16F87 datasheet(129/228 Pages) MICROCHIP

129 / 228 page

 2005 Microchip Technology Inc.

DS30487C-page 127

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.

CVRCON: COMPARATOR VOLTAGE REFERENCE CONTROL REGISTER

(ADDRESS 9Dh)

R/W-0

U-0

R/W-0

CVREN

CVROE

CVRR

—

CVR3

CVR2

CVR1

CVR0

bit 7

bit 0

bit 7

CVREN: Comparator Voltage Reference Enable bit

= CVREF circuit powered on

= CVREF circuit powered down

bit 6

CVROE: Comparator VREF Output Enable bit

= CVREF voltage level is output on the RA2/AN2/CVREF/VREF- pin(1)

= CVREF voltage level is disconnected from the RA2/AN2/CVREF/VREF- pin(1)

bit 5

CVRR: Comparator VREF Range Selection bit(1)

= 0.00 CVRSRC to 0.625 CVRSRC with CVRSRC/24 step size

= 0.25 CVRSRC to 0.72 CVRSRC with CVRSRC/32 step size

bit 4

Unimplemented: Read as ‘0’

bit 3-0

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

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

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