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Sheet目录3753

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135

8018P–AVR–08/10

ATmega169P

16. Timer/Counter0 and Timer/Counter1 Prescalers

Timer/Counter1 and Timer/Counter0 share the same prescaler module, but the Timer/Counters

can have different prescaler settings. The description below applies to both Timer/Counter1 and

Timer/Counter0.

16.1

Prescaler Reset

The prescaler is free running, that is, operates independently of the Clock Select logic of the

Timer/Counter, and it is shared by Timer/Counter1 and Timer/Counter0. 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 (6 > CSn2:0 > 1). The number of system clock

cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system

clock cycles, where N equals the prescaler divisor (8, 64, 256, or 1024).

It is possible to use the prescaler reset for synchronizing the Timer/Counter to program execu-

tion. However, care must be taken if the other Timer/Counter that shares the same prescaler

also uses prescaling. A prescaler reset will affect the prescaler period for all Timer/Counters it is

connected to.

16.2

Internal Clock Source

The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 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. The prescaled clock has a frequency of either f

CLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or

CLK_I/O/1024.

16.3

External Clock Source

An external clock source applied to the T1/T0 pin can be used as Timer/Counter clock

(clk

T1/clkT0). The T1/T0 pin is sampled once every system clock cycle by the pin synchronization

logic. The synchronized (sampled) signal is then passed through the edge detector. Figure 16-1

on page 135 shows a functional equivalent block diagram of the T1/T0 synchronization and

edge detector logic. The registers are clocked at the positive edge of the internal system clock

(clk

I/O). The latch is transparent in the high period of the internal system clock.

The edge detector generates one clk

T1/clkT0 pulse for each positive (CSn2:0 = 7) or negative

(CSn2:0 = 6) edge it detects.

Figure 16-1. T1/T0 Pin Sampling

The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles

from an edge has been applied to the T1/T0 pin to the counter is updated.

Tn_sync

(To Clock

Select Logic)

Edge Detector

Synchronization

clk

I/O

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