Datasheet

154
7728G–AVR–06/10
ATtiny87/ATtiny167
Figure 14-4. Two-wire Mode Operation, Simplified Diagram
Figure 14-4 shows two USI units operating in Two-wire mode, one as Master and one as
Slave. It is only the physical layer that is shown since the system operation is highly depen-
dent of the communication scheme used. The main differences between the Master and Slave
operation at this level, is the serial clock generation which is always done by the Master, and
only the Slave uses the clock control unit. Clock generation must be implemented in software,
but the shift operation is done automatically by both devices. Note that only clocking on nega-
tive edge for shifting data is of practical use in this mode. The slave can insert wait states at
start or end of transfer by forcing the SCL clock low. This means that the Master must always
check if the SCL line was actually released after it has generated a positive edge.
Since the clock also increments the counter, a counter overflow can be used to indicate that
the transfer is completed. The clock is generated by the master by toggling the USCK pin via
the PORT Register.
The data direction is not given by the physical layer. A protocol, like the one used by the
TWI-bus, must be implemented to control the data flow.
Figure 14-5. Two-wire Mode, Typical Timing Diagram
MASTER
SLAVE
Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0
SDA
SCL
Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0
Two-wire Clock
Control Unit
HOLD
SCL
PORTxn
SDA
SCL
VCC
PS
ADDRESS
1 - 7 8 9
R/W ACK ACK
1 - 8 9
DATA ACK
1 - 8 9
DATA
SDA
SCL
A B D EC F