Rainbow Electronics ATmega64L - Manual (Page 27)

27
ATmega16(L)
2466B–09/01
Note: 1. This option should not be used when operating close to the maximum frequency of
the device.
Calibrated Internal RC
Oscillator The calibrated internal RC oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All
frequencies are nominal values at 5V and 25
°C. This clock may be selected as the sys-
tem clock by programming the CKSEL fuses as shown in Table 9. If selected, it will
operate with no external components. The CKOPT fuse should always be unpro-
grammed when using this clock option. During reset, hardware loads the calibration byte
into the OSCCAL register and thereby automatically calibrates the RC oscillator. At 5V,
25
°C and 1.0 MHz oscillator frequency selected, 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 reset time-out.
For more information on the pre-programmed calibration value, see the section “Calibra-
tion Byte” on page 255.
Note: 1. The device is shipped with this option selected.
When this oscillator is selected, start-up times are determined by the SUT fuses as
shown in Table 10. XTAL1 and XTAL2 should be left unconnected (NC).
Note: 1. The device is shipped with this option selected.
Table 8. Start-up Times for the External RC Oscillator Clock Selection
SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) Recommended Usage
00 18 CK – BOD enabled
01 18 CK 4 ms Fast rising power
10 18 CK 64 ms Slowly rising power
11 6 CK (1) 4 ms Fast rising power or BOD enabled
Table 9. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0 Nominal Frequency (MHz)
0001 (1) 1.0
0010 2.0
0011 4.0
0100 8.0
Table 10. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1..0
Start-up Time from
Power-down and
Power-save
Additional Delay
from Reset
(V
CC
= 5.0V) Recommended Usage
00 6 CK – BOD enabled
01 6 CK 4 ms Fast rising power
10 (1) 6 CK 64 ms Slowly rising power
11 Reserved