Datasheet

45
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
10.7 Calibrated Internal RC Oscillator
By default, the Internal RC Oscillator provides an approximate 8MHz clock. Though voltage and temperature
dependent, this clock can be very accurately calibrated by the user. See Table 31-1 on page 359 and “Internal
Oscillator Speed” on page 392 for more details. The device is shipped with the CKDIV8 Fuse programmed. See
“System Clock Prescaler” on page 47 for more details.
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in Table 10-9. 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 calibrates the RC Oscillator. The accuracy of this cali-
bration is shown as Factory calibration in Table 31-1 on page 359.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Register” on page 48, it is pos-
sible to get a higher calibration accuracy than by using the factory calibration. The accuracy of this calibration is
shown as User calibration in Table 31-1 on page 359.
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 calibration value, see the section “Calibration
Byte” on page 328.
Notes: 1. The device is shipped with this option selected.
2. If 8MHz frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8 Fuse can be pro-
grammed 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 Table 10-10.
Note: 1. The device is shipped with this option selected.
10.8 128kHz Internal Oscillator
The 128kHz internal Oscillator is a low power Oscillator providing a clock of 128kHz. The frequency is nominal at
3V and 25C. This clock may be select as the system clock by programming the CKSEL Fuses to “11” as shown in
Table 10-11.
Note: 1. Note that the 128kHz oscillator is a very low power clock source, and is not designed for high accuracy.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in Table 10-12 on
page 46.
Table 10-9. Internal Calibrated RC Oscillator Operating Modes
(1)(2)
Frequency Range [MHz] CKSEL3:0
7.3 - 8.1 0010
Table 10-10. 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 6CK 14CK 00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms
(1)
10
Reserved 11
Table 10-11. 128kHz Internal Oscillator Operating Modes
(1)
Nominal Frequency CKSEL3:0
128kHz 0011