AT89LP51RB2 datasheet(42/254 Pages) ATMEL

42 / 254 page

3722A–MICRO–10/11

AT89LP51RB2/RC2/IC2 Preliminary

6.1

Crystal Oscillator A

When enabled, internal inverting oscillator amplifier A is connected between XTAL1A and

XTAL2A for connection to an external quartz crystal or ceramic resonator. The oscillator may

operate in either high-speed or low-power mode. Low-power mode is intended for crystals of 12

MHz or less and consumes less power than the higher speed mode. The configuration as shown

in Figure 6-3 applies for both high and low power oscillators. Note that in some cases, external

capacitors C1 and C2 may be reduced due to the on-chip capacitance of the XTAL1A and

XTAL2A inputs (approximately 10 pF each). When using the crystal oscillator, P4.6 and P4.7 will

have their inputs and outputs disabled. Also, XTAL2A in crystal oscillator mode should not be

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

An optional 5 M

Ω on-chip resistor can be connected between XTAL1A and GND. This resistor

can improve the startup characteristics of the oscillator especially at higher frequencies. The

resistor can be enabled/disabled with the R1 User Fuse (See “User Configuration Fuses” on

page 188.)

Figure 6-3.

Crystal Oscillator A Connections

Note:

C1, C2 = 5–15 pF for Crystals

= 5–15 pF for Ceramic Resonators

Table 6-1.

Clock Source A Settings

Clock Source A

Fuse 1

Clock Source A

Fuse 0

Selected Clock Source

High Speed Crystal Oscillator A (f > 12 MHz)

Low Power Crystal Oscillator A (f

≤12 MHz)

External Clock on XTAL1A

Internal 8.0 MHz RC Oscillator

Table 6-2.

Clock Source B Settings (AT89LP51IC2 Only)

Clock Source B

Fuse 1

Clock Source B

Fuse 0

Selected Clock Source

–

Low Frequency Crystal Oscillator B (32 kHz)

External Clock on XTAL1B

Internal 8.0 MHz RC Oscillator

~10 pF

~5 M

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