214
7679H–CAN–08/08
AT90CAN32/64/128
• 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 210. The value of TWPS1.0 is used
in the equation.
18.6.4 TWI Data Register – TWDR
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 TWI Serial Bus.
18.6.5 TWI (Slave) Address Register – TWAR
• Bits 7.1 – TWA: TWI (Slave) Address Register
These seven bits constitute the slave address of the TWI unit. The TWAR should be loaded with
the 7-bit slave address 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.
• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
Table 18-2. TWI Bit Rate Prescaler
TWPS1 TWPS0 Prescaler Value
001
014
1016
1164
Bit 76543210
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
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
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