Datasheet

150
7728G–AVR–06/10
ATtiny87/ATtiny167
A transparent latch is inserted between the USI Data Register Output and output pin, which
delays the change of data output to the opposite clock edge of the data input sampling. The
serial input is always sampled from the Data Input (DI) pin independent of the configuration.
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
condition is detected, or after the counter overflows.
14.3 Functional Descriptions
14.3.1 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 imple-
mented in software if necessary. Pin names used by this mode are: DI, DO, and USCK.
Figure 14-2. Three-wire Mode Operation, Simplified Diagram
Figure 14-2 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.
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