P89LPC921 datasheet(15/45 Pages) PHILIPS

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Philips Semiconductors

P89LPC920/921/922

8-bit microcontrollers with two-clock 80C51 core

Product data

Rev. 06 — 21 November 2003

15 of 45

9397 750 12285

8.2.6

Clock output

The P89LPC920/921/922 supports a user-selectable clock output function on the

XTAL2/CLKOUT pin when crystal oscillator is not being used. This condition occurs if

another clock source has been selected (on-chip RC oscillator, watchdog oscillator,

external clock input on X1) and if the Real-Time clock is not using the crystal

oscillator as its clock source. This allows external devices to synchronize to the

P89LPC920/921/922. This output is enabled by the ENCLK bit in the TRIM register.

The frequency of this clock output is 1

⁄

2 that of the CCLK. If the clock output is not

needed in Idle mode, it may be turned off prior to entering Idle, saving additional

power.

8.3 On-chip RC oscillator option

The P89LPC920/921/922 has a 6-bit TRIM register that can be used to tune the

frequency of the RC oscillator. During reset, the TRIM value is initialized to a factory

pre-programmed value to adjust the oscillator frequency to 7.373 MHz,

±1% at room

temperature. End-user applications can write to the Trim register to adjust the on-chip

RC oscillator to other frequencies.

8.4 Watchdog oscillator option

The watchdog has a separate oscillator which has a frequency of 400 kHz. This

oscillator can be used to save power when a high clock frequency is not needed.

8.5 External clock input option

In this conﬁguration, the processor clock is derived from an external source driving

the XTAL1/P3.1 pin. The rate may be from 0 Hz up to 12 MHz. The XTAL2/P3.0 pin

may be used as a standard port pin or a clock output.

Fig 5.

Block diagram of oscillator control.

÷2

002aaa424

RTC

CPU

WDT

BAUD RATE

GENERATOR

DIVM

CCLK

UART

OSCCLK

I2C

PCLK

TIMER 0 and

TIMER 1

High freq.

Med. freq.

Low freq.

XTAL1

XTAL2

OSCILLATOR

WATCHDOG

OSCILLATOR

(7.3728 MHz)

(400 kHz)

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