ATtiny261-ESSZ datasheet(139/227 Pages) ATMEL

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139

7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

• 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 USICS1..0 bits are set to zero and by doing so the soft-

ware clock strobe option is selected. The output will change immediately when the clock strobe

is executed, i.e., in the same instruction cycle. The value shifted into the USI Data 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 17-2).

• 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 DDB2 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.

17.5.5

USIPP – USI Pin Position

• Bits 7:1 – Res: Reserved Bits

These bits are reserved bits in the ATtiny261/461/861 and always reads as zero.

• Bit 0 – USIPOS: USI Pin Position

Setting this bit to one changes the USI pin position. As default pins PB2..PB0 are used for the

USI pin functions, but when writing this bit to one the USIPOS bit is set the USI pin functions are

on pins PA2..PA0.

Table 17-2.

Relations between the USICS1..0 and USICLK Setting

USICS1

USICS0

USICLK

USI Data 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)

Bit

76543210

0x11 (0x31)

-------

USIPOS

USIPP

Read/Write

RRRRRRR

R/W

Initial Value

00000000

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