ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 41

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

External Clock
2490G–AVR–03/04
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.
Table 15. Internal RC Oscillator Frequency Range
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.
Table 16. Start-up Times for the External Clock Selection
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
SUT1..0
OSCCAL Value
00
01
10
11
0x00
0x7F
0xFF
Start-up Time from Power-
down and Power-save
Min Frequency in Percentage of
6 CK
6 CK
6 CK
Nominal Frequency (%)
EXTERNAL
SIGNAL
CLOCK
100
50
75
Additional Delay from
Reset (V
Reserved
4.1 ms
65 ms
CC
–
= 5.0V)
Max Frequency in Percentage of
Nominal Frequency (%)
ATmega64(L)
Recommended Usage
BOD enabled
Fast rising power
Slowly rising power
100
150
200
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