PIC16F688 Datasheet, PDF (61/174 Pages) Microchip Technology – 14-Pin Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

PIC16F688
7.6 Comparator Reference
The comparator module also allows the selection of an
internally generated voltage reference for one of the
comparator inputs. The VRCON register, Register 7-3,
controls the voltage reference module shown in
Figure 7-6.
7.6.1
CONFIGURING THE VOLTAGE
REFERENCE
The voltage reference can output 32 distinct voltage
levels, 16 in a high range and 16 in a low range.
The following equation determines the output voltages:
EQUATION 7-1:
VRR = 1 (low range): CVREF = (VR3:VR0/24) x VDD
VRR = 0 (high range):
CVREF = (VDD/4) + (VR3:VR0 x VDD/32)
7.6.2
VOLTAGE REFERENCE
ACCURACY/ERROR
The full range of VSS to VDD cannot be realized due to
the construction of the module. The transistors on the
top and bottom of the resistor ladder network
(Figure 7-6) keep CVREF from approaching VSS or
VDD. The exception is when the module is disabled by
clearing the VREN bit (VRCON<7>). When disabled,
the reference voltage is VSS when VR<3:0> = 0000.
This allows the comparators to detect a zero-crossing
and not consume CVREF module current.
The voltage reference is VDD derived and therefore, the
CVREF output changes with fluctuations in VDD. The
tested absolute accuracy of the comparator voltage
reference can be found in Section 14.0 “Electrical
Specifications”.
FIGURE 7-6:
COMPARATOR VOLTAGE REFERENCE BLOCK DIAGRAM
16 Stages
VDD
VREN
CVREF to
Comparator
Input
8R
R
R
16-1 Analog
MUX
R
R
8R
VRR
VR<3:0>
 2004 Microchip Technology Inc.
Preliminary
DS41203B-page 59