ATMEGA644PV ATMEL [ATMEL Corporation], ATMEGA644PV Datasheet - Page 36

• Manufacturer Part Number: ATMEGA644PV
• Description: 8-bit Microcontroller with 16/32/64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

7.6
36
Calibrated Internal RC Oscillator
ATmega164P/324P/644P
By default, the Internal RC Oscillator provides an approximate 8 MHz clock. Though voltage and
temperature dependent, this clock can be very accurately calibrated by the the user. See
26-3 on page 328
The device is shipped with the CKDIV8 Fuse programmed. See
page 38
This clock may be selected as the system clock by programming the CKSEL Fuses as shown in
Table
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
Table 26-3 on page
By changing the OSCCAL register from SW, see
page
The accuracy of this calibration is shown as User calibration in
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
Table 7-10.
Notes:
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 7-11 on page
Table 7-11.
Note:
Power Conditions
BOD enabled
Fast rising power
Slowly rising power
40, it is possible to get a higher calibration accuracy than by using the factory calibration.
7-10. If selected, it will operate with no external components. During reset, hardware loads
1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8 MHz frequency exceeds the specification of the device (depends on V
1.
for more details.
Fuse can be programmed in order to divide the internal frequency by 8.
The device is shipped with this option selected.
Internal Calibrated RC Oscillator Operating Modes
Start-up times for the Internal Calibrated RC Oscillator clock selection
Frequency Range
and
328.
36.
”Internal Oscillator Speed” on page 354
7.3 - 8.1
Start-up Time from Power-
”Calibration Byte” on page
down and Power-save
(2)
(MHz)
Reserved
6 CK
6 CK
6 CK
”OSCCAL – Oscillator Calibration Register” on
294.
Additional Delay from
Reset (V
Table 26-3 on page
14CK + 65 ms
and
14CK + 4.1 ms
(1)(3)
”System Clock Prescaler” on
page 378
14CK
CC
CKSEL3..0
= 5.0V)
0010
(1)
CC
for more details.
), the CKDIV8
8011D–AVR–02/07
328.
SUT1..0
00
01
10
11
Table