P89LPC921 datasheet(28/45 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

28 of 45

9397 750 12285

8.20 Analog comparators

Two analog comparators are provided on the P89LPC920/921/922. Input and output

options allow use of the comparators in a number of different conﬁgurations.

Comparator operation is such that the output is a logical one (which may be read in a

greater than the negative input (selectable from a pin or an internal reference

voltage). Otherwise the output is a zero. Each comparator may be conﬁgured to

cause an interrupt when the output value changes.

The overall connections to both comparators are shown in Figure 10. The

comparators function to VDD = 2.4 V.

When each comparator is ﬁrst enabled, the comparator output and interrupt ﬂag are

not guaranteed to be stable for 10 microseconds. The corresponding comparator

interrupt should not be enabled during that time, and the comparator interrupt ﬂag

must be cleared before the interrupt is enabled in order to prevent an immediate

interrupt service.

When a comparator is disabled the comparator’s output, COx, goes HIGH. If the

comparator output was LOW and then is disabled, the resulting transition of the

comparator output from a LOW to HIGH state will set the comparator ﬂag, CMFx.

This will cause an interrupt if the comparator interrupt is enabled. The user should

therefore disable the comparator interrupt prior to disabling the comparator.

Additionally, the user should clear the comparator ﬂag, CMFx, after disabling the

comparator.

8.20.1

Internal reference voltage

An internal reference voltage generator may supply a default reference when a single

comparator input pin is used. The value of the internal reference voltage, referred to

as VREF, is 1.23 V ±10%.

Fig 10. Comparator input and output connections.

Comparator 1

CP1

CN1

(P0.4) CIN1A

(P0.3) CIN1B

(P0.5) CMPREF

VREF

OE1

Change Detect

CO1

CMF1

Interrupt

002aaa422

CMP1 (P0.6)

Change Detect

CMF2

Comparator 2

OE2

CO2

CMP2 (P0.0)

CP2

CN2

(P0.2) CIN2A

(P0.1) CIN2B

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