P89LPC912_913_914_5 © NXP B.V. 2007. All rights reserved.
Product data sheet Rev. 05 — 28 September 2007 45 of 66
NXP Semiconductors
P89LPC912/913/914
8-bit microcontrollers with two-clock 80C51 core
8.20 Analog comparators
Two analog comparators are provided on the P89LPC912/913/914. Input and output
options allow use of the comparators in a number of different configurations. Comparator
operation is such that the output is a logic 1 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. Each comparator may be configured to cause an interrupt when the
output value changes. Comparator 1 may be output to a port pin.
The overall connections to both comparators are shown in Figure 21. 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.
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
V
ref(bg)
, is 1.23 V ± 3%.
8.22 Comparator interrupt
Each comparator has an interrupt flag contained in its configuration register. This flag is
set whenever the comparator output changes state. The flag may be polled by software or
may be used to generate an interrupt. The two comparators use one common interrupt
vector. If both comparators enable interrupts, after entering the interrupt service routine,
the user needs to read the flags to determine which comparator caused the interrupt.
Possible comparator configurations are shown in Figure 22.
Fig 21. Comparator input and output connections
(P0.4) CIN1A
(P0.5) CMPREF
(P0.2) CIN2A
CMF1
CMF2
CN1
CN2
CO1
OE1
CMP1 (P0.6)
EC
002aaa496
V
ref(bg)
comparator 1
comparator 2
interrupt
change detect
change detect
