Datasheet

32
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
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 8.0MHz 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 35 for more details. This clock may be selected as the system clock by programming the CKSEL Fuses as
shown in the table below. 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 8MHz ±1%. The oscillator can be calibrated to any frequency in the
range 7.3 - 8.1MHz 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 section “Calibration Byte” on page 356
Notes: 1. The device is shipped with this option selected.
2. 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 the table.
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 automatically written to this register
during chip reset, giving an oscillator frequency of 8.0MHz at 25°C. The application software can write this
Reserved 1 11
Table 6-6. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection
Power Conditions
Start-up Time from
Power-down and
Power-save
Additional Delay from
Reset
(V
CC
= 5.0V) CKSEL0 SUT1..0
Table 6-7. Internal Calibrated RC Oscillator Operating Modes
Frequency Range [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 6CK 14CK 00
Fast rising power 6CK 14CK + 4.1ms 01
Slowly rising power 6CK 14CK + 65ms 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