External Clock - Atmel ATmega128 Manual

External Clock

2467S–AVR–07/09
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. Internal RC Oscillator Frequency Range.
Min Frequency in Percentage of
OSCCAL Value Nominal Frequency (%)
$00
$7F
$FF
To drive the device from an external clock source, XTAL1 should be driven as shown in
21. To run the device on an external clock, the CKSEL fuses must be programmed 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
Start-up Time from Power-
SUT1..0 down and Power-save
00 6 CK
01 6 CK
10 6 CK
11
When applying an external clock, it is required to avoid sudden changes in the applied clock fre-
quency 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 required to ensure that the
MCU is kept in Reset during such changes in the clock frequency.
Table 15.
50
75
100
EXTERNAL
CLOCK
SIGNAL
Additional Delay from
Reset (V
CC
–
4.1 ms
65 ms
Reserved
ATmega128
Max Frequency in Percentage of
Nominal Frequency (%)
100
150
200
= 5.0V) Recommended Usage
BOD enabled
Fast rising power
Slowly rising power
Figure
43