ATmega48PA/88PA/168PA/328P
Table 8-16. Start-up Times for the External Clock Selection
Start-up Time from Power-
Additional Delay from
Reset (VCC = 5.0V)
Power Conditions
down and Power-save
SUT1..0
00
BOD enabled
6 CK
6 CK
14CK
Fast rising power
14CK + 4.1 ms
14CK + 65 ms
01
Slowly rising power
6 CK
10
Reserved
11
When applying an external clock, it is required to avoid sudden changes in the applied clock fre-
quency 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.
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 ”System Clock Prescaler” on page
35 for details.
8.9
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 cir-
cuits 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.
8.10 Timer/Counter Oscillator
ATmega48PA/88PA/168PA/328P uses the same crystal oscillator for Low-frequency Oscillator
and Timer/Counter Oscillator. See ”Low Frequency Crystal Oscillator” on page 32 for details on
the oscillator and crystal requirements.
ATmega48PA/88PA/168PA/328P 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.
Applying an external clock source to TOSC1 can be done if EXTCLK in the ASSR Register is
written to logic one. See ”Asynchronous Operation of Timer/Counter2” on page 155 for further
description on selecting external clock as input instead of a 32.768 kHz watch crystal.
8.11 System Clock Prescaler
The ATmega48PA/88PA/168PA/328P has a system clock prescaler, and the system clock can
be divided by setting the ”CLKPR – Clock Prescale Register” on page 377. 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. clkI/O, clkADC, clkCPU, and clkFLASH
are divided by a factor as shown in Table 28-3 on page 318.
35
8161D–AVR–10/09
ATMEL [ ATMEL ]
