AT90PWM2 datasheet(33/365 Pages) ATMEL

33 / 365 page

AT90PWM2/3

4317B–AVR–02/05

Notes:

1. These options should only be used when not operating close to the maximum fre-

quency 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 fre-

quency stability at start-up. They can also be used with crystals when not operating

close to the maximum frequency of the device, and if frequency stability at start-up is

not important for the application.

Calibrated Internal RC

Oscillator

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

is nominal value at 3V and 25

°C. The device is shipped with the CKDIV8 Fuse pro-

grammed. 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

3. 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

section “Calibration Byte” on page 291.

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.

Table 6. Start-up Times for the Oscillator Clock Selection

CKSEL0

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

CC = 5.0V)

Recommended Usage

258 CK

(1)

14CK + 4.1 ms

Ceramic resonator, fast

rising power

258 CK

(1)

14CK + 65 ms

Ceramic resonator,

slowly rising power

010

1K CK

(2)

14CK

Ceramic resonator,

BOD enabled

011

1K CK

(2)

14CK + 4.1 ms

Ceramic resonator, fast

rising power

100

1K CK

(2)

14CK + 65 ms

Ceramic resonator,

slowly rising power

16K CK

14CK

Crystal Oscillator, BOD

enabled

16K CK

14CK + 4.1 ms

Crystal Oscillator, fast

rising power

16K CK

14CK + 65 ms

Crystal Oscillator,

slowly rising power

Table 7. Internal Calibrated RC Oscillator Operating Modes(1)(3)

Frequency Range

(2) (MHz)

CKSEL3..0

7.3 - 8.1

0010

AT90PWM2 Datasheet(PDF) 33 Page - ATMEL Corporation

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