Rainbow Electronics ATmega162V - Manual (Page 36)

36
ATmega162/V
2513E–AVR–09/03
Notes: 1. These options should only be used when not operating close to the maximum fre-
quency 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 fre-
quency stability at start-up. They can also be used with crystals when not operating
close to the maximum frequency 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-fre-
quency 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 internal 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 V
CC
), the CKDIV8 Fuse must be programmed in order to divide the
internal frequency by 8 during start-up. See “System Clock Prescaler” on page 39 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 compo-
nents. During Reset, hardware loads the calibration byte into the OSCCAL Register and
thereby automatically calibrates the RC Oscillator. At 3V and 25
°
C, this calibration gives
a frequency within ± 1% of the nominal frequency. When this Oscillator is used as the
chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the
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
00 0 ms 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
Internal Capacitors
Enabled?
Start-up Time from
Power-down and
Power-save Recommended Usage
00 (1) No 1K CK
01 No 32K CK Stable Frequency at start-up
10 (1) Yes 1K CK
11 Yes 32K CK Stable Frequency at start-up