2545M–AVR–09/07

ATmega48/88/168

8.5 Low Frequency Crystal Oscillator

The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low Fre-

quency Crystal Oscillator. The crystal should be connected as shown in Figure 8-2. When this

Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in

Table 8-7.

Note: 1. These options should only be used if frequency stability at start-up is not important for the

application.

8.6 Calibrated Internal RC Oscillator

By default, the Internal RC OScillator provides an approximate 8.0 MHz clock. Though voltage

and temperature dependent, this clock can be very accurately calibrated by the user. The device

is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page 37 for

more details.

This clock may be selected as the system clock by programming the CKSEL Fuses as shown in

Table 8-8. If selected, it will operate with no external components. During reset, hardware loads

the pre-programmed calibration value into the OSCCAL Register and thereby automatically cal-

ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in

Table 28-1 on page 307.

By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on

page 38, it is possible to get a higher calibration accuracy than by using the factory calibration.

The accuracy of this calibration is shown as User calibration in Table 28-1 on page 307.

When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the

Watchdog Timer and for the Reset Time-out. For more information on the pre-programmed cali-

bration value, see the section “Calibration Byte” on page 289.

Table 8-7. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

= 5.0V) CKSEL0 SUT1..0

BOD enabled 1K CK 14CK

(1)

000

Fast rising power 1K CK 14CK + 4.1 ms

(1)

001

Slowly rising power 1K CK 14CK + 65 ms

(1)

010

Reserved 0 11

BOD enabled 32K CK 14CK 1 00

Fast rising power 32K CK 14CK + 4.1 ms 1 01

Slowly rising power 32K CK 14CK + 65 ms 1 10

Reserved 1 11

2545M–AVR–09/07

ATmega48/88/168

Notes: 1. The device is shipped with this option selected.

2. If 8 MHz frequency exceeds the specification of the device (depends on V

), the CKDIV8

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

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

Table 8-9 on page 35.

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.

2. The device is shipped with this option selected.

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 8-10.

Note: 1. Note that the 128 kHz oscillator is a very low power clock source, and is not designed for a

high accuracy.

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

Table 8-11.

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.

Table 8-8. Internal Calibrated RC Oscillator Operating Modes

(1)(2)

Frequency Range (MHz) CKSEL3..0

7.3 - 8.1 0010

Table 8-9. Start-up times for the internal calibrated RC Oscillator clock selection

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

= 5.0V) SUT1..0

BOD enabled 6 CK 14CK

(1)

Fast rising power 6 CK 14CK + 4.1 ms 01

Slowly rising power 6 CK 14CK + 65 ms

(2)

Reserved 11

Table 8-10. 128 kHz Internal Oscillator Operating Modes

Nominal Frequency CKSEL3..0

128 kHz 0011

Table 8-11. 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 01

Slowly rising power 6 CK 14CK + 64 ms 10

Reserved 11

2545M–AVR–09/07

ATmega48/88/168

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 36. To run the device on an external clock, the CKSEL Fuses must be programmed

to “0000” (see Table 8-12).

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 8-13.

When applying an external clock, it is required to avoid sudden changes in the applied clock fre-

quency 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.

Note that the System Clock Prescaler can be used to implement run-time changes of the internal

clock frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on page

37 for details.

8.9 Clock Output Buffer

The device can output the system clock on the CLKO pin. To enable the output, the CKOUT

Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other cir-

cuits on the system. The clock also will be output during reset, and the normal operation of I/O

pin will be overridden when the fuse is programmed. Any clock source, including the internal RC

Table 8-12. Crystal Oscillator Clock Frequency

Frequency CKSEL3..0

0 - 20 MHz 0000

Table 8-13. Start-up Times for the External Clock Selection

Power Conditions

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

= 5.0V) SUT1..0

BOD enabled 6 CK 14CK 00

Fast rising power 6 CK 14CK + 4.1 ms 01

Slowly rising power 6 CK 14CK + 65 ms 10

Reserved 11

EXTERNAL

CLOCK

SIGNAL

XTAL2

XTAL1

GND

Part #	ATMEGA48-20AU
Description	MCU 8BIT ATMEGA RISC 4KB FLASH 3.3V/5V 32TQFP - Trays
Category	IC
Availability	Out of Stock
Qty	0

ATMEGA48-20AU

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