ATtiny87-15XZ datasheet(32/282 Pages) ATMEL

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7728D–AVR–07/09

ATtiny87/ATtiny167

–‘Recover System Clock Source’,

–‘Enable Watchdog in Automatic Reload Mode’.

• Command status return. The ‘Request Clock Availability ’ command returns status via the

CLKRDY bit in the CLKCSR register. The ‘Recover System Clock Source ’ command

returns a code of the current clock source in the CLKSELR register. This information is

used in the supervisory software routines as shown in Section 4.3.7 on page 33.

4.3.2

CLKSELR Register

4.3.2.1

Fuses Substitution

At reset, bits of the Low Fuse Byte are copied into the CLKSELR register. The content of this

Byte. CKSEL3..0, SUT1..0 and CKOUT fuses correspond respectively to CSEL3..0, CSUT1:0

and ~(COUT) bits of the CLKSELR register as shown in Figure 4-5 on page 32.

4.3.2.2

Source Selection

The available codes of clock source are given in Table 4-1 on page 25.

Figure 4-5.

Fuses substitution and Clock Source Selection

The CLKSELR register contains the CSEL, CSUT and COUT values which will be used by the

‘Enable/Disable Clock Source’, ‘Request for Clock Availability’ or ‘Clock Source Switching’

commands.

4.3.2.3

Source Recovering

The ‘Recover System Clock Source’ command updates the CKSEL field of CLKSELR register

(See “System Clock Source Recovering” on page 33.).

4.3.3

Enable/Disable Clock Source

The ‘Enable Clock Source’ command selects and enables a clock source configured by the

settings in the CLKSELR register. CSEL3..0 will select the clock source and CSUT1:0 will

select the start-up time (just as CKSEL and SUT fuse bits do). To be sure that a clock source

is operating, the ‘Request for Clock Availability ’ command must be executed after the ‘Enable

CLKSELR

Fuse:

Fuse Low Byte

SEL-1

SEL-0

SEL-2

SEL-n

CKSEL[3..0]

SUT[1..0]

EN-1

EN-0

EN-2

EN-n

CKOUT

Reset

SCLKRq

(*)

SCLKRq

(*): Command of Clock Control & Status Register

Selected

Configuration

Clock

Switch

Current

Configuration

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