ATTIny2313-16MI ATMEL Corporation, ATTIny2313-16MI Datasheet - Page 143

• Manufacturer Part Number: ATTIny2313-16MI
• Description: 8-bit AVR Microcontroller with 2K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

Two-wire Mode
2543C–AVR–12/03
Note that the first two instructions is for initialization only and needs only to be executed
once.These instructions sets Three-wire mode and positive edge Shift Register clock.
The loop is repeated until the USI Counter Overflow Flag is set.
The USI Two-wire mode does not incorporate slew rate limiting on outputs and input
noise filtering. Pin names used by this mode are SCL and SDA.
Figure 62. Two-wire Mode Operation, Simplified Diagram
Figure 62 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
dependent of the communication scheme used. The main differences between the Mas-
ter 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 negative 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.
SLAVE
MASTER
Bit7
Bit7
Bit6
Bit6
Bit5
Bit5
Bit4
Bit4
Bit3
Bit3
Bit2
Bit2
Bit1
Bit1
Bit0
Bit0
Two-wire Clock
Control Unit
PORTxn
ATtiny2313/V
HOLD
SCL
SDA
SCL
SDA
SCL
VCC
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