Rainbow-electronics ATmega128L Uživatelský manuál stáhnout pdf (Strana 39)

ATmega128(L)

2467B–09/01

External Clock To drive the device from an external clock source, XTAL1 should be driven as shown in

Figure 20. To run the device on an external clock, the CKSEL fuses must be pro-

grammed to “0000”. By programming the CKOPT fuse, the user can enable an internal

36 pF capacitor between XTAL1 and GND.

Figure 20. External Clock Drive Configuration

When this clock source is selected, start-up times are determined by the SUT fuses as

shown in

Table 16.

Timer/Counter Oscillator For AVR microcontrollers with Timer/Counter Oscillator pins (TOSC1 and TOSC2), the

crystal is connected directly between the pins. No external capacitors are needed. The

oscillator is optimized for use with a 32.768 kHz watch crystal. Applying an external

clock source to TOSC1 is not recommended.

XTAL Divide Control Register

– XDIV

The XTAL Divide Control Register is used to divide the Source clock frequency by a

number in the range 2 - 129. This feature can be used to decrease power consumption

when the requirement for processing power is low.

Table 15. Internal RC Oscillator Frequency Range.

OSCCAL Value

Min Frequency in Percentage of

Nominal Frequency (%)

Max Frequency in Percentage of

Nominal Frequency (%)

$00 50 100

$7F 75 150

$FF 100 200

Table 16. Start-up Times for the External Clock Selection

SUT1..0

Start-up Time from Power-

down and Power-save

Additional Delay from

Reset (V

= 5.0V) Recommended Usage

00 6 CK – BOD enabled

01 6 CK 4 ms Fast rising power

10 6 CK 64 ms Slowly rising power

11 Reserved

EXTERNAL

CLOCK

SIGNAL

Bit 76543210

XDIVEN XDIV6 XDIV5 XDIV4 XDIV3 XDIV2 XDIV1 XDIV0 XDIV

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

Initial value00000000

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 330 331

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