ATMEGA162_1 Datasheet, PDF (38/324 Pages) ATMEL Corporation – 8-bit Microcontroller with 16K Bytes In-System Programmable Flash

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.
Table 9. Start-up Delay from Reset when Low-frequency Crystal Oscillator is Selected
SUT1:0
00
Additional Delay from Reset (V_CC = 5.0V)
0 ms
Recommended Usage
Fast rising power or BOD enabled
01
4.1 ms
Fast rising power or BOD enabled
10
65 ms
Slowly rising power
11
Reserved
Table 10. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL1:0
00⁽¹⁾
01
10⁽¹⁾
11
Internal Capacitors
Enabled?
No
No
Yes
Yes
Start-up Time from
Power-down and
Power-save
1K CK
32K CK
1K CK
32K CK
Recommended Usage
Stable Frequency at start-up
Stable Frequency at start-up
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
V_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
38 ATmega162/V
2513K–AVR–07/09