ATmega16U4 datasheet(32/408 Pages) ATMEL

32 / 408 page

7766D–AVR–01/09

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

⋅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 351

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.

Note:

1. The device is shipped with this option selected.

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-

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 (VCC = 5.0V)

SUT1..0

BOD enabled

6 CK

14CK

Fast rising power

6 CK

14CK + 4.1 ms

Slowly rising power

6 CK

14CK + 65 ms(1)

Reserved

Bit

7654

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

ATmega16U4 Datasheet(PDF) 32 Page - ATMEL Corporation

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