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8127B–AVR–08/09

ATtiny4/5/9/10

11.3 Clock Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock source

is selected by the Clock Select logic which is controlled by the Clock Select (CS02:0) bits

located in the Timer/Counter control Register B (TCCR0B). For details on clock sources and

prescaler, see section “Prescaler”.

11.3.1 Prescaler

The Timer/Counter can be clocked directly by the system clock (by setting the CS2:0 = 1). This

provides the fastest operation, with a maximum Timer/Counter clock frequency equal to system

clock frequency (f

CLK_I/O

). Alternatively, one of four taps from the prescaler can be used as a

clock source.

See Figure 11-2 for an illustration of the prescaler unit.

Figure 11-2. Prescaler for Timer/Counter0

Note: 1. The synchronization logic on the input pins (T0) is shown in Figure 11-3 on page 55.

The prescaled clock has a frequency of f

CLK_I/O

/8, f

CLK_I/O

/64, f

CLK_I/O

/256, or f

CLK_I/O

/1024. See

Table 11-6 on page 76 for details.

Prescaler Reset

The prescaler is free running, i.e., operates independently of the Clock Select logic of the

Timer/CounterCounter, and it is shared by the Timer/Counter Tn. Since the prescaler is not

affected by the Timer/Counter’s clock select, the state of the prescaler will have implications for

situations where a prescaled clock is used. One example of prescaling artifacts occurs when the

timer is enabled and clocked by the prescaler (CS2:0 = 2, 3, 4, or 5). The number of system

PSR10

Clear

clk

I/O

Synchronization

1 2 ... 49 50 51 52 53 54 55 56 57 58 59 ... 165 166

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