ATmega162 datasheet(38/324 Pages) ATMEL

38 / 324 page

2513K–AVR–07/09

ATmega162/V

Notes:

1. These options should only be used when not operating close to the maximum frequency of the

device, and only if frequency stability at start-up is not important for the application. These

options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability

at start-up. They can also be used with crystals when not operating close to the maximum fre-

quency of the device, and if frequency stability at start-up is not important for the application.

Low-frequency

Crystal Oscillator

To use a 32.768 kHz watch crystal as the clock source for the device, the Low-frequency Crystal

Oscillator must be selected by setting the CKSEL Fuses to “0100”, “0101”, “0110” or “0111”. The

crystal should be connected as shown in Figure 19. If CKSEL equals “0110” or “0111”, the inter-

nal capacitors on XTAL1 and XTAL2 are enabled, thereby removing the need for external

capacitors. The internal capacitors have a nominal value of 10 pF.

When this Oscillator is selected, start-up times are determined by the SUT Fuses (real time-out

from Reset) and CKSEL0 (number of clock cycles) as shown in Table 9 and Table 10.

Note:

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

application.

Calibrated Internal

RC Oscillator

The calibrated internal RC Oscillator provides a fixed 8.0 MHz clock. The frequency is nominal

value at 3V and 25

°C. If 8.0 MHz frequency exceed the specification of the device (depends on

CC), the CKDIV8 Fuse must be programmed in order to divide the internal frequency by 8 dur-

ing start-up. See “System Clock Prescaler” on page 41 for more details. This clock may be

selected as the system clock by programming the CKSEL Fuses as shown in Table 11. If

selected, it will operate with no external components. During Reset, hardware loads the calibra-

tion byte into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. At

3V and 25

°C, this calibration gives a frequency within ±10% of the nominal frequency. Using cal-

ibration methods as described in application notes available at www.atmel.com/avr it is possible

to achieve ±2% accuracy at any given V

CC and Temperature. When this Oscillator is used as the

chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the Reset

Table 9. Start-up Delay from Reset when Low-frequency Crystal Oscillator is Selected

SUT1:0

Additional Delay from Reset (V

CC = 5.0V)

Recommended Usage

0 ms

Fast rising power or BOD enabled

4.1 ms

Fast rising power or BOD enabled

65 ms

Slowly rising power

Reserved

Table 10. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

CKSEL1:0

Internal Capacitors

Enabled?

Start-up Time from

Power-down and

Power-save

Recommended Usage

00(1)

1K CK

32K CK

Stable Frequency at start-up

10(1)

Yes

1K CK

Yes

32K CK

Stable Frequency at start-up

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