Datasheet

32
7766E–AVR–04/10
ATmega16U4/ATmega32U4
Note: 1. These options should only be used if frequency stability at start-up is not important for the
application.
6.5 Calibrated Internal RC Oscillator
The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. This frequency is
nominal value at 3V and 25°C. The device is shipped with the CKDIV8 Fuse programmed. See
“System Clock Prescaler” on page 37 for more details. This clock may be selected as the system
clock by programming the CKSEL Fuses as shown in Table 6-7. If selected, it will operate with
no external components. During reset, hardware loads the calibration byte into the OSCCAL
Register and thereby automatically calibrates the RC Oscillator. At 3V and 25°C, this calibration
gives a frequency of 8 MHz ± 1%. The oscillator can be calibrated to any frequency in the range
7.3 - 8.1 MHz within ±1% accuracy, by changing the OSCCAL register. 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 sec-
tion “Calibration Byte” on page 349
Notes: 1. The device is shipped with this option selected.
2. The frequency ranges are preliminary values. Actual values are TBD.
3. If 8 MHz frequency exceeds the specification of the device (depends on V
CC
), the CKDIV8
Fuse can be programmed in order to divide the internal frequency by 8.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 6-5 on page 31.
6.5.1 Oscillator Calibration Register – OSCCAL
• Bits 7..0 – CAL7..0: Oscillator Calibration Value
The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to
remove process variations from the oscillator frequency. The factory-calibrated value is automat-
ically written to this register during chip reset, giving an oscillator frequency of 8.0 MHz at 25°C.
The application software can write this register to change the oscillator frequency. The calibra-
tion range is +/- 40% and linear (calibration step ~0.4%). With typical process at 25°C the code
should be 127 for 8 MHz. Input value of 0x00 gives the lowest frequency, and 0xFF the highest.
Table 6-7. Internal Calibrated RC Oscillator Operating Modes
(1)(3)
Frequency Range
(2)
(MHz) CKSEL[3:0]
7.3 - 8.1 0010
Table 6-8. Start-up times for the internal calibrated RC Oscillator clock selection
Power Conditions
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V) SUT1..0
BOD enabled 6 CK 14CK 00
Fast rising power 6 CK 14CK + 4.1 ms 01
Slowly rising power 6 CK 14CK + 65 ms 10
Reserved 11
Bit 76543210
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