User Manual

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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
• Bit 4 – TWSTO: TWI STOP Condition Bit
Writing the TWSTO bit to one in Master mode will generate a STOP condition on the 2-wire Serial Bus. When the
STOP condition is executed on the bus, the TWSTO bit is cleared automatically. 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 – Reserved
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.
22.9.3 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
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 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 – Reserved
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.
Bit 76543210
(0xB9) TWS7 TWS6 TWS5 TWS4 TWS3 – TWPS1 TWPS0 TWSR
Read/Write R R R R R R R/W R/W
Initial Value11111000
Table 22-7. TWI Bit Rate Prescaler
TWPS1 TWPS0 Prescaler Value
001
014
1016
1164