Functional Descriptions - Atmel ATtiny25 Manual

16.2

Functional Descriptions

16.2.1 Three-wire Mode
7598H–AVR–07/09
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.
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 16-2. Three-wire Mode Operation, Simplified Diagram
Bit7 Bit6 Bit5 Bit4
SLAVE
Bit7 Bit6 Bit5 Bit4
MASTER
Figure 16-2 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.
Bit3 Bit2 Bit1 Bit0
Bit3 Bit2 Bit1 Bit0
ATtiny25/45/85
DO
DI
USCK
DO
DI
USCK
PORTxn
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