DOC7707 datasheet(32/307 Pages) ETC1

32 / 307 page

7707F–AVR–11/10

AT90USB82/162

Notes:

1. These options should only be used when not operating close to the maximum frequency of the

device, and only if frequency stability at start-up is not important for the application. These

options are not suitable for crystals.

2. These options are intended for use with ceramic resonators and will ensure frequency stability

at start-up. They can also be used with crystals when not operating close to the maximum fre-

quency of the device, and if frequency stability at start-up is not important for the application.

Note:

The device is shipped with this option selected.

6.5

Calibrated Internal RC Oscillator

The calibrated internal RC Oscillator by default provides a 8.0 MHz clock. The fre-

quency is nominal value at 3.0V and 25°C. At 3.0V and 25°C, this calibration gives a

frequency of 8 MHz ± 10%. The device is shipped with the CKDIV8 Fuse programmed.

See “System Clock Prescaler” on page 34 for more details. This clock may be selected

as the system clock by programming the CKSEL Fuses as shown in Table 6-6. 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 3.3V 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

section “Calibration Byte” on page 246

Notes:

1. The device is shipped with this option selected.

2. The frequency ranges are preliminary values. Actual values are TBD.

Crystal Oscillator, BOD

enabled

16K CK

14CK

Crystal Oscillator, fast

rising power

16K CK

14CK + 4.1 ms

Crystal Oscillator, slowly

rising power

16K CK

14CK + 65 ms

Table 6-5.

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

Table 6-4.

Start-up Times for the Low Power Crystal Oscillator Clock Selection (Continued)

Oscillator Source /

Power Conditions

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

CC = 5.0V)

CLKSEL

0SUT1..0

Table 6-6.

Internal Calibrated RC Oscillator Operating Modes(1)(3)

Frequency Range(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

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