Rainbow Electronics ATtiny861_V - Manual (Page 127)

127
2588B–AVR–11/06
ATtiny261/461/861
The 4-bit counter can be both read and written via the data bus, and can generate an overflow
interrupt. Both the USI Data Register and the counter are clocked simultaneously by the same
clock source. This allows the counter to count the number of bits received or transmitted and
generate an interrupt when the transfer is complete. Note that when an external clock source is
selected the counter counts both clock edges. In this case the counter counts the number of
edges, and not the number of bits. The clock can be selected from three different sources: The
USCK pin, Timer/Counter0 Compare Match or from software.
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 con-
dition is detected, or after the counter overflows.
17.3 Functional Descriptions
17.3.1 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 17-2. Three-wire Mode Operation, Simplified Diagram
Figure 17-2 shows two USI units operating in Three-wire mode, one as Master and one as
Slave. The two USI Data Register 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.
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