Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Data Sheet
Product codes
MEGA328P-XMINI
36
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to
ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the next
can lead to unpredictable behavior. If changes of more than 2% is required, ensure that the MCU is kept in
Reset during the changes.
ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the next
can lead to unpredictable behavior. If changes of more than 2% is required, ensure that the MCU is kept in
Reset during the changes.
Note that the System Clock Prescaler can be used to implement run-time changes of the internal clock
frequency while still ensuring stable operation. Refer to
frequency while still ensuring stable operation. Refer to
for details.
9.9
Clock Output Buffer
The device can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to be
programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. The clock
also will be output during reset, and the normal operation of I/O pin will be overridden when the fuse is
programmed. Any clock source, including the internal RC Oscillator, can be selected when the clock is output on
CLKO. If the System Clock Prescaler is used, it is the divided system clock that is output.
programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. The clock
also will be output during reset, and the normal operation of I/O pin will be overridden when the fuse is
programmed. Any clock source, including the internal RC Oscillator, can be selected when the clock is output on
CLKO. If the System Clock Prescaler is used, it is the divided system clock that is output.
9.10
Timer/Counter Oscillator
ATmega48A/PA/88A/PA/168A/PA/328/P uses the same crystal oscillator for Low-frequency Oscillator and
Timer/Counter Oscillator. See
Timer/Counter Oscillator. See
crystal requirements.
ATmega48A/PA/88A/PA/168A/PA/328/P share the Timer/Counter Oscillator Pins (TOSC1 and TOSC2) with
XTAL1 and XTAL2. When using the Timer/Counter Oscillator, the system clock needs to be four times the
oscillator frequency. Due to this and the pin sharing, the Timer/Counter Oscillator can only be used when the
Calibrated Internal RC Oscillator is selected as system clock source.
XTAL1 and XTAL2. When using the Timer/Counter Oscillator, the system clock needs to be four times the
oscillator frequency. Due to this and the pin sharing, the Timer/Counter Oscillator can only be used when the
Calibrated Internal RC Oscillator is selected as system clock source.
Applying an external clock source to TOSC1 can be done if EXTCLK in the ASSR Register is written to logic
one. See
one. See
for further description on selecting external
clock as input instead of a 32.768kHz watch crystal.
9.11
System Clock Prescaler
The ATmega48A/PA/88A/PA/168A/PA/328/P has a system clock prescaler, and the system clock can be
divided by setting the
divided by setting the
7. This feature can be used to decrease
the system clock frequency and the power consumption when the requirement for processing power is low. This
can be used with all clock source options, and it will affect the clock frequency of the CPU and all synchronous
peripherals. clk
can be used with all clock source options, and it will affect the clock frequency of the CPU and all synchronous
peripherals. clk
I/O
, clk
ADC
, clk
CPU
, and clk
FLASH
.
When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occurs in the
clock system. It also ensures that no intermediate frequency is higher than neither the clock frequency
corresponding to the previous setting, nor the clock frequency corresponding to the new setting. The ripple
counter that implements the prescaler runs at the frequency of the undivided clock, which may be faster than the
CPU's clock frequency. Hence, it is not possible to determine the state of the prescaler - even if it were
readable, and the exact time it takes to switch from one clock division to the other cannot be exactly predicted.
clock system. It also ensures that no intermediate frequency is higher than neither the clock frequency
corresponding to the previous setting, nor the clock frequency corresponding to the new setting. The ripple
counter that implements the prescaler runs at the frequency of the undivided clock, which may be faster than the
CPU's clock frequency. Hence, it is not possible to determine the state of the prescaler - even if it were
readable, and the exact time it takes to switch from one clock division to the other cannot be exactly predicted.
Table 9-16.
Start-up Times for the External Clock Selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V)
SUT1...0
BOD enabled
6 CK
14CK
00
Fast rising power
6 CK
14CK + 4.1ms
01
Slowly rising power
6 CK
14CK + 65ms
10
Reserved
11