Atmel ATmega48A Manual page 144

ATmega48A/PA/88A/PA/168A/PA/328/P
Enabling and disabling of the clock input must be done when T1/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 sys-
tem clock frequency (f < f /2) given a 50/50% duty cycle. Since the edge detector uses
ExtClk clk_I/O
sampling, the maximum frequency of an external clock it can detect is half the sampling fre-
quency (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 f /2.5.
clk_I/O
An external clock source can not be prescaled.
(1)
Figure 17-2. Prescaler for Timer/Counter0 and Timer/Counter1
clk
I/O
Clear
PSRSYNC
T0
Synchronization
T1
Synchronization
clk clk
T1
T0
T1/T0)
Note: 1. The synchronization logic on the input pins ( is shown in Figure 17-1.
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