AT90PWM2 datasheet(31/365 Pages) ATMEL

31 / 365 page

AT90PWM2/3

4317B–AVR–02/05

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 illus-

trated Table 3. 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 starting normal operation. The Watchdog Oscillator is used for tim-

ing 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 “Watchdog Oscillator Frequency vs. VCC” on page 342.

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.

Table 3. Device Clocking Options Select(1)

Device Clocking Option

CKSEL3..0

External Crystal/Ceramic Resonator

1111 - 1000

PLL output divided by 4 : 16 MHz

0001

Calibrated Internal RC Oscillator

0010

External Clock

0000

Reserved

0111- 0100, 0011

Table 4. Number of Watchdog Oscillator Cycles

Typ Time-out (VCC = 5.0V)

Typ Time-out (VCC = 3.0V)

Number of Cycles

4.1 ms

4.3 ms

4K (4,096)

65 ms

69 ms

64K (65,536)

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