AT89LP428 datasheet(21/149 Pages) ATMEL

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3654A–MICRO–8/09

AT89LP428/828

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:

1. C2

= 0–10 pF for Crystals

= 0–10 pF for Ceramic Resonators

= 4–5 M

6.2

External Clock Source

The external clock option disables the oscillator amplifier and allows XTAL1 to be driven directly

by an external clock source as shown in Figure 6-2. XTAL2 may be left unconnected, used as

general-purpose I/O P4.1, or configured to output a divided version of the system clock.

Figure 6-2.

External Clock Drive Configuration

~10 pF

XTAL2 (P4.1)

XTAL1 (P4.0)

GND

NC, GPIO, or

CLKOUT

External

Oscillator

Signal

AT89LP428 数据表(PDF) 21 Page - ATMEL Corporation

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