ATMEGA640 datasheet(45/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

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 VCC), 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

(VCC = 5.0V)

SUT1:0

BOD enabled

6CK

14CK

Fast rising power

6CK

14CK + 4.1ms

Slowly rising power

6CK

14CK + 65ms(1)

Reserved

Table 10-11. 128kHz Internal Oscillator Operating Modes(1)

Nominal Frequency

CKSEL3:0

128kHz

0011

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