AT90PWM2 datasheet(33/361 Pages) ATMEL

33 / 361 page

4317J–AVR–08/10

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 26-1 on page 301.

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

page 33, 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 26-1 on page 301.

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 282.

Notes:

1. The device is shipped with this option selected.

2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), 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 33.

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 (V

CC = 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

76543210

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

Device Specific Calibration Value

AT90PWM2 Datasheet(PDF) 33 Page - ATMEL Corporation

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