Rainbow-electronics ATmega128L Uživatelský manuál stáhnout pdf (Strana 37)

ATmega128(L)

2467B–09/01

Note: 1. These options should only be used if frequency stability at start-up is not important

for the application.

External RC Oscillator For timing insensitive applications, the external RC configuration shown in Figure 19

can be used. The frequency is roughly estimated by the equation f = 1/(3RC). C should

be at least 22 pF. By programming the CKOPT fuse, the user can enable an internal

36 pF capacitor between XTAL1 and GND, thereby removing the need for an external

capacitor. For more information on oscillator operation and details on how to choose R

and C, refer to the External RC Oscillator Application Note.

Figure 19. External RC Configuration

The oscillator can operate in four different modes, each optimized for a specific fre-

quency range. The operating mode is selected by the fuses CKSEL3..0 as shown in

Table 11.

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

shown in

Table 12.

Note: 1. This option should not be used when operating close to the maximum frequency of

the device.

Table 11. External RC Oscillator Operating Modes

CKSEL3..0 Frequency Range (MHz)

0101 - 0.9

0110 0.9 - 3.0

0111 3.0 - 8.0

1000 8.0 - 12.0

Table 12. Start-Up Times for the External RC Oscillator Clock Selection

SUT1..0

Start-up Time from

Power-down and

Power-save

Additional Delay

from Reset

= 5.0V) Recommended usage

00 18 CK - BOD enabled

01 18 CK 4 ms Fast rising power

10 18 CK 64 ms Slowly rising power

11 6 CK

(1)

4 ms Fast rising power or BOD enabled

XTAL2

XTAL1

GND

1 2 ... 32 33 34 35 36 37 38 39 40 41 42 ... 330 331

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Rainbow-electronics ATmega128L Uživatelský manuál Strana 37