AT89LP51RB2 datasheet(44/254 Pages) ATMEL

44 / 254 page

3722A–MICRO–10/11

AT89LP51RB2/RC2/IC2 Preliminary

Figure 6-5.

Crystal Oscillator B Connections

Note:

C1, C2 = 10–20 pF for Crystals

= 10–20 pF for Ceramic Resonators

6.5

External Clock Source B (AT89LP51IC2)

The external clock option of AT89LP51IC2 disables the oscillator amplifier B and allows XTAL1B

to be driven directly by an external clock source as shown in Figure 6-6. XTAL2B may be left

unconnected or used as general purpose I/O P4.2.

Figure 6-6.

External Clock A Drive Configuration

6.6

Dual Oscillator Support (AT89LP51IC2)

The AT89LP51IC2 has the ability to switch between two different selectable system clock

sources under software control as shown in Figure 6-2 on page 41.

•OSCA can be a high frequency crystal, external clock or the internal 8 MHz oscillator

•OSCB can be a low frequency crystal, external clock or the internal 8 MHz oscillator

Several operating modes are available and programmable by software:

• Switch system clock source from OSCA to OSCB and vice-versa

•Power down OSCA or OSCB to reduce consumption

•Boot from a fast responding, less accurate oscillator and switch later to a more accurate but

slow to stabilize oscillator.

Selection of which oscillator drives the system clock is controlled by the CKS bit in CKSEL. In

order to switch to a different oscillator, that oscillator must be enabled with the OscAEn or Osc-

~10 pF

XTAL2B

XTAL1B

XTAL2B (P4.2)

XTAL1B (P1.0)

GND

NC GPIO

EXTERNAL

OSCILLATOR

SIGNAL

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