ATMEGA406_0607 datasheet(30/263 Pages) ATMEL

30 / 263 page

2548E–AVR–07/06

ATmega406

• Bit 0 - ACS: Asynchronous Clock Select

The ACS bit is used to selected the source of the asynchronous clock for the Coulomb Counter

ADC and Wake-up Timer. The Slow RC Oscillator is selected when this bit is cleared (zero). The

32 kHz Crystal Oscillator is selected when this bit is set (one).

The selected clock source and oscillator enable conditions are illustrated in Table 7-4.

Recommended algorithm for switching from the RC Oscillator to the Crystal Oscillator as the

asynchronous clock for the Coulomb Counter ADC and Wake-up Timer:

Enable the Crystal Oscillator by setting the XOE bit (one).

Enable the Wake-up Timer, select a two second timeout, and reset the Wake-up Timer

(”Wake-up Timer” on page 49 for details).

Wait for the Wake-up Timer time-out.

Switch to the Crystal Oscillator by setting the ACS bit (one) while keeping the XOE bit set

(one).

Optional: Wait for another Wake-up Timer time-out, to ensure the Crystal Oscillator is

operating correctly. This can be done by enabling another timer interrupt with significantly

longer time-out, and checking that the Wake-up Timer time-out occurs first.

Recommended algorithm for switching from the Crystal Oscillator to the RC Oscillator as the

asynchronous clock for the Coulomb Counter ADC and Wake-up Timer:

Switch to the RC Oscillator by clearing the ACS bit (zero) while keeping the XOE bit set

(one).

Disable the Crystal Oscillator by clearing the XOE bit (zero) while keeping the ACS bit

cleared (zero).

Table 7-4.

Asynchronous Clock Source and Oscillator Enable Conditions

Sleep Mode

32 kHz Crystal

Oscillator Enable

Slow RC

Oscillator Enable

Power-off or Power-down

Other Sleep Modes

XOE

ACS & (CADEN | WUTEN)

Active Mode

XOE

ATMEGA406_0607 Datasheet(PDF) 30 Page - ATMEL Corporation