89LPC925 datasheet(26/49 Pages) PHILIPS

26 / 49 page

Philips Semiconductors

P89LPC924/925

8-bit microcontrollers with accelerated two-clock 80C51 core

Product data

Rev. 03 — 15 December 2004

26 of 49

9397 750 14471

8.15 Power reduction modes

The P89LPC924/925 supports three different power reduction modes. These modes

are Idle mode, Power-down mode, and total Power-down mode.

8.15.1

Idle mode

Idle mode leaves peripherals running in order to allow them to activate the processor

when an interrupt is generated. Any enabled interrupt source or reset may terminate

Idle mode.

8.15.2

Power-down mode

The Power-down mode stops the oscillator in order to minimize power consumption.

The P89LPC924/925 exits Power-down mode via any reset, or certain interrupts. In

Power-down mode, the power supply voltage may be reduced to the RAM keep-alive

voltage VRAM. This retains the RAM contents at the point where Power-down mode

was entered. SFR contents are not guaranteed after VDD has been lowered to VRAM,

therefore it is highly recommended to wake up the processor via reset in this case.

VDD must be raised to within the operating range before the Power-down mode is

exited.

Some chip functions continue to operate and draw power during Power-down mode,

increasing the total power used during Power-down. These include: Brownout detect,

Watchdog Timer, Comparators (note that Comparators can be powered-down

separately), and Real-Time Clock (RTC)/System Timer. The internal RC oscillator is

disabled unless both the RC oscillator has been selected as the system clock AND

the RTC is enabled.

8.15.3

Total Power-down mode

This is the same as Power-down mode except that the brownout detection circuitry

and the voltage comparators are also disabled to conserve additional power. The

internal RC oscillator is disabled unless both the RC oscillator has been selected as

the system clock and the RTC is enabled. If the internal RC oscillator is used to clock

the RTC during Power-down, there will be high power consumption. Please use an

external low frequency clock to achieve low power with the Real-Time Clock running

during Power-down.

8.16 Reset

The P1.5/RST pin can function as either an active-LOW reset input or as a digital

input, P1.5. The RPE (Reset Pin Enable) bit in UCFG1, when set to ‘1’, enables the

external reset input function on P1.5. When cleared, P1.5 may be used as an input

pin.

Remark: During a power-up sequence, the RPE selection is overridden and this pin

will always function as a reset input. An external circuit connected to this pin

should not hold this pin LOW during a power-on sequence as this will keep the

device in reset. After power-up this input will function either as an external reset

input or as a digital input as deﬁned by the RPE bit. Only a power-up reset will

temporarily override the selection deﬁned by RPE bit. Other sources of reset will not

override the RPE bit.

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