ATMEGA325-16AJ ATMEL [ATMEL Corporation], ATMEGA325-16AJ Datasheet - Page 28

• Manufacturer Part Number: ATMEGA325-16AJ
• Description: 8-bit Microcontroller with In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Low-frequency Crystal
Oscillator
Calibrated Internal RC
Oscillator
28
ATmega325/3250/645/6450
Notes:
To use a 32.768 kHz watch crystal as the clock source for the device, the low-frequency
crystal Oscillator must be selected by setting the CKSEL Fuses to “0110” or “0111”. The
crystal should be connected as shown in Figure 13. When this Oscillator is selected,
start-up times are determined by the SUT Fuses as shown in Table 7 and CKSEL1..0 as
shown in Table 8.
Table 7. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
Table 8. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
Note:
The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The frequency
is nominal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse pro-
grammed. See “System Clock Prescaler” on page 31 for more details. This clock may
be selected as the system clock by programming the CKSEL Fuses as shown in Table
9. If selected, it will operate with no external components. During reset, hardware loads
the calibration byte into the OSCCAL Register and thereby automatically calibrates the
RC Oscillator. At 3V and 25°C, this calibration gives a frequency of 8 MHz ± 1%. The
oscillator can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1%
accuracy, by changing the OSCCAL register. 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 259.
Table 9. Internal Calibrated RC Oscillator Operating Modes
Notes:
SUT1..0
CKSEL3..0
0110
00
01
10
11
0111
(1)
1. These options should only be used when not operating close to the maximum fre-
2. These options are intended for use with ceramic resonators and will ensure fre-
1. This option should only be used if frequency stability at start-up is not important for
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
quency of the device, and only if frequency stability at start-up is not important for the
application. These options are not suitable for crystals.
quency stability at start-up. They can also be used with crystals when not operating
close to the maximum frequency of the device, and if frequency stability at start-up is
not important for the application.
the application
CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8.
Additional Delay from Reset (V
Frequency Range
Power-down and Power-save
7.3 - 8.1
14CK + 4.1 ms
14CK + 65 ms
Start-up Time from
14CK
(2)
32K CK
1K CK
(MHz)
CC
Reserved
= 5.0V)
Recommended Usage
Fast rising power or BOD enabled
Slowly rising power
Stable frequency at start-up
(1)(3)
Recommended Usage
Stable frequency at start-up
CKSEL3..0
0010
2570A–AVR–09/04
CC
), the