ATEMGA128L View Datasheet(PDF) - Atmel Corporation
Part Name
Description
MFG CO.
'ATEMGA128L' PDF : 395 Pages View PDF
ATmega128
Calibrated Internal RC
Oscillator
The Calibrated Internal RC Oscillator provides a fixed 1.0, 2.0, 4.0, or 8.0 MHz clock. All
frequencies are nominal values at 5V and 25°C. This clock may be selected as the sys-
tem clock by programming the CKSEL fuses as shown in Table 13. If selected, it will
operate with no external components. The CKOPT fuse should always be unpro-
grammed when using this clock option. During Reset, hardware loads the calibration
byte for the 1MHz oscillator into the OSCCAL Register and thereby automatically cali-
brates the RC Oscillator. At 5V, 25°C and 1.0 MHz Oscillator frequency selected, this
calibration gives a frequency within ± 3% of the nominal frequency. Using calibration
methods as described in application notes available at www.atmel.com/avr it is possible
to achieve ± 1% accuracy at any given VCC and Temperature. 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 292.
Table 13. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0
0001(1)
Nominal Frequency (MHz)
1.0
0010
2.0
0011
4.0
0100
8.0
Note: 1. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT fuses as
shown in Table 14. XTAL1 and XTAL2 should be left unconnected (NC).
Table 14. Start-up Times for the Internal Calibrated RC Oscillator 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
10(1)
6 CK
6 CK
4.1 ms
65 ms
Fast rising power
Slowly rising power
11
Reserved
Notes: 1. The device is shipped with this option selected.
Oscillator Calibration Register
– OSCCAL
Bit
Read/Write
Initial Value
7
CAL7
R/W
6
CAL6
R/W
5
4
3
2
CAL5
CAL4
CAL3
CAL2
R/W
R/W
R/W
R/W
Device Specific Calibration Value
1
CAL1
R/W
0
CAL0
R/W
OSCCAL
Note: OSCCAL Register is not available in ATmega103 compatibility mode.
• Bits 7..0 – CAL7..0: Oscillator Calibration Value
Writing the calibration byte to this address will trim the Internal Oscillator to remove pro-
cess variations from the Oscillator frequency. During Reset, the 1 MHz calibration value
which is located in the signature row high byte (address 0x00) is automatically loaded
into the OSCCAL Register. If the internal RC is used at other frequencies, the calibration
values must be loaded manually. This can be done by first reading the signature row by
a programmer, and then store the calibration values in the Flash or EEPROM. Then the
value can be read by software and loaded into the OSCCAL Register. When OSCCAL is
41
2467O–AVR–10/06
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