Datasheet

ATmega48A/PA/88A/PA/168A/PA/328/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002061A-page 44
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 9-14.
Note: 1. If the RSTDISBL fuse is programmed, this start-up time will be increased to
14CK + 4.1ms to ensure programming mode can be entered.
9.8 External Clock
To drive the device from an external clock source, XTAL1 should be driven as shown in Figure 9-4. To run the
device on an external clock, the CKSEL Fuses must be programmed to “0000” (see Table 9-15).
Figure 9-4. External Clock Drive Configuration
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 9-16.
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.
Table 9-14. Start-up Times for the 128kHz Internal Oscillator
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset SUT1...0
BOD enabled 6 CK 14CK
(1)
00
Fast rising power 6 CK 14CK + 4ms 01
Slowly rising power 6 CK 14CK + 64ms 10
Reserved 11
Table 9-15. Crystal Oscillator Clock Frequency
Frequency CKSEL3...0
0 - 20MHz 0000
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
PB7
EXTERNAL
CLOCK
SIGNAL
XTAL2
XTAL1
GND