PIC14000-04 Microchip Technology, PIC14000-04 Datasheet - Page 68

• Manufacturer Part Number: PIC14000-04
• Description: 28-Pin Programmable Mixed Signal Controller
• Manufacturer: Microchip Technology

PIC14000
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.
FIGURE 9-3:
DS40122B-page 68
From AN5 Level
Shift Network
From AN1 Level
Shift Network
~5 A
~
~
0.85V
0.15V
CMBOE
COMPARATOR AND PROGRAMMABLE REFERENCE BLOCK DIAGRAM
(ONE OF TWO SHOWN)
Coarse Adjust
PREFx<7:3>
Programmable
Reference
PREFx<7:3>
Channel B only
(1 of 32)
(1 of 32)
Analog
Analog
Mux
Mux
CMOFF
Preliminary
Fine Tune Adjust
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.
PREFx<2:0>
CPOLx
+
(1-of-8)
Analog
Mux
To CMxOUT bit,
CMCON register
From Other
Comparator
1996 Microchip Technology Inc.
To A/D
Converter
RC0/REFA or
RD3/REFB
RC1/CMPA or
RD2/CMPB
CMxOE
CMIF bit
PIR1<7>