ATMEGA32(2008) Просмотр технического описания (PDF) - Atmel Corporation

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ATMEGA32
(Rev.:2008)

8-bit Microcontroller with 32K Bytes In-System Programmable Flash

Atmel Corporation 

ATmega32(L)

ADC Clock – clkADC

Clock Sources

Default Clock

Source

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 accurate ADC conversion

results.

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.

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

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 “Register Sum-

mary” on page 327.

Table 3. Number of Watchdog Oscillator Cycles

Typ Time-out (VCC = 5.0V)

4.1 ms

Typ Time-out (VCC = 3.0V)

4.3 ms

65 ms

69 ms

Number of Cycles

4K (4,096)

64K (65,536)

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

therefore the 1 MHz Internal RC Oscillator with longest startup time. This default setting ensures

that all users can make their desired clock source setting using an In-System or Parallel

Programmer.

25

2503N–AVR–06/08

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