PIC16F684-ESLASHST datasheet(62/164 Pages) MICROCHIP

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PIC16F684

DS41202C-page 60

Preliminary

 2004 Microchip Technology Inc.

8.6

Comparator Reference

The comparator module also allows the selection of an

internally generated voltage reference for one of the

comparator inputs. The VRCON register (Register 8-3)

controls the voltage reference module shown in

Figure 8-6.

8.6.1

CONFIGURING THE VOLTAGE

REFERENCE

The voltage reference can output 32 distinct voltage

levels, 16 in a high range and 16 in a low range.

The following equation determines the output voltages:

EQUATION 8-1:

8.6.2

VOLTAGE REFERENCE

ACCURACY/ERROR

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

the construction of the module. The transistors on the

top and bottom of the resistor ladder network

(Figure 8-6) keep CVREF from approaching VSS or

VDD. The exception is when the module is disabled by

clearing the VREN bit (VRCON<7>). When disabled,

the reference voltage is VSS when VR<3:0> is ‘0000’

and the VRR (VRCON<5>) bit is set. This allows the

comparators to detect a zero-crossing and not

consume CVREF module current.

The 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 15.0 “Electrical

Specifications”.

FIGURE 8-6:

COMPARATOR VOLTAGE REFERENCE BLOCK DIAGRAM

VRR = 1 (low range): CVREF = (VR3:VR0/24) X VDD

VRR = 0 (high range):

CVREF = (VDD/4) + (VR3:VR0 X VDD/32)

VRR

VR3:VR0

16-1 Analog

CVREF to

16 Stages

Comparator

Input

VREN

VDD

MUX

VR3:VR0 = ‘0000’

VREN

VRR

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