ATEMGA128L View Datasheet(PDF) - Atmel Corporation
Part Name
Description
MFG CO.
'ATEMGA128L' PDF : 395 Pages View PDF
External Clock
zero, the lowest available frequency is chosen. Writing non-zero values to this register
will increase the frequency of the Internal Oscillator. Writing $FF 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.
Min Frequency in Percentage of
OSCCAL Value
Nominal Frequency (%)
$00
50
$7F
75
$FF
100
Max Frequency in Percentage of
Nominal Frequency (%)
100
150
200
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
EXTERNAL
CLOCK
SIGNAL
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
SUT1..0
00
Start-up Time from Power-
down and Power-save
6 CK
Additional Delay from
Reset (VCC = 5.0V)
–
Recommended Usage
BOD enabled
01
6 CK
4.1 ms
Fast rising power
10
6 CK
65 ms
Slowly rising power
11
Reserved
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
required to ensure that the MCU is kept in Reset during such changes in the clock
frequency.
42 ATmega128
2467O–AVR–10/06
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