ATtiny261-ESMZ datasheet(96/227 Pages) ATMEL

96 / 227 page

7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

16.3

Counter Unit

The main part of the Timer/Counter1 is the programmable bi-directional counter unit. Figure 16-3

shows a block diagram of the counter and its surroundings.

Figure 16-3. Counter Unit Block Diagram

Signal description (internal signals):

count

TCNT1 increment or decrement enable.

direction

Select between increment and decrement.

clear

Clear TCNT1 (set all bits to zero).

clk

Timer/Counter clock, referred to as clk

T1 in the following.

top

Signalize that TCNT1 has reached maximum value.

bottom

Signalize that TCNT1 has reached minimum value (zero).

Depending of the mode of operation used, the counter is cleared, incremented, or decremented

at each timer clock (clk

T1). The timer clock is generated from an synchronous system clock or an

asynchronous PLL clock using the Clock Select bits (CS13:0) and the PCK Enable bit (PCKE).

When no clock source is selected (CS13:0 = 0) the timer is stopped. However, the TCNT1 value

can be accessed by the CPU, regardless of whether clk

T1 is present or not. A CPU write over-

rides (has priority over) all counter clear or count operations.

The counting sequence of the Timer/Counter1 is determined by the setting of the WGM10 and

PWM1x bits located in the Timer/Counter1 Control Registers (TCCR1A, TCCR1C and

TCCR1D). For more details about advanced counting sequences and waveform generation, see

“Modes of Operation” on page 102. The Timer/Counter Overflow Flag (TOV1) is set according to

the mode of operation selected by the PWM1x and WGM10 bits. The Overflow Flag can be used

for generating a CPU interrupt.

16.3.1

Counter Initialization for Asynchronous Mode

To change Timer/Counter1 to the asynchronous mode follow the procedure below:

1. Enable PLL.

2. Wait 100 µs for PLL to stabilize.

3. Poll the PLOCK bit until it is set.

4. Set the PCKE bit in the PLLCSR register which enables the asynchronous mode.

DATA BUS

TCNT1

Control Logic

count

TOV1

top

Timer/Counter1 Count Enable

( From Prescaler )

bottom

direction

clear

PCK

PCKE

clk

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