PIC16F882 datasheet(96/288 Pages) MICROCHIP

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PIC16F882/883/884/886/887

DS41291D-page 94

Preliminary

8.10

Comparator Voltage Reference

The Comparator Voltage Reference module provides

an internally generated voltage reference for the com-

parators. The following features are available:

• Independent from Comparator operation

• Two 16-level voltage ranges

• Output clamped to VSS

• Ratiometric with VDD

• Fixed Reference (0.6V)

The VRCON register (Register 8-5) controls the

Voltage Reference module shown in Figure 8-8.

The voltage source is selectable through both ends of

the 16 connection resistor ladder network. Bit VRSS of

the VRCON register selects either the internal or

external voltage source.

The PIC16F883/884/886/887 allows the CVREF signal

to be output to the RA2 pin of PORTA under certain

configurations only. For more details, see Figure 8-9.

8.10.1

INDEPENDENT OPERATION

The comparator voltage reference is independent of

the comparator configuration. Setting the VREN bit of

the VRCON register will enable the voltage reference.

8.10.2

OUTPUT VOLTAGE SELECTION

The

CVREF voltage reference has 2 ranges with 16

voltage levels in each range. Range selection is

controlled by the VRR bit of the VRCON register. The

16 levels are set with the VR<3:0> bits of the VRCON

The CVREF output voltage is determined by the following

equations:

EQUATION 8-1:

CVREF OUTPUT VOLTAGE

The full range of VSS to VDD cannot be realized due to

the construction of the module. See Figure 8-8.

8.10.3

OUTPUT CLAMPED TO VSS

The CVREF output voltage can be set to Vss with no

power consumption by configuring VRCON as follows:

•VREN = 0

•VRR = 1

•VR<3:0> = 0000

This allows the comparator to detect a zero-crossing

while not consuming additional CVREF module current.

8.10.4

OUTPUT RATIOMETRIC TO VDD

The comparator 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 17.0

“Electrical Specifications”.

8.10.5

FIXED VOLTAGE REFERENCE

The fixed voltage reference is independent of VDD, with

a nominal output voltage of 0.6V. This reference can be

enabled by setting the FVREN bit of the SRCON

the HFINTOSC oscillator is active.

8.10.6

FIXED VOLTAGE REFERENCE

STABILIZATION PERIOD

When the fixed Voltage Reference module is enabled,

it will require some time for the reference and its

amplifier circuits to stabilize. The user program must

include a small delay routine to allow the module to

settle. See the electrical specifications section for the

minimum delay requirement.

8.10.7

VOLTAGE REFERENCE

SELECTION

Multiplexers on the output of the Voltage Reference

module enable selection of either the CVREF or fixed

voltage reference for use by the comparators.

Setting the C1VREN bit of the VRCON register enables

current to flow in the CVREF voltage divider and selects

the CVREF voltage for use by C1. Clearing the C1VREN

bit selects the fixed voltage for use by C1.

Setting the C2VREN bit of the VRCON register enables

current to flow in the CVREF voltage divider and selects

the CVREF voltage for use by C2. Clearing the C2VREN

bit selects the fixed voltage for use by C2.

When both the C1VREN and C2VREN bits are cleared,

current flow in the CVREF voltage divider is disabled

minimizing the power drain of the voltage reference

peripheral.

VRR

1 (low range):

VRR

0 (high range):

CVREF

(VLADDER/4) +

CVREF

(VR<3:0>/24)

VLADDER

(VR<3:0>

VLADDER/32)

VLADDER

VDD

or ([VREF+] - [VREF-]) or VREF+

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