Datasheet

30
7734Q–AVR–02/12
AT90PWM81/161
The switch between 8MHz and 1MHz is done by the CKRC81 bit in MCUCR register. See
“MCUCR - MCU Control Register” on page 42 for more details.The RC oscillator can be
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)
The device is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on
page 39 for more details. This clock may be selected as the system clock by programming the
CKSEL Fuses or CSEL field as shown in Table 5-1 on page 28. If selected, it will operate with no
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
shown as Factory calibration in Table 22-1 on page 270.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on
page 39, it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as User calibration in Table 22-1 on page 270.
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-
bration value, see the section “Calibration Byte” on page 252.
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 V
CC
), 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
Table 5-4 on page 30.
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
(1)(3)
.
Frequency range
(2)
(MHz) CKSEL3..0
7.6 - 8.4 0010
0.95 - 1.05
(4)
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 14CK
(1)
00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms
(2)
10
Reserved 11