ATMEGA64-16AI datasheet(44/393 Pages) ATMEL

44 / 393 page

ATmega64(L)

2490L–AVR–10/06

value can be read by software and loaded into the OSCCAL Register. When OSCCAL is

zero, the lowest available frequency is chosen. Writing non-zero values to this register

will increase the frequency of the internal Oscillator. Writing 0xFF to the register gives

the highest available frequency. The calibrated Oscillator is used to time EEPROM and

Flash access. If EEPROM or Flash is written, do not calibrate to more than 10% above

the nominal frequency. Otherwise, the EEPROM or Flash write may fail. Note that the

Oscillator is intended for calibration to 1.0, 2.0, 4.0, or 8.0 MHz. Tuning to other values is

not guaranteed, as indicated in Table 15.

External Clock

To drive the device from an external clock source, XTAL1 should be driven as shown in

Figure 21. To run the device on an external clock, the CKSEL Fuses must be pro-

grammed to “0000”. By programming the CKOPT Fuse, the user can enable an internal

36 pF capacitor between XTAL1 and GND.

Figure 21. External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT Fuses as

shown in Table 16.

When applying an external clock, it is required to avoid sudden changes in the applied

clock frequency to ensure stable operation of the MCU. A variation in frequency of more

than 2% from one clock cycle to the next can lead to unpredictable behavior. It is

Table 15. Internal RC Oscillator Frequency Range

OSCCAL Value

Min Frequency in Percentage of

Nominal Frequency (%)

Max Frequency in Percentage of

Nominal Frequency (%)

0x00

100

0x7F

150

0xFF

100

200

Table 16. Start-up Times for the External Clock Selection

SUT1..0

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (VCC = 5.0V)

Recommended Usage

6 CK

–

BOD enabled

6 CK

4.1 ms

Fast rising power

6 CK

65 ms

Slowly rising power

Reserved

EXTERNAL

CLOCK

SIGNAL

ATMEGA64-16AI Datasheet(PDF) 44 Page - ATMEL Corporation

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