ATMEGA649 datasheet(199/375 Pages) ATMEL

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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

No Clock

Software clock strobe

(USICLK)

Software clock strobe

(USICLK)

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)

ATMEGA649 Datasheet(PDF) 199 Page - ATMEL Corporation

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