Microchip Technology DM164134 Data Sheet

© 2006 Microchip Technology Inc.

DS41159E-page 251

A single comparator is shown in Figure 21-2 along with
the relationship between the analog input levels and
the digital output. When the analog input at V

IN

+ is less

than the analog input V

IN

-, the output of the comparator

is a digital low level. When the analog input at V

IN

+ is

greater than the analog input V

IN

-, the output of the

comparator is a digital high level. The shaded areas of
the output of the comparator in Figure 21-2 represent
the uncertainty due to input offsets and response time.

An external or internal reference signal may be used
depending on the comparator operating mode. The
analog signal present at V

IN

- is compared to the signal

at V

IN

+ and the digital output of the comparator is

adjusted accordingly (Figure 21-2).

21.3.1

EXTERNAL REFERENCE SIGNAL

When external voltage references are used, the
comparator module can be configured to have the com-
parators operate from the same or different reference
sources. However, threshold detector applications may
require the same reference. The reference signal must
be between V

SS

 and V

DD

 and can be applied to either

pin of the comparator(s).

21.3.2

INTERNAL REFERENCE SIGNAL

The comparator module also allows the selection of an
internally generated voltage reference for the compara-
tors.  Section 22.0 “Comparator Voltage Reference
Module” contains a detailed description of the module
that provides this signal. The internal reference signal is
used when comparators are in mode CM<2:0> = 110
(Figure 21-1). In this mode, the internal voltage
reference is applied to the V

IN

+ pin of both comparators.

Response time is the minimum time, after selecting a
new reference voltage or input source, before the
comparator output has a valid level. If the internal ref-
erence is changed, the maximum delay of the internal
voltage reference must be considered when using the
comparator outputs. Otherwise, the maximum delay of
the comparators should be used (Section 27.0
“Electrical Characteristics”).

The comparator outputs are read through the CMCON
register. These bits are read-only. The comparator
outputs may also be directly output to the RE1 and RE2
I/O pins. When enabled, multiplexors in the output path
of the RE1 and RE2 pins will switch and the output of
each pin will be the unsynchronized output of the
comparator. The uncertainty of each of the
comparators is related to the input offset voltage and
the response time given in the specifications.
Figure 21-3 shows the comparator output block
diagram.

The TRISE bits will still function as an output enable/
disable for the RE1 and RE2 pins while in this mode.

The polarity of the comparator outputs can be changed
using the C2INV and C1INV bits (CMCON<4:5>).         

-

+

V

IN

+

V

IN

-

Output

V

IN–

V

IN+

Output

Output

V

IN

+

V

IN

-

Note 1: When reading the Port register, all pins

configured as analog inputs will read as a
‘0’. Pins configured as digital inputs will
convert an analog input according to the
Schmitt Trigger input specification.

2: Analog levels on any pin defined as a dig-

ital input may cause the input buffer to
consume more current than is specified.