ATA8741-PXQW【PDF 42/238 页】MCU W/TRANSMITTER ASK/FSK 24QFN

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Sheet目录52

38页 39页 40页 41页 [42页]43页 44页 45页 46页

9140CINDCO02/10

ATA8741

14.2 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

14-2 on page 42.

14.3 Default Clock Source

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

clock source setting is therefore the Internal RC Oscillator running at 8.0 MHz with longest

start-up time and an initial system clock prescaling of 8, resulting in 1.0 MHz system clock. This

default setting ensures that all users can make their desired clock source setting using an

In-System or High-voltage Programmer.

14.4 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 14-2. 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 14-3 on page 43. For ceramic resonators, the capacitor val-

ues given by the manufacturer should be used.

Table 14-1. Device Clocking Options Select

(1)

Device Clocking Option

CKSEL3..0

External Clock

0000

Calibrated Internal RC Oscillator 8.0 MHz

0010

Watchdog Oscillator 128 kHz

0100

External Low-frequency Oscillator

0110

External Crystal/Ceramic Resonator

1000-1111

Reserved

0101, 0111, 0011,0001

Table 14-2. Number of Watchdog Oscillator Cycles

Typ Time-out

Number of Cycles

4 ms

512

64 ms

8K (8,192)

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