ATMEGA48V_06 ATMEL [ATMEL Corporation], ATMEGA48V_06 Datasheet - Page 138

• Manufacturer Part Number: ATMEGA48V_06
• Description: 8-bit Microcontroller with 8K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

138
ATmega48/88/168
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
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
An external clock source can not be prescaled.
Figure 15-2. Prescaler for Timer/Counter0 and Timer/Counter1
Note:
PSRSYNC
clk
1. The synchronization logic on the input pins (
T0
T1
I/O
Synchronization
Synchronization
ExtClk
< f
clk_I/O
/2) given a 50/50% duty cycle. Since the edge detector uses
clk
Clear
T1
T1/T0)
is shown in
(1)
Figure
15-1.
clk
T0
clk_I/O
2545J–AVR–12/06
/2.5.