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ATmega8A [DATASHEET]
8159E–AVR–02/2013
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 sam-
pling. The external clock must be guaranteed to have less than half the system clock frequency (f
ExtClk < 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 rec-
ommended that maximum frequency of an external clock source is less than f
clk_I/O/2.5.
An external clock source can not be prescaled.
Figure 16-2. Prescaler for Timer/Counter0 and Timer/Counter1
Note:
1. The synchronization logic on the input pins (
16.5
Register Description
16.5.1
SFIOR – Special Function IO Register
Bit 0 – PSR10: Prescaler Reset Timer/Counter1 and Timer/Counter0
When this bit is written to one, the Timer/Counter1 and Timer/Counter0 prescaler will be reset. The bit will be
cleared by hardware after the operation is performed. Writing a zero to this bit will have no effect. Note that
Timer/Counter1 and Timer/Counter0 share the same prescaler and a reset of this prescaler will affect both timers.
This bit will always be read as zero.
PSR10
Clear
clk
T1
clk
T0
T1
T0
clk
I/O
Synchronization
Bit
7
6
5
4
3
2
1
0
–
ACME
PUD
PSR2
PSR10
SFIOR
Read/Write
R
R/W
Initial Value
0