ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 207

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

TWI Data Register – TWDR
TWI (Slave) Address Register
– TWAR
2490G–AVR–03/04
• Bits 1..0 – TWPS: TWI Prescaler Bits
These bits can be read and written, and control the bit rate prescaler.
Table 87. TWI Bit Rate Prescaler
To calculate bit rates, see “Bit Rate Generator Unit” on page 202. The value of
TWPS1..0 is used in the equation.
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 Two-wire Serial Bus.
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.
Bit
Read/Write
Initial Value
Bit
Read/Write
Initial Value
TWPS1
0
0
1
1
TWD7
TWA6
R/W
R/W
7
1
7
1
TWD6
TWA5
R/W
R/W
6
1
6
1
TWD5
TWPS0
TWA4
R/W
R/W
5
1
5
1
0
1
0
1
TWD4
TWA3
R/W
R/W
4
1
4
1
TWD3
TWA2
R/W
R/W
3
1
3
1
TWD2
TWA1
R/W
R/W
2
1
2
1
Prescaler Value
ATmega64(L)
TWD1
TWA0
R/W
R/W
1
1
1
1
16
64
1
4
TWGCE
TWD0
R/W
R/W
0
1
0
0
TWDR
TWAR
207