AT89LP51RB2 datasheet(45/254 Pages) ATMEL

45 / 254 page

3722A–MICRO–10/11

AT89LP51RB2/RC2/IC2 Preliminary

BEn bits in OSCCON. The oscillator selection at reset is controlled by the Oscillator Select user

fuse (See Section 24.2 on page 188). This fuse is also shadowed in the OSC bit of the boot-

loader Hardware Security Byte. The fuse sets the CKS, OscAEn and OscBEn bits as shown in

Table 6-3.

6.6.1

Normal Operation

Only a single oscillator source can drive the system clock at any one time. Under normal condi-

tions it is always possible to dynamically switch from OSCA to OSCB or vice-versa by changing

the CKS bit. The procedure is as follows:

Enable the desired oscillator by setting the OscAEn or OscBEn bits in OSCCON

Wait for the oscillator to stabilize. This can be a very long time when using the 32 kHz

oscillator. The application software must ensure that the delay is long enough for the

operating conditions

Change CKS to switch the system clock source. This takes at most 2 periods of each

oscillator

Disable the previous oscillator by clearing the OscAEn or OscBEn bits in OSCCON

Note that unlike AT89C51IC2, the OSCB source is affected by both X2 and the CKRL

divider. When changing the clock source, the X2 and CKRL values may need to be

updated to achieve the desired frequency

The clock system hardware will prevent the disabling of the current active oscillator and will pre-

vent switching to a disabled oscillator. However, the hardware will not prevent switching to an

oscillator before it has stabilized. The application software must ensure enough delay between

enabling an oscillator and switching to that oscillator so that the oscillator source can stabilize.

This is generally only an issue when using one of the crystal oscillators.

6.6.2

Idle Operation

Any enabled oscillator will continue to function during Idle mode. Power can be reduced by dis-

abling the alternate oscillator before entering Idle mode. Once in Idle mode, the oscillator source

cannot be changed until the mode is exited. An interrupt exit from Idle will leave the oscillator

control bits (OscAEn, OscBEn and CKS) unchanged. Any reset will exit Idle mode and place

these bits in their default states as determined by the user fuse.

6.6.3

Power-down Operation

All oscillators are stopped during Power-down mode. Once in Power-down mode, the oscillator

source cannot be changed until the mode is exited. An interrupt exit from Power-down will leave

the oscillator control bits (OscAEn, OscBEn and CKS) unchanged. Any reset will exit Power-

down mode and place these bits in their default states as determined by the user fuse.

Table 6-3.

Oscillator Reset States

Control Bit

Oscillator Select Fuse

(HSB.OSC)

00H (0)

FFH (1)

OscAEn (OSCCON.0)

OscBEn (OSSCON.1)

CKS (CKSEL.0)

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