ATtiny261A datasheet(132/292 Pages) ATMEL

132 / 292 page

132

8197A–AVR–10/09

ATtiny261A/461A/861A

The 4-bit counter increments by one for each clock generated either by the external clock edge

detector, by a Timer/Counter0 Compare Match, or by software using USICLK or USITC strobe

bits. The clock source depends of the setting of the USICS1:0 bits. For external clock operation

a special feature is added that allows the clock to be generated by writing to the USITC strobe

bit. This feature is enabled by write a one to the USICLK bit while setting an external clock

source (USICS1 = 1).

Note that even when no wire mode is selected (USIWM1:0 = 0) the external clock input

(USCK/SCL) are can still be used by the counter.

13.5.4

USICR – USI Control Register

The Control Register includes interrupt enable control, wire mode setting, Clock Select setting,

and clock strobe.

• Bit 7 – USISIE: Start Condition Interrupt Enable

Setting this bit to one enables the Start Condition detector interrupt. If there is a pending inter-

rupt when the USISIE and the Global Interrupt Enable Flag is set to one, this will immediately be

executed. Refer to the USISIF bit description on page 131 for further details.

• Bit 6 – USIOIE: Counter Overflow Interrupt Enable

Setting this bit to one enables the Counter Overflow interrupt. If there is a pending interrupt when

the USIOIE and the Global Interrupt Enable Flag is set to one, this will immediately be executed.

Refer to the USIOIF bit description on page 131 for further details.

• Bit 5:4 – USIWM1:0: Wire Mode

These bits set the type of wire mode to be used, as shown in Table 13-1 on page 132.

Basically, only the function of the outputs are affected by these bits. Data and clock inputs are

not affected by the mode selected and will always have the same function. The counter and USI

Data Register can therefore be clocked externally, and data input sampled, even when outputs

are disabled.

Bit

654

0x0D (0x2D)

USISIE

USIOIE

USIWM1

USIWM0

USICS1

USICS0

USICLK

USITC

USICR

Read/Write

R/W

Initial Value

Table 13-1.

Relationship between USIWM1:0 and USI Operation

USIWM1

USIWM0

Description

Outputs, clock hold, and start detector disabled.

Port pins operate as normal.

Three-wire mode. Uses DO, DI, and USCK pins.

The Data Output (DO) pin overrides the corresponding bit in the PORTA

When the port pin is set as input the pin pull-up is controlled by the PORTA bit.

The Data Input (DI) and Serial Clock (USCK) pins do not affect the normal port

operation. When operating as master, clock pulses are software generated by

toggling the PORTA register, while the data direction is set to output. The

USITC bit in the USICR Register can be used for this purpose.

ATtiny261A Datasheet(PDF) 132 Page - ATMEL Corporation

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