199
ATmega128(L)
2467B–09/01
• 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 activated 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 2-wire Serial Bus. The different
status codes are described later in this chapter. 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 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. See
“Bit Rate
Generator Unit” on page 196
for calculating bit rates.
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 register 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
Bit 76543210
TWS7 TWS6 TWS5 TWS4 TWS3 – TWPS1 TWPS0 TWSR
Read/Write R R R R R R R/W R/W
Initial value11111000
Bit 76543210
TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0 TWDR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value11111111
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