes control over the I/O pins connected to the SCL and SDA pins, enabling the

slew-rate limiters and spike filters. If this bit is written to zero, the TWI is switched off and all TWI

transmissions are terminated, regardless of any ongoing operation.

• Bit 1 – Res: Reserved Bit

This bit is a reserved bit and will always read as zero.

• 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.

TWI Status Register –

TWSR

• Bits 7..3 – TWS: TWI Status

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

codes are described later in this section. 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 pres-

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 204. The value of TWPS1..0 is

used in the equation.

Bit

765

4321

TWS7

TWS6

TWS5

TWS4

TWS3

–

TWPS1

TWPS0

TWSR

Read/Write

RRRR

R/W

Initial Value

111

1100

Table 87. TWI Bit Rate Prescaler

TWPS1

TWPS0

Prescaler Value

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