PIC14000 datasheet(68/153 Pages) MICROCHIP

68 / 153 page

PIC14000

DS40122B-page 68

Preliminary

Two registers PREFA (9Bh) and PREFB (9Ch) are

used to select the reference output voltages. The

PREFx<7:3> bits select the output from the coarse lad-

der, while PREFx<2:0> bits are for the ﬁne-tune adjust-

ment. Table 9-1 and Table 9-2 show the reference

decoding.

These voltages are visible at either RC0/REFA or

RD3/REFB pins by setting the CMAOE (CMCON<1>)

or CMBOE (CMCON<5>) bits. Setting CMxOE does

not affect the reference voltages. It only enables the pin

function regardless of the port TRIS register setting.

These outputs are not buffered, so they cannot directly

drive any DC loads.

The reference outputs are also connected to two inde-

pendent comparators, COMPA and COMPB. Thus, the

references can be used to set the comparator trip-

points. The A/D converter can also monitor the refer-

ence outputs via A/D channels 8 and 9. Refer to

Section 8 for the description of the A/D operation.

The programmable reference output is designed to

track the output from the level shift network. However,

there will always be some mismatch due to component

drift. For best accuracy, the A/D should be used to peri-

odically calibrate the references to the desired

set-point.

FIGURE 9-3:

COMPARATOR AND PROGRAMMABLE REFERENCE BLOCK DIAGRAM

(ONE OF TWO SHOWN)

From AN1 Level

Shift Network

CMOFF

RC1/CMPA or

RC0/REFA or

µA

PREFx<7:3>

CPOLx

To A/D

Converter

Analog

Mux

(1 of 32)

Analog

Mux

(1-of-8)

PREFx<2:0>

~0.15V

~0.85V

Fine Tune Adjust

Coarse Adjust

To CMxOUT bit,

CMCON register

Programmable

Analog

Mux

(1 of 32)

RD3/REFB

CMxOE

RD2/CMPB

From Other

Comparator

CMIF bit

PIR1<7>

Reference

From AN5 Level

Shift Network

CMBOE

Channel B only

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