ATmega169P datasheet(210/393 Pages) ATMEL

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210

8018N–AVR–08/09

ATmega169P

Table 20-2 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 Table 20-2).

• 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 20-2.

Relations between the USICS1:0 and USICLK Setting

USICS1

USICS0

USICLK

Shift Register Clock Source

4-bit Counter Clock Source

No Clock

Software clock strobe

(USICLK)

Software clock strobe

(USICLK)

01X

Timer/Counter0 Compare

Match

Timer/Counter0 Compare

Match

External, positive edge

External, both edges

External, negative edge

External, both edges

External, positive edge

Software clock strobe (USITC)

External, negative edge

Software clock strobe (USITC)

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