PIC16F87XA
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 tran-
sistor. This reference will only be as accurate as the
values of CVRSRC and VSAT.
13.0 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
110. A programmable register controls the function of
the reference generator. Register 13-1 lists the bit func-
tions of the CVRCON register.
The output of the reference generator may be con-
nected to the RA2/AN2/VREF-/CVREF pin. 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 refer-
ence generator function.
As shown in Figure 13-1, 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
REGISTER 13-1: CVRCON CONTROL REGISTER (ADDRESS 9Dh)
R/W-0
R/W-0
R/W-0
CVRR
U-0
—
R/W-0
CVR3
R/W-0
CVR2
R/W-0
CVR1
R/W-0
CVR0
CVREN
CVROE
bit 7
bit 0
bit 7
bit 6
bit 5
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 RA2/AN2/VREF-/CVREF pin
0= CVREF voltage level is disconnected from RA2/AN2/VREF-/CVREF pin
CVRR: Comparator VREF Range Selection bit
1= 0 to 0.75 CVRSRC, with CVRSRC/24 step size
0= 0.25 CVRSRC to 0.75 CVRSRC, with CVRSRC/32 step size
bit 4
Unimplemented: Read as ‘0’
bit 3-0
CVR3:CVR0: Comparator VREF Value Selection bits 0 ≤ VR3:VR0 ≤ 15
When CVRR = 1:
CVREF = (VR<3:0>/ 24) • (CVRSRC)
When CVRR = 0:
CVREF = 1/4 • (CVRSRC) + (VR3:VR0/ 32) • (CVRSRC)
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
2001 Microchip Technology Inc.
Advance Information
DS39582A-page 139
MICROCHIP [ MICROCHIP ]
