Page 39 - Making PIC Microcontroller Instruments and Controllers

plC
OF THE ilCU
G MICBOCHIP
26 UNDEBSTANDI TECHNOLOGY'S 16FA77Ar A DESCRIPIION
Set up the pin used on ihe MCU for analog mode

Select the pin to be sed for input
Select what the excitation voltage soxrce will bc (inlernal or erlcrnal)

On rhe 16F877A. only l6 pins can be used with the POT command, and these l6 pirs
havc been assigned lhe aliases from PINo to PINI5 in thc lnclude file BSIDEFS-BAS
(see the PBP marual). For the 16F877A, these are the pins on PORTB (0 to 7) and
PORTC (8 to 15). Other PIC MCU have dillqent designations based on their pin counts
See pagc 26 of the PICBAS]C PRO nanual for more information
The POT command is:
POT Pin, Scale, NUBR

qe
P,, is the pir numbef hdrc been di.cr s.ing
S.?le is the adjustmcnt forvarious RC constanls.Ifthe RC constant is large, the value
oI Scale sho ld be small. Scale is detemincd experimen|ally with a potenliometef
in place ofthe resistive load. At the low end o[ the resistance, the value ofscale shou]d
be 0, and at the high end il shouldbe 255.
NMrli is the variable lhc result will be placcd in.

Values belween 5 and 50K ohms may be read with a 0.1 uF capacitor, as shown in
the Conpiler manual under the POT command.

Capacitance Capacilance can be measurcd by deternining how long i1 lakes to
charge a capacitor through an accuralely calibratcd resistor or by sctting up an oscilla
tor with the two components and ncasuring the oscillalor frequency.
yorfage Vollage is measured by setting up an appropriate dividing network with pre-
cision resistors and measurirg the vollage across an approprraie resrslance

Frequency The PIC 16F877A can neasurc trequencies directly (as made possible
by the compiler software). Thc timers and counlcrs within the MCU are used to sct the
mcJ.urelrenr inlen aL\ 3nd (' 'unrin! \arJs."e.

Ree.ting Siwitches Swilches can be read from thc lines of any port set up as an input
port. De-bouncjng must be perlbrmcd either in hardware or in soltware to avoid lalse
readlngs. (The BUTTON comnand provides flexible de bouncing capabilities.)
Make sure tha! other hardware lhat may be connecled to the pins does not interlere
with the switch lunction and iis detection.
Peading Switches in a iltatix Switches armnged in a matrix can be read by set
dng and reading ihe rows and columN in thc matrix. The technique activales one row
ofbutlons at a time by making it high or low and then seeing iI any of the columns have
been affected. A detailed desciption ofhow this is done is in Chapler 5.