Data Sheet

233
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
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 213. The value of TWPS1...0 is used in the
equation.
22.9.4 TWDR – TWI Data Register
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 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.
22.9.5 TWAR – TWI (Slave) Address Register
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 multi master systems, TWAR must be set in masters which can be addressed as Slaves by other
Masters.
Table 22-7. TWI Bit Rate Prescaler
TWPS1 TWPS0 Prescaler Value
001
014
1016
1164
Bit 76543210
(0xBB) 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 Value 1 1 1 1 1 1 1 1
Bit 76543210
(0xBA)
TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE TWAR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 1 1 1 1 1 1 1 0