P89LPC904FD datasheet(28/41 Pages) PHILIPS

28 / 41 page

Philips Semiconductors

P89LPC904

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

Preliminary data

Rev. 02 — 25 June 2004

28 of 41

9397 750 13521

8.20 Analog comparators

Two analog comparators are provided on the P89LPC904. Comparator operation is

such that the output is a logic 1 (which may be read in a register) when the positive

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

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

an interrupt when the output value changes.

The connections to the comparator are shown in Figure 8. The comparator functions

to VDD = 2.4 V.

When the comparator is ﬁrst enabled, the comparator’s interrupt ﬂag is not

guaranteed to be stable for 10 microseconds. The 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.

8.21 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 8.

Comparator input and output connections.

Comparator 1

CN1

(P0.4) CIN1A

(P0.5) CMPREF

VREF

Change Detect

CO1

CMF1

Interrupt

Change Detect

CMF2

Comparator 2

CO2

CN2

(P0.2) CIN2A

002aaa785

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