ATmega328P datasheet(34/448 Pages) ATMEL

34 / 448 page

8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

8.7

128 kHz Internal Oscillator

The 128 kHz internal Oscillator is a low power Oscillator providing a clock of 128 kHz. 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 Table 8-13.

Note:

1. Note that the 128 kHz 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

Table 8-14.

Note:

1. If the RSTDISBL fuse is programmed, this start-up time will be increased to

14CK + 4.1 ms to ensure programming mode can be entered.

8.8

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in Figure

8-4 on page 34. To run the device on an external clock, the CKSEL Fuses must be programmed

to “0000” (see Table 8-15).

Figure 8-4.

External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT Fuses as shown in

Table 8-16.

Table 8-13.

128 kHz Internal Oscillator Operating Modes

Nominal Frequency(1)

CKSEL3..0

128 kHz

0011

Table 8-14.

Start-up Times for the 128 kHz 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)

Fast rising power

6 CK

14CK + 4 ms

Slowly rising power

6 CK

14CK + 64 ms

Reserved

Table 8-15.

Crystal Oscillator Clock Frequency

Frequency

CKSEL3..0

0 - 20 MHz

0000

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

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