Datasheet

Figure 12-4. External Clock Drive Configuration
EXTERNAL
CLOCK
SIGNAL
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in the
following table.
Table 12-10. Start-up Times for the External Clock Selection
SUT1:0 Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V)
Recommended Usage
00 6 CK – BOD enabled
01 6 CK 4.1ms Fast rising power
10 6 CK 65ms Slowly rising power
11 Reserved
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. It is required to ensure that the MCU is kept in Reset during such
changes in the clock frequency.
12.9. Timer/Counter Oscillator
For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the crystal is
connected directly between the pins. No external capacitors are needed. The Oscillator is optimized for
use with a 32.768kHz watch crystal. Applying an external clock source to TOSC1 is not recommended.
Note: 1. The Timer/Counter Oscillator uses the same type of crystal oscillator as Low-Frequency
Oscillator and the internal capacitors have the same nominal value of 36pF.
12.10. Register Description
Atmel ATmega32A [DATASHEET]
Atmel-8155I-ATmega32A_Datasheet_Complete-08/2016
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