41
7679H–CAN–08/08
AT90CAN32/64/128
When this Oscillator is selected, start-up times are determined by the SUT1..0 fuses as shown in
Table 5-5 and CKSEL1..0 fuses as shown in Table 5-6.
Note: 1. These options should only be used if frequency stability at start-up is not important for the
application
5.6 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
V
CC
), the CKDIV8 Fuse must be programmed in order to divide the internal frequency by 8 dur-
ing start-up. The device is shipped with the CKDIV8 Fuse programmed. See “System Clock
Prescaler” on page 44. for more details. This clock may be selected as the system clock by pro-
gramming the CKSEL Fuses as shown in Table 5-7. If selected, it will operate with no external
components. During reset, hardware loads the calibration byte into the OSCCAL Register and
thereby automatically calibrates the RC Oscillator. At 5V and 25°C, this calibration gives a fre-
quency 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 V
CC and temperature. When this Oscillator is used as the chip clock, the Watchdog Oscil-
lator 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 339.
Note: 1. The device is shipped with this option selected.
Table 5-5. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
SUT1..0 Additional Delay from Reset (V
CC
= 5.0V) Recommended Usage
00 14 CK Fast rising power or BOD enabled
01 14 CK + 4.1 ms Slowly rising power
10 14 CK + 65 ms Stable frequency at start-up
11 Reserved
Table 5-6. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL3..0
Start-up Time from
Power-down and Power-save
Recommended Usage
0100
(1)
1K CK
0101 32K CK Stable frequency at start-up
0110
(1)
1K CK
0111 32K CK Stable frequency at start-up
Table 5-7. Internal Calibrated RC Oscillator Operating Modes
(1)
CKSEL3..0 Nominal Frequency
0010 8.0 MHz
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