11. 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.
11.1 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 (fCLK_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 fCLK_I/O/8,
f
CLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024.
11.2 Prescaler Reset
The prescaler is free running, i.e., 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 execution. 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.
11.3 External Clock Source
An external clock source applied to the Tn pin can be used as Timer/Counter clock (clkT1/clkT0). The Tn 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 11-1 shows a functional equivalent block diagram of the Tn/T0 synchronization and edge detector
logic. The registers are clocked at the positive edge of the internal system clock (clkI/O). The latch is transparent in the high
period of the internal system clock.
The edge detector generates one clkT1/clkT0 pulse for each positive (CSn2:0 = 7) or negative (CSn2:0 = 6) edge it detects.
Figure 11-1. Tn Pin Sampling
Tn_sync
Tn
D
Q
D
Q
D
Q
(to Clock
Select Logic)
LE
clkI/O
Synchronization
Edge Detector
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 Tn/T0 pin to the counter is updated.
Enabling and disabling of the clock input must be done when Tn/T0 has been stable for at least one system clock cycle,
otherwise it is a risk that a false Timer/Counter clock pulse is generated.
Each half period of the external clock applied must be longer than one system clock cycle to ensure correct sampling. The
external clock must be guaranteed to have less than half the system clock frequency (fExtClk < fclk_I/O/2) given a 50/50% duty
cycle. Since the edge detector uses sampling, the maximum frequency of an external clock it can detect is half the sampling
frequency (Nyquist sampling theorem). However, due to variation of the system clock frequency and duty cycle caused by
Oscillator source (crystal, resonator, and capacitors) tolerances, it is recommended that maximum frequency of an external
clock source is less than fclk_I/O/2.5.
An external clock source can not be prescaled.
ATmega16/32/64/M1/C1 [DATASHEET]
75
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
