Datasheet

208
2490R–AVR–02/2013
ATmega64(L)
• 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.
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 sta-
ble 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 Two-wire Serial Bus.
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 multimaster systems, TWAR must be set in masters which
can be addressed as slaves by other masters.
The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an
associated address comparator that looks for the slave address (or general call address if
enabled) in the received serial address. If a match is found, an interrupt request is generated.
• Bits 7..1 – TWA: TWI (Slave) Address Register
These seven bits constitute the slave address of the TWI unit.
Table 87. TWI Bit Rate Prescaler
TWPS1 TWPS0 Prescaler Value
00 1
01 4
10 16
11 64
Bit 76543210
(0x73) 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
Bit 76543210
(0x72) 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 Value11111110