Rainbow Electronics ATmega3290P_V - Manual (Page 199)

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ATmega329/3290/649/6490
2552H–AVR–11/06
• Bit 3..2 – USICS1..0: Clock Source Select
These bits set the clock source for the Shift 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 Shift 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 90 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 immedi-
ately when the clock strobe is executed, 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 Table 90).
• 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, writing 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 90. 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)