107
7598H–AVR–07/09
ATtiny25/45/85
shows the relationship between the USICS1..0 and USICLK setting and clock source
used for the Shift Register and the 4-bit counter.
• Bit 1 – USICLK: Clock Strobe
Writing a one to this bit location strobes the Shift Register to shift one step and the counter to
increment by one, provided that the USICS1..0 bits are set to zero and by doing so the software
clock strobe option is selected. The output will change immediately when the clock strobe is exe-
cuted, i.e., in the same instruction cycle. The value shifted into the Shift Register is sampled the
previous instruction cycle. The bit will be read as zero.
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
• 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 DDRE4 must be set as output (to one). This feature allows easy clock
generation when implementing master devices. The bit will be read as zero.
When an external clock source is selected (USICS1 = 1) and the USICLK bit is set to one, writ-
ing to the USITC strobe bit will directly clock the 4-bit counter. This allows an early detection of
when the transfer is done when operating as a master device.
Table 16-2.
Relations between the USICS1..0 and USICLK Setting
USICS1
USICS0
USICLK
Shift Register Clock Source
4-bit Counter Clock Source
0
0
0
No Clock
No Clock
0
0
1
Software clock strobe
(USICLK)
Software clock strobe
(USICLK)
0
1
X
Timer/Counter0 Compare
Match
Timer/Counter0 Compare
Match
1
0
0
External, positive edge
External, both edges
1
1
0
External, negative edge
External, both edges
1
0
1
External, positive edge
Software clock strobe (USITC)
1
1
1
External, negative edge
Software clock strobe (USITC)
