134
2588F–AVR–06/2013
ATtiny261/461/861
The 4-bit counter increments by one for each clock generated either by the external clock edge
detector, by a Timer/Counter0 Compare Match, or by software using USICLK or USITC strobe
bits. The clock source depends of the setting of the USICS1:0 bits. For external clock operation
a special feature is added that allows the clock to be generated by writing to the USITC strobe
bit. This feature is enabled by write a one to the USICLK bit while setting an external clock
source (USICS1 = 1).
Note that even when no wire mode is selected (USIWM1:0 = 0) the external clock input
(USCK/SCL) are can still be used by the counter.
13.5.4
USICR – USI Control Register
The Control Register includes interrupt enable control, wire mode setting, Clock Select setting,
and clock strobe.
Bit 7 – USISIE: Start Condition Interrupt Enable
Setting this bit to one enables the Start Condition detector interrupt. If there is a pending inter-
rupt when the USISIE and the Global Interrupt Enable Flag is set to one, this will immediately be
executed. Refer to the USISIF bit description on page
133 for further details.
Bit 6 – USIOIE: Counter Overflow Interrupt Enable
Setting this bit to one enables the Counter Overflow interrupt. If there is a pending interrupt when
the USIOIE and the Global Interrupt Enable Flag is set to one, this will immediately be executed.
Refer to the USIOIF bit description on page
133 for further details.
Bits 5:4 – USIWM1:0: Wire Mode
Basically, only the function of the outputs are affected by these bits. Data and clock inputs are
not affected by the mode selected and will always have the same function. The counter and USI
Data Register can therefore be clocked externally, and data input sampled, even when outputs
are disabled.
Bit
7
654
3
2
1
0
USISIE
USIOIE
USIWM1
USIWM0
USICS1
USICS0
USICLK
USITC
USICR
Read/Write
R/W
W
Initial Value
0
Table 13-1.
Relationship between USIWM1:0 and USI Operation
USIWM1
USIWM0
Description
00
Outputs, clock hold, and start detector disabled.
Port pins operate as normal.
01
Three-wire mode. Uses DO, DI, and USCK pins.
The Data Output (DO) pin overrides the corresponding bit in the PORTA
register. However, the corresponding DDRA bit still controls the data direction.
When the port pin is set as input the pin pull-up is controlled by the PORTA bit.
The Data Input (DI) and Serial Clock (USCK) pins do not affect the normal port
operation. When operating as master, clock pulses are software generated by
toggling the PORTA register, while the data direction is set to output. The
USITC bit in the USICR Register can be used for this purpose.