PIC18FXX20
20.3.2
INTERNAL REFERENCE SIGNAL
20.2 Comparator Operation
The comparator module also allows the selection of an
internally generated voltage reference for the
comparators. Section 21.0 contains a detailed descrip-
tion of the Comparator Voltage Reference Module that
provides this signal. The internal reference signal is
used when comparators are in mode CM<2:0> = 110
(Figure 20-1). In this mode, the internal voltage
reference is applied to the VIN+ pin of both comparators.
A single comparator is shown in Figure 20-2, along with
the relationship between the analog input levels and
the digital output. When the analog input at VIN+ is less
than the analog input VIN-, the output of the comparator
is a digital low level. When the analog input at VIN+ is
greater than the analog input VIN-, the output of the
comparator is a digital high level. The shaded areas of
the output of the comparator in Figure 20-2 represent
the uncertainty, due to input offsets and response time.
20.4 Comparator Response Time
20.3 Comparator Reference
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 26.0).
An external or internal reference signal may be used,
depending on the comparator operating mode. The
analog signal present at VIN- is compared to the signal
at VIN+, and the digital output of the comparator is
adjusted accordingly (Figure 20-2).
FIGURE 20-2:
SINGLE COMPARATOR
20.5 Comparator Outputs
The comparator outputs are read through the CMCON
Register. These bits are read only. The comparator
outputs may also be directly output to the RF1 and RF2
I/O pins. When enabled, multiplexors in the output path
of the RF1 and RF2 pins will switch and the output of
each pin will be the unsynchronized output of the com-
parator. The uncertainty of each of the comparators is
related to the input offset voltage and the response time
given in the specifications. Figure 20-3 shows the
comparator output block diagram.
The TRISA bits will still function as an output
enable/disable for the RF1 and RF2 pins while in this
mode.
The polarity of the comparator outputs can be changed
using the C2INV and C1INV bits (CMCON<4:5>).
VIN+
VIN-
+
Output
–
VIN-
V
IN–
VIN+
V
IN+
Output
Output
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.
20.3.1
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 VSS and VDD, and can be applied to either
pin of the comparator(s).
2: Analog levels on any pin defined as a dig-
ital input, may cause the input buffer to
consume more current than is specified.
2003 Microchip Technology Inc.
Advance Information
DS39609A-page 225
MICROCHIP [ MICROCHIP ]
