AT89LP6440 datasheet(30/194 Pages) ATMEL

30 / 194 page

3706A–MICRO–9/09

AT89LP6440 - Preliminary

System Clock

The system clock is generated directly from one of three selectable clock sources. The three

sources are the on-chip crystal oscillator, external clock source, and internal RC oscillator. The

on-chip crystal oscillator may also be configured for low or high speed operation. The clock

source is selected by the Clock Source User Fuses as shown in Table 6-1. See “User Configura-

tion Fuses” on page 164. By default, no internal clock division is used to generate the CPU clock

from the system clock. However, the system clock divider may be used to prescale the system

clock. The choice of clock source also affects the start-up time after a POR, BOD or Power-

down event (See “Reset” on page 32 or “Power-down Mode” on page 36)

6.1

Crystal Oscillator

When enabled, the internal inverting oscillator amplifier is connected between XTAL1 and

XTAL2 for connection to an external quartz crystal or ceramic resonator. The oscillator may

operate in either high-speed or low-speed mode. Low-speed mode is intended for 32.768 kHz

watch crystals and consumes less power than high-speed mode. The configuration as shown in

Figure 6-1 applies for both high and low speed oscillators. Note that the internal structure of the

device adds about 10 pF of capacitance to both XTAL1 and XTAL2, so that in some cases an

external capacitor may NOT be required. It is recommended that a resistor R1 be connected to

XTAL1, instead of load capacitor C1, for improved startup performance. The total capacitance

on XTAL1 or XTAL2, including the external load capacitor plus internal device load, board trace

and crystal loadings, should not exceed 20 pF. When using the crystal oscillator, P4.0 and P4.1

will have their inputs and outputs disabled. Also, XTAL2 in crystal oscillator mode should not be

used to directly drive a board-level clock without a buffer.

Figure 6-1.

Crystal Oscillator Connections

Note:

= 0–10 pF for Crystals

= 0–10 pF for Ceramic Resonators

= 4–5 M

Table 6-1.

Clock Source Settings

Clock Source

Fuse 1

Clock Source

Fuse 0

Selected Clock Source

High Speed Crystal Oscillator (f > 500 kHz)

Low Speed Crystal Oscillator (f

≤100 kHz)

External Clock on XTAL1

Internal 8 MHz RC Oscillator

~10 pF

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