Datasheet

43
2490R–AVR–02/2013
ATmega64(L)
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 calibration byte into the OSCCAL Register
and thereby automatically calibrates the RC Oscillator. At 5V, 25C and 1.0 MHz Oscillator fre-
quency selected, this calibration gives a frequency within ±3% of the nominal frequency. Using
run-time calibration methods as described in application notes available at www.atmel.com/avr it
is possible to achieve ±1% accuracy at any given V
CC
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 preprogrammed calibration value, see the sec-
tion “Calibration Byte” on page 293.
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.
OSCCAL – Oscillator
Calibration Register
(1)
Note: 1. The 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 process vari-
ations 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 calibra-
tion values in the Flash or EEPROM. Then the value can be read by software and loaded into
the OSCCAL Register. When OSCCAL is zero, the lowest available frequency is chosen. Writing
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 4.1 ms Fast rising power
10
(1)
6 CK 65 ms Slowly rising power
11 Reserved
Bit 76543210
(0x6F) 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