ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 241

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

22.9.3
9223B–AVR–09/11
TWSR – TWI Status Register
• Bit 5 – TWSTA: TWI START Condition Bit
The application writes the TWSTA bit to one when it desires to become a Master on the 2-wire
Serial Bus. The TWI hardware checks if the bus is available, and generates a START condi-
tion on the bus if it is free. However, if the bus is not free, the TWI waits until a STOP condition
is detected, and then generates a new START condition to claim the bus Master status.
TWSTA must be cleared by software when the START condition has been transmitted.
• 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 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 unad-
dressed 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.
• 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.
Bit
(0xB9)
Read/Write
Initial Value
Atmel ATmega48PA/88PA/168PA [Preliminary]
TWS7
R
7
1
TWS6
R
6
1
TWS5
R
5
1
TWS4
R
4
1
TWS3
R
3
1
R
2
–
0
TWPS1
R/W
1
0
TWPS0
R/W
0
0
TWSR
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