ATMEGA2561V ATMEL [ATMEL Corporation], ATMEGA2561V Datasheet - Page 44

• Manufacturer Part Number: ATMEGA2561V
• Description: 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Low Frequency Crystal
Oscillator
Calibrated Internal RC
Oscillator
44
ATmega640/1280/1281/2560/2561
The device can utilize a 32.768 kHz watch crystal as clock source by a dedicated Low
Frequency Crystal Oscillator. The crystal should be connected as shown in Figure 21.
When this Oscillator is selected, start-up times are determined by the SUT Fuses and
CKSEL0 as shown in Table 16.
Table 16. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Note:
By defaylt, the Internal RC Oscillator provides an approximate 8 MHz clock. Though volt-
age and temperature dependent, this clock can be very accurately calibrated by the
user. See Table 172 on page 384 and “Internal Oscillator Speed” on page 409 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 17. 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 calibration is
shown as Factory calibration in Table 172 on page 384.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator Calibration Reg-
ister” on page 48, 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
172 on page 384.
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 345.
Table 17. Internal Calibrated RC Oscillator Operating Modes
Notes:
When this Oscillator is selected, start-up times are determined by the SUT Fuses as
shown in Table 18 on page 45.
Power Conditions
BOD enabled
Fast rising power
Slowly rising power
BOD enabled
Fast rising power
Slowly rising power
1. These options should only be used if frequency stability at start-up is not important
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
for the application.
CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8.
Frequency Range
7.3 - 8.1
Start-up Time from
Power-down and
Reserved
Reserved
Power-save
32K CK
32K CK
32K CK
(2)
1K CK
1K CK
1K CK
(MHz)
Additional Delay
14CK + 4.1 ms
14CK + 65 ms
14CK + 4.1 ms
14CK + 65 ms
(V
from Reset
CC
14CK
14CK
= 5.0V)
(1)
(1)(3)
(1)
(1)
CKSEL3:0
CKSEL0
0010
0
0
0
0
1
1
1
1
2549K–AVR–01/07
SUT1:0
CC
00
01
10
11
00
01
10
11
), the