35
8271D–AVR–05/11
ATmega48A/PA/88A/PA/168A/PA/328/P
9.7
128kHz Internal Oscillator
The 128kHz internal Oscillator is a low power Oscillator providing a clock of 128kHz. The fre-
quency is nominal at 3V and 25
°
C. This clock may be select as the system clock by
programming the CKSEL Fuses to “11” as shown in
Note:
1. Note that the 128kHz oscillator is a very low power clock source, and is not designed for high
accuracy.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
.
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
. To run the device on an external clock, the CKSEL Fuses must be programmed to “0000”
(see
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 9-13.
128kHz Internal Oscillator Operating Modes
Nominal Frequency
CKSEL3...0
128kHz
0011
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
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
PB7
EXTERNAL
CLOCK
S
IGNAL
XTAL2
XTAL1
GND