attiny25v ATMEL Corporation, attiny25v Datasheet - Page 121

• Manufacturer Part Number: attiny25v
• Description: Microcontroller With 2/4/8k Bytes In-system Programmable Flash
• Manufacturer: ATMEL Corporation

2586K–AVR–01/08
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 3:2 – USICS1:0: Clock Source Select
These bits set the clock source for the USI Data Register and counter. The data output latch
ensures that the output is changed at the opposite edge of the sampling of the data input
(DI/SDA) when using external clock source (USCK/SCL). When software strobe or
Timer/Counter0 Compare Match clock option is selected, the output latch is transparent and
therefore the output is changed immediately.
Clearing the USICS1:0 bits enables software strobe option. When using this option, writing a
one to the USICLK bit clocks both the USI Data Register and the counter. For external clock
source (USICS1 = 1), the USICLK bit is no longer used as a strobe, but selects between external
clocking and software clocking by the USITC strobe bit.
Table 15-2
used for the USI Data Register and the 4-bit counter.
Table 15-2.
• Bit 1 – USICLK: Clock Strobe
Writing a one to this bit location strobes the USI Data Register to shift one step and the counter
to increment by one, provided that the software clock strobe option has been selected by writing
USICS1:0 bits to zero. The output will change immediately when the clock strobe is executed,
i.e., during the same instruction cycle. The value shifted into the USI Data Register is sampled
the previous instruction cycle.
When an external clock source is selected (USICS1 = 1), the USICLK function is changed from
a clock strobe to a Clock Select Register. Setting the USICLK bit in this case will select the
USITC strobe bit as clock source for the 4-bit counter (see
The bit will be read as zero.
• Bit 0 – USITC: Toggle Clock Port Pin
Writing a one to this bit location toggles the USCK/SCL value either from 0 to 1, or from 1 to 0.
The toggling is independent of the setting in the Data Direction Register, but if the PORT value is
to be shown on the pin the corresponding DDR pin must be set as output (to one). This feature
allows easy clock generation when implementing master devices.
USICS1
0
0
0
1
1
1
1
shows the relationship between the USICS1:0 and USICLK setting and clock source
USICS0
Relationship between the USICS1:0 and USICLK Setting
0
0
1
0
1
0
1
USICLK
X
0
1
0
0
1
1
Clock Source
No Clock
Software clock strobe (USICLK)
Timer/Counter0 Compare Match
External, positive edge
External, negative edge
External, positive edge
External, negative edge
Table
4-bit Counter Clock Source
No Clock
Software clock strobe (USICLK)
Timer/Counter0 Compare Match
External, both edges
External, both edges
Software clock strobe (USITC)
Software clock strobe (USITC)
15-2).
ATtiny25/45/85
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