ATtiny5 datasheet(21/168 Pages) ATMEL

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8127C–AVR–10/09

ATtiny4/5/9/10

6.5

6.5.1

CLKMSR – Clock Main Settings Register

• Bit 7:2 – Res: Reserved Bits

These bits are reserved and always read zero.

• Bit 1:0 – CLKMS[1:0]: Clock Main Select Bits

These bits select the main clock source of the system. The bits can be written at run-time to

switch the source of the main clock. The clock system ensures glitch free switching of the main

clock source.

The main clock alternatives are shown in Table 6-3.

To avoid unintentional switching of main clock source, a protected change sequence must be

followed to change the CLKMS bits, as follows:

1. Write the signature for change enable of protected I/O register to register CCP

2. Within four instruction cycles, write the CLKMS bits with the desired value

6.5.2

OSCCAL – Oscillator Calibration Register

• Bits 7:0

– CAL[7:0]: Oscillator Calibration Value

The oscillator calibration register is used to trim the calibrated internal oscillator and remove pro-

cess variations from the oscillator frequency. A pre-programmed calibration value is

automatically written to this register during chip reset, giving the factory calibrated frequency as

specified in Table 16-2, “Calibration Accuracy of Internal RC Oscillator,” on page 118.

The application software can write this register to change the oscillator frequency. The oscillator

can be calibrated to frequencies as specified in Table 16-2, “Calibration Accuracy of Internal RC

Oscillator,” on page 118. Calibration outside the range given is not guaranteed.

The CAL[7:0] bits are used to tune the frequency of the oscillator. A setting of 0x00 gives the

lowest frequency, and a setting of 0xFF gives the highest frequency.

Bit

765

432

0x37

–

CLKMS1

CLKMS0

CLKMSR

Read/Write

R/W

Initial Value

Table 6-3.

Selection of Main Clock

CLKM1

CLKM0

Main Clock Source

Calibrated Internal 8 MHzOscillator

Internal 128 kHz Oscillator (WDT Oscillator)

External clock

Reserved

Bit

765

432

0x39

CAL7

CAL6

CAL5

CAL4

CAL3

CAL2

CAL1

CAL0

OSCCAL

Read/Write

R/W

Initial Value

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