When this Oscillator is selected, start-up times are determined by the SUT1..0 fuses as
shown in Table 9 and CKSEL1..0 fuses as shown in Table 10.
Table 9. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0
00
Additional Delay from Reset (VCC = 5.0V)
Recommended Usage
14CK
Fast rising power or BOD enabled
Slowly rising power
01
14CK + 4.1 ms
14CK + 65 ms
Reserved
10
Stable frequency at start-up
11
Table 10. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
Start-up Time from
CKSEL3..0
0100(1)
0101
Power-down and Power-save
Recommended Usage
1K CK
32K CK
1K CK
Stable frequency at start-up
Stable frequency at start-up
0110(1)
0111
32K CK
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 MHz frequency exceeds the specification of the
device (depends on VCC), the CKDIV8 Fuse must be programmed in order to divide the
internal frequency by 8 during start-up. The device is shipped with the CKDIV8 Fuse
programmed. 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, hard-
ware loads the calibration byte into the OSCCAL Register and thereby automatically
calibrates the RC Oscillator. At 5V and 25°C, this calibration gives a frequency within ±
10% of the nominal frequency. Using calibration methods as described in application
notes available at www.atmel.com/avr it is possible to achieve ± 2% accuracy at any
given VCC and temperature. When this Oscillator is used as the chip clock, the Watch-
dog 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 “Calibration
Byte” on page 328.
Table 11. Internal Calibrated RC Oscillator Operating Modes(1)
CKSEL3..0
Nominal Frequency
0010
8.0 MHz
Note:
1. The device is shipped with this option selected.
38
AT90CAN128
4250E–CAN–12/04
ATMEL [ ATMEL ]
