Philips Semiconductors
P89LPC924/925
8-bit microcontrollers with accelerated two-clock 80C51 core
Product data Rev. 03 — 15 December 2004 33 of 49
9397 750 14471
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
8.21 Analog comparators
Two analog comparators are provided on the P89LPC924/925. 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
register and/or routed to a pin) when the positive input (one of two selectable pins) is
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 V
DD
= 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.
8.21.1 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 V
REF
, 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
V
REF
OE1
Change Detect
CO1
CMF1
Interrupt
002aaa422
CMP1 (P0.6)
EC
Change Detect
CMF2
Comparator 2
OE2
CO2
CMP2 (P0.0)
CP2
CN2
(P0.2) CIN2A
(P0.1) CIN2B
