34
4317K–AVR–03/2013
AT90PWM2/3/2B/3B
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 7-6. If selected, it will operate with no external components. During reset, hardware loads
the pre-programmed calibration value into the OSCCAL Register and thereby automatically cal-
ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in
page 34, 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. If 8 MHz 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.
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 + 4.1 ms to ensure programming mode can be entered.
2. The device is shipped with this option selected.
Table 7-8.
Oscillator Calibration Register – OSCCAL
Bits 7..0 – CAL7..0: Oscillator Calibration Value
The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to
remove process variations from the oscillator frequency. The factory-calibrated value is automat-
ically written to this register during chip reset, giving an oscillator frequency of 8.0 MHz at 25°C.
The application software can write this register to change the oscillator frequency. The oscillator
can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1% accuracy. Calibration
outside that range is not guaranteed.
Table 7-6.
Internal Calibrated RC Oscillator Operating Modes
(1)(2)Frequency Range (MHz)
CKSEL3..0
7.3 - 8.1
0010
Table 7-7.
Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC = 5.0V)
SUT1..0
BOD enabled
6 CK
00
Fast rising power
6 CK
14CK + 4.1 ms
01
Slowly rising power
6 CK
10
Reserved
11
Bit
76543210
CAL7
CAL6
CAL5
CAL4
CAL3
CAL2
CAL1
CAL0
OSCCAL
Read/Write
R/W
Initial Value
Device Specific Calibration Value