PIC18F23K20-ESLASHML datasheet(289/456 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS41303G-page 289

PIC18F2XK20/4XK20

21.0

VOLTAGE REFERENCES

There

are

two

independent

voltage

references

available:

• Programmable Comparator Voltage Reference

• 1.2V Fixed Voltage Reference

21.1

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

• 1.2 Fixed Reference Voltage (FVR)

The CVRCON register (Register 21-1) controls the

Voltage Reference module shown in Figure 21-1.

21.1.1

INDEPENDENT OPERATION

The comparator voltage reference is independent of

the comparator configuration. Setting the CVREN bit of

the CVRCON register will enable the voltage reference

by allowing current to flow in the CVREF voltage divider.

When both the CVREN bit is cleared, current flow in the

CVREF voltage divider is disabled minimizing the power

drain of the voltage reference peripheral.

21.1.2

OUTPUT VOLTAGE SELECTION

The

CVREF voltage reference has 2 ranges with 16

voltage levels in each range. Range selection is

controlled by the CVRR bit of the CVRCON register.

The 16 levels are set with the CVR<3:0> bits of the

CVRCON register.

The CVREF output voltage is determined by the

following equations:

EQUATION 21-1:

CVREF OUTPUT VOLTAGE

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

the construction of the module. See Figure 21-1.

21.1.3

OUTPUT CLAMPED TO VSS

The CVREF output voltage can be set to Vss with no

power consumption by configuring CVRCON as

follows:

• CVREN = 0

•CVRR = 1

•CVR<3:0> = 0000

This allows the comparator to detect a zero-crossing

while not consuming additional CVREF module current.

21.1.4

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 26.0

“Electrical Characteristics”.

21.1.5

VOLTAGE REFERENCE OUTPUT

The CVREF voltage reference can be output to the

device CVREF pin by setting the CVROE bit of the CVR-

CON register to ‘1’. Selecting the reference voltage for

output on the CVREF pin automatically overrides the

digital output buffer and digital input threshold detector

functions of that pin. Reading the CVREF pin when it

has been configured for reference voltage output will

always return a ‘0’.

Due to the limited current drive capability, a buffer must

be used on the voltage reference output for external

connections to CVREF. Figure 21-2 shows an example

buffering technique.

21.1.6

OPERATION DURING SLEEP

When the device wakes up from Sleep through an

interrupt or a Watchdog Timer time-out, the contents of

the CVRCON register are not affected. To minimize

current consumption in Sleep mode, the voltage

reference should be disabled.

21.1.7

EFFECTS OF A RESET

A device Reset affects the following:

• Comparator voltage reference is disabled

• Fixed voltage reference is disabled

•CVREF is removed from the CVREF pin

• The high-voltage range is selected

• The CVR<3:0> range select bits are cleared

CVRR

1 (low range):

CVRR

0 (high range):

CVREF = (CVRSRC/24) X CVR<3:0> + VREF-

CVRSRC

VDD

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

CVREF = (CVRSRC/32) X (8 + CVR<3:0>) + VREF-

Note: VREF- is 0 when CVRSS = 0

PIC18F23K20-E/ML Datasheet(PDF) 289 Page - Microchip Technology

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