ATtiny87-15XZ datasheet(34/282 Pages) ATMEL

34 / 282 page

7728D–AVR–07/09

ATtiny87/ATtiny167

It is strongly recommended to run this sequence only once the interrupts have been disabled.

The user (code) is responsible for the correct implementation of the clock switching sequence.

Here is a “light” C-code that describes such a sequence of commands.

Warning:

In the ATtiny87/167, only one among the three external clock sources can be enabled at a

given time. Moreover, the enables of the external clock and of the external low-frequency

oscillator are shared with the asynchronous timer.

4.3.8

Clock Monitoring

A safe system needs to monitor its clock sources. Two domains need to be monitored:

- Clock sources for peripherals,

- Clocks sources for system clock generation.

C Code Example

void ClockSwiching (unsigned char clk

_number, unsigned char sut) {

#define CLOCK

_RECOVER

0x05

#define CLOCK

_ENABLE

0x02

#define CLOCK

_SWITCH

0x04

#define CLOCK

_DISABLE

0x01

unsigned char previous

_clk, temp;

// Disable interrupts

temp = SREG; asm ("cli");

// Save the current system clock source

CLKCSR = 1 << CLKCCE;

CLKCSR = CLOCK

_RECOVER;

_clk = CLKSELR & 0x0F;

// Enable the new clock source

CLKSELR = ((sut << 4 ) & 0x30) | (clk

_number & 0x0F);

CLKCSR = 1 << CLKCCE;

CLKCSR = CLOCK

_ENABLE;

// Wait for clock validity

while ((CLKCSR & (1 << CLKRDY)) == 0);

// Switch clock source

CLKCSR = 1 << CLKCCE;

CLKCSR = CLOCK

_SWITCH;

// Wait for effective switching

while (1){

CLKCSR = 1 << CLKCCE;

CLKCSR = CLOCK

_RECOVER;

if ((CLKSELR & 0x0F) == (clk

_number & 0x0F)) break;

}

// Shut down unneeded clock source

if (previous_clk != (clk_number & 0x0F)) {

CLKSELR = previous

_clk;

CLKCSR = 1 << CLKCCE;

CLKCSR = CLOCK

_DISABLE;

}

// Re-enable interrupts

SREG = temp;

}

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