ATMEGA649 datasheet(26/375 Pages) ATMEL

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ATmega329/3290/649/6490

2552H–AVR–11/06

ADC Clock – clk

ADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and

I/O clocks in order to reduce noise generated by digital circuitry. This gives more accu-

rate ADC conversion results.

Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as

shown below. The clock from the selected source is input to the AVR clock generator,

and routed to the appropriate modules.

Note:

1. For all fuses “1” means unprogrammed while “0” means programmed.

The various choices for each clocking option is given in the following sections. When the

CPU wakes up from Power-down or Power-save, the selected clock source is used to

time the start-up, ensuring stable Oscillator operation before instruction execution starts.

When the CPU starts from reset, there is an additional delay allowing the power to reach

a stable level before commencing normal operation. The Watchdog Oscillator is used

for timing this real-time part of the start-up time. The number of WDT Oscillator cycles

used for each time-out is shown in Table 4. The frequency of the Watchdog Oscillator is

voltage dependent as shown in “ATmega329/3290/649/6490 Typical Characteristics –

Preliminary Data” on page 320.

Default Clock Source

The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed.

The default clock source setting is the Internal RC Oscillator with longest start-up time

and an initial system clock prescaling of 8. This default setting ensures that all users can

make their desired clock source setting using an In-System or Parallel programmer.

Crystal Oscillator

XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can

be configured for use as an On-chip Oscillator, as shown in Figure 13. Either a quartz

crystal or a ceramic resonator may be used.

C1 and C2 should always be equal for both crystals and resonators. The optimal value

of the capacitors depends on the crystal or resonator in use, the amount of stray capac-

itance, and the electromagnetic noise of the environment. Some initial guidelines for

choosing capacitors for use with crystals are given in Table 5. For ceramic resonators,

the capacitor values given by the manufacturer should be used.

Table 3. Device Clocking Options Select(1)

Device Clocking Option

CKSEL3..0

External Crystal/Ceramic Resonator

1111 - 1000

External Low-frequency Crystal

0111 - 0110

Calibrated Internal RC Oscillator

0010

External Clock

0000

Reserved

0011, 0001, 0101, 0100

Table 4. Number of Watchdog Oscillator Cycles

Typ Time-out (V

CC = 5.0V)

Typ Time-out (V

CC = 3.0V)

Number of Cycles

4.1 ms

4.3 ms

4K (4,096)

65 ms

69 ms

64K (65,536)

ATMEGA649 Datasheet(PDF) 26 Page - ATMEL Corporation

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