Rainbow-electronics ATmega32L Manuel d'utilisateur télécharger pdf (Page 23)

ATmega32(L)

2503C–AVR–10/02

Asynchronous Timer Clock –

clk

ASY

The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked

directly from an external 32 kHz clock crystal. The dedicated clock domain allows using

this Timer/Counter as a real-time counter even when the device is in sleep mode.

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 as 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 3. The frequency of the Watchdog

Oscillator is voltage dependent as shown in “ATmega32 Typical Characteristics – Pre-

liminary Data” on page 292.

Default Clock Source The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source

setting is therefore the Internal RC Oscillator with longest startup time. This default set-

ting ensures that all users can make their desired clock source setting using an In-

System or Parallel Programmer.

Table 2. Device Clocking Options Select

(1)

Device Clocking Option CKSEL3..0

External Crystal/Ceramic Resonator 1111 - 1010

External Low-frequency Crystal 1001

External RC Oscillator 1000 - 0101

Calibrated Internal RC Oscillator 0100 - 0001

External Clock 0000

Table 3. Number of Watchdog Oscillator Cycles

Typ Time-out (V

= 5.0V) Typ Time-out (V

= 3.0V) Number of Cycles

4.1 ms 4.3 ms 4K (4,096)

65 ms 69 ms 64K (65,536)

1 2 ... 18 19 20 21 22 23 24 25 26 27 28 ... 312 313

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Rainbow-electronics ATmega32L Manuel d'utilisateur Page 23

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