Rainbow Electronics Atmega169L - Manual (Page 178)

178
ATmega169V/L
2514A–AVR–08/02
The Two-wire clock control unit can generate an interrupt when a start condition is
detected on the Two-wire bus. It can also generate wait states by holding the clock pin
low after a start condition is detected, or after the counter overflows.
Functional Descriptions
Three-wire Mode The USI Three-wire mode is compliant to the Serial Peripheral Interface (SPI) mode 0
and 1, but does not have the slave select (SS) pin functionality. However, this feature
can be implemented in software if necessary. Pin names used by this mode are: DI, DO,
and USCK.
Figure 76. Three-wire Mode Operation, Simplified Diagram
Figure 76 shows two USI units operating in Three-wire mode, one as Master and one as
Slave. The two Shift Registers are interconnected in such way that after eight USCK
clocks, the data in each register are interchanged. The same clock also increments the
USI’s 4-bit counter. The Counter Overflow (interrupt) Flag, or USIOIF, can therefore be
used to determine when a transfer is completed. The clock is generated by the Master
device software by toggling the USCK pin via the PORT Register or by writing a one to
the USITC bit in USICR.
Figure 77. Three-wire Mode, Timing Diagram
SLAVE
MASTER
Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0
DO
DI
USCK
Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0
DO
DI
USCK
PORTxn
MSB
MSB
6 5 4 3 2 1 LSB
1 2 3 4 5 6 7 8
6 5 4 3 2 1 LSB
USCK
USCK
DO
DI
D C B A E
CYCLE ( Reference )