ATmega48 datasheet(32/340 Pages) ATMEL

32 / 340 page

7530H–AVR–02/09

ATmega48/88/168 Automotive

6.7

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 6-10.

Note:

1. The frequency is preliminary value. Actual value is TBD.

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

Table 6-11.

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.

6.8

External Clock

The device can utilize a external clock source as shown in Figure 6-4. To run the device on an

external clock, the CKSEL Fuses must be programmed as shown in Table 6-12.

Notes:

1. The frequency ranges are preliminary values. Actual values are TBD.

2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8

Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured

that the resulting divided clock meets the frequency specification of the device.

Table 6-10.

128 kHz Internal Oscillator Operating Modes

Nominal Frequency

CKSEL3..0

128 kHz

0011

Table 6-11.

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 6-12.

Full Swing Crystal Oscillator operating modes

(2)

Frequency Range(1) (MHz)

CKSEL3..0

Recommended Range for Capacitors C1

and C2 (pF)

0 - 100

0000

12 - 22

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