PIC16F87_05 Datasheet, PDF (129/228 Pages) Microchip Technology – 18/20/28-Pin Enhanced Flash Microcontrollers with nanoWatt Technology

PIC16F87/88
14.0 COMPARATOR VOLTAGE
REFERENCE MODULE
The comparator voltage reference generator is a 16-tap
resistor ladder network that provides a fixed voltage
reference when the comparators are in mode ‘010’. A
programmable register controls the function of the refer-
ence generator. Register 14-1 lists the bit functions of
the CVRCON register.
As shown in Figure 14-1, the resistor ladder is seg-
mented to provide two ranges of CVREF values and has
a power-down function to conserve power when the
reference is not being used. The comparator reference
supply voltage (also referred to as CVRSRC) comes
directly from VDD. It should be noted, however, that the
voltage at the top of the ladder is CVRSRC – VSAT,
where VSAT is the saturation voltage of the power
switch transistor. This reference will only be as
accurate as the values of CVRSRC and VSAT.
The output of the reference generator may be
connected to the RA2/AN2/CVREF/VREF- pin (VREF- is
available on the PIC16F88 device only). This can be
used as a simple D/A function by the user if a very high-
impedance load is used. The primary purpose of this
function is to provide a test path for testing the
reference generator function.
REGISTER 14-1:
CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER
(ADDRESS 9Dh)
R/W-0
R/W-0
R/W-0
U-0
R/W-0 R/W-0 R/W-0 R/W-0
CVREN CVROE CVRR
—
CVR3 CVR2 CVR1 CVR0
bit 7
bit 0
bit 7
bit 6
bit 5
bit 4
bit 3-0
CVREN: Comparator Voltage Reference Enable bit
1 = CVREF circuit powered on
0 = CVREF circuit powered down
CVROE: Comparator VREF Output Enable bit
1 = CVREF voltage level is output on the RA2/AN2/CVREF/VREF- pin⁽¹⁾
0 = CVREF voltage level is disconnected from the RA2/AN2/CVREF/VREF- pin⁽¹⁾
CVRR: Comparator VREF Range Selection bit⁽¹⁾
1 = 0.00 CVRSRC to 0.625 CVRSRC with CVRSRC/24 step size
0 = 0.25 CVRSRC to 0.72 CVRSRC with CVRSRC/32 step size
Unimplemented: Read as ‘0’
CVR<3:0>: Comparator VREF Value Selection 0 ≤ VR3:VR0 ≤ 15 bits⁽¹⁾
When CVRR = 1:
CVREF = (VR<3:0>/24) • (CVRSRC)
When CVRR = 0:
CVREF = 1/4 • (CVRSRC) + (VR3:VR0/32) • (CVRSRC)
Note 1: VREF is available on the PIC16F88 device only.
Legend:
R = Readable bit
-n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
 2005 Microchip Technology Inc.
DS30487C-page 127