ATMEGA6450-16AI(2004) データシートの表示（PDF） - Atmel Corporation

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ATMEGA6450-16AI
(Rev.:2004)

8-bit Microcontroller with In-System Programmable Flash

Atmel Corporation 

Low-frequency Crystal

Oscillator

Notes:

1. These options should only be used when not operating close to the maximum fre-

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.

2. These options are intended for use with ceramic resonators and will ensure fre-

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.

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

SUT1..0

00

Additional Delay from Reset (VCC = 5.0V) Recommended Usage

14CK

Fast rising power or BOD enabled

01

14CK + 4.1 ms

Slowly rising power

10

14CK + 65 ms

Stable frequency at start-up

11

Reserved

Table 8. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection

CKSEL3..0

0110(1)

Start-up Time from

Power-down and Power-save

1K CK

Recommended Usage

0111

32K CK

Stable frequency at start-up

Note: 1. This option should only be used if frequency stability at start-up is not important for

the application

Calibrated Internal RC

Oscillator

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(1)(3)

Frequency Range(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

Notes:

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 VCC), the

CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8.

28 ATmega325/3250/645/6450

2570A–AVR–09/04

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