AT90PWM3 datasheet(33/361 Pages) ATMEL

33 / 361 page

4317I–AVR–01/08

AT90PWM2/3/2B/3B

This clock may be selected as the system clock by programming the CKSEL Fuses as shown in

Table 7-6. If selected, it will operate with no external components. During reset, hardware loads

the pre-programmed calibration value into the OSCCAL Register and thereby automatically cal-

ibrates the RC Oscillator. The accuracy of this calibration is shown as Factory calibration in

Table 26-2 on page 302.

By changing the OSCCAL register from SW, see “Oscillator Calibration Register – OSCCAL” on

page 34, it is possible to get a higher calibration accuracy than by using the factory calibration.

The accuracy of this calibration is shown as User calibration in Table 26-2 on page 302.

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 cali-

bration value, see the section “Calibration Byte” on page 283.

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

Table 7-7 on page 34.

Note:

1. If the RSTDISBL fuse is programmed, this start-up time will be increased to

14CK + 4.1 ms to ensure programming mode can be entered.

2. The device is shipped with this option selected.

Table 7-8.

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 oscillator

can be calibrated to any frequency in the range 7.3 - 8.1 MHz within ±1% accuracy. Calibration

outside that range is not guaranteed.

Table 7-6.

Internal Calibrated RC Oscillator Operating Modes(1)(2)

Frequency Range (MHz)

CKSEL3..0

7.3 - 8.1

0010

Table 7-7.

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

(1)

Fast rising power

6 CK

14CK + 4.1 ms

Slowly rising power

6 CK

14CK + 65 ms

(1)

Reserved

Bit

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

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