P89LPC921 datasheet(20/45 Pages) PHILIPS

20 / 45 page

Philips Semiconductors

P89LPC920/921/922

8-bit microcontrollers with two-clock 80C51 core

Product data

Rev. 06 — 21 November 2003

20 of 45

9397 750 12285

8.12.7

Additional port features

After power-up, all pins are in Input-Only mode. Please note that this is different

from the LPC76x series of devices.

• After power-up, all I/O pins except P1.5, may be conﬁgured by software.

• Pin P1.5 is input only. Pins P1.2 and P1.3 and are conﬁgurable for either input-only

or open-drain.

Every output on the P89LPC920/921/922 has been designed to sink typical LED

drive current. However, there is a maximum total output current for all ports which

must not be exceeded. Please refer to Table 8 “DC electrical characteristics” for

detailed speciﬁcations.

All ports pins that can function as an output have slew rate controlled outputs to limit

noise generated by quickly switching output signals. The slew rate is factory-set to

approximately 10 ns rise and fall times.

8.13 Power monitoring functions

The P89LPC920/921/922 incorporates power monitoring functions designed to

prevent incorrect operation during initial power-up and power loss or reduction during

operation. This is accomplished with two hardware functions: Power-on Detect and

Brownout detect.

8.13.1

Brownout detection

The Brownout detect function determines if the power supply voltage drops below a

certain level. The default operation is for a Brownout detection to cause a processor

reset, however it may alternatively be conﬁgured to generate an interrupt.

Brownout detection may be enabled or disabled in software.

If Brownout detection is enabled, the operating voltage range for VDD is 2.7 V to 3.6 V,

and the brownout condition occurs when VDD falls below the brownout trip voltage,

VBO (see Table 8 “DC electrical characteristics”), and is negated when VDD rises

above VBO. If brownout detection is disabled, the operating voltage range for VDD is

2.4 V to 3.6 V. If the P89LPC920/921/922 device is to operate with a power supply

that can be below 2.7 V, BOE should be left in the unprogrammed state so that the

device can operate at 2.4 V, otherwise continuous brownout reset may prevent the

device from operating.

For correct activation of Brownout detect, the VDD rise and fall times must be

observed. Please see Table 8 “DC electrical characteristics” for speciﬁcations.

8.13.2

Power-on detection

The Power-on Detect has a function similar to the Brownout detect, but is designed to

work as power comes up initially, before the power supply voltage reaches a level

where Brownout detect can work. The POF ﬂag in the RSTSRC register is set to

indicate an initial power-up condition. The POF ﬂag will remain set until cleared by

software.

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