Rainbow Electronics ATmega128L - Manual (Page 38)

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ATmega128(L)
2467B–09/01
Calibrated Internal RC
Oscillator The calibrated internal RC oscillator provides a fixed 1.0 MHz, 2.0 MHz, 4.0 MHz, or 8.0
MHz clock. All frequencies are nominal values at 5V and 25
°C. This clock may be
selected as the system 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 unprogrammed when using this clock option. During reset, hardware loads the cali-
bration byte into the OSCCAL register and thereby automatically calibrates the RC
oscillator. At 5V, 25
°C and 1.0 MHz oscillator frequency selected, this calibration gives a
frequency within ± 1% of the nominal frequency. 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 282.
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).
Note: 1. The device is shipped with this option selected.
Oscillator Calibration Register
– 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. This is done automatically during chip
reset. 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
$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 cali-
brate 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 MHz, 2.0 MHz,
4.0 MHz, or 8.0 MHz. Tuning to other values is not guaranteed, as indicated in
Table 15.
Table 13. Internal Calibrated RC Oscillator Operating Modes
CKSEL3..0 Nominal Frequency (MHz)
0001 (1) 1.0
0010 2.0
0011 4.0
0100 8.0
Table 14. Start-up Times for the Internal Calibrated RC Oscillator Clock Selection
SUT1..0
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) Recommended Usage
00 6 CK - BOD enabled
01 6 CK 4 ms Fast rising power
10 (1) 6 CK 64 ms Slowly rising power
11 Reserved
Bit 7 6 5 4 3 2 1 0
CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial value Device specific calibration value