30
7734Q–AVR–02/12
AT90PWM81/161
The switch between 8MHz and 1MHz is done by the CKRC81 bit in MCUCR register. See
accessed by two CKSEL or CSEL configurations. At reset, the CKRC81 bit is initialised with the
value compatible with CKSEL value (1 for CKSEL3..0 = 0110, 0 for all other values).
The RC oscillator is active for any CKSEL3..0 or CSEL3..0 configuration where it is used as sys-
tem clock or PLL source clock. The RC oscillator is diabled in the following CKSEL3..0 or
CSEL3..0 cases:
0011 (128k oscillator)
0100, 0101 (PLL/4 system clock driven by external clock or oscillator)
1100, 1101 (External oscillator)
page 39 for more details. This clock may be selected as the system clock by programming the
external components. During reset, hardware loads the calibration byte into the OSCCAL Regis-
ter and thereby automatically calibrates the RC Oscillator. The accuracy of this calibration is
page 39, it is possible to get a higher calibration accuracy than by using the factory calibration.
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 cali-
Notes:
1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8.
4. Switch between 8MHz and 1MHz is done by CKRC81 bit in MCUCR register.
When this oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Note:
1. If the RSTDISBL fuse is programmed, this start-up time will be increased to
14CK + 4.1ms to ensure programming mode can be entered.
2. The device is shipped with this option selected.
Table 5-3.
Internal calibrated RC oscillator operating modes
Frequency range (2) (MHz) CKSEL3..0
7.6 - 8.4
0010
0010
Table 5-4.
Start-up times for the internal calibrated RC Oscillator clock selection.
Power conditions
Start-up time from power-
down
Additional delay from
reset (V
CC = 5.0V)
SUT1..0
BOD enabled
6CK
00
Fast rising power
6CK
14CK + 4.1ms
01
Slowly rising power
6CK
10
Reserved
11