Page 41 - Making PIC Microcontroller Instruments and Controllers

iECH||O!OGY'S PIC i6Fa7?A: A DESCBIPIIOI{
30 U DERSIAI{Dn{G IVIICROCHIP OF THE iilCu

All the pins have TTL level inputs and full CMOS level output drivers. This makes
il casy to connecl these lines direclly to standard logic components (meaning that r,t&-
dlh no intermediate rcsistors are needcd between components if TTL or CMOS com
ponents are interconnected).
PORTA (tetisnations are somewhat complicated. Pins A0, A 1 , A2, A3 ' (skip A4)' and
Pin A5 can be configured as analog inpuls by setting rhe ADCONI register appropri_
ately. Pin A3 is also used as a voltage input fbr comparing with the analog voltage
inputs on other pins. Pin PORTA4, is used as input for the timerTIMERo.lt is called
ToCKlforTimer0clocklnput.ThisisPin6oflhcPlCandisusedastheinputpjnlor
TIMERo only when configured as such. It is a Schmitt trigger input with open drain
ourput. Open drain means it acts like the contacts of a liny relay Scbmitt trjggered inputs
have increased noise immunity (becausc of the hystelesis between state changes)
The two registers that control PORIA arc TRISA and ADCONI
ADCON1 controls theA to-D and voltage reference functjons of PORTA The set
jn
ting of the vadous bits select acomplicated set ofconditioN that are described detail
and in this book. (Inlhcpreceding discussjon,
in atablein the datasheet whenADCONl
was setto 7.00000111, we were accessing this feature.)
Pitr A4 hds speciel needs when useA as an output.lt can be pulled down low but will
fioat when ser high. lt must be pulled up with a ( LoK to I 00K) resislor to tie it high when
This pin has an open drajn output rather thar the usual bipolar state
this is necessary.
of the other pins. As noted earliet this pin is skipped in the A-to-D conversion table.
(Ignoring this fact is a source olmany common mistakes, so keep this in mind.)

PORTB

PORTB is a full 8 bjt-wide bidireclional po|t.
Intcmal circuitry (meaning built into the MCU) allows all the pins on PORTB to be
pulled up to a high slate (very weakly) by setting pin 7 of t}le option register
(OPTION REG.?) to 0. Only those pins designated at iDputs are affected. These pull
ups are disabled on starlup and on reset.
Pins 83. 86. and 87 are useal for the low voltage programning ofthe PlC. Bit 3 in
TRISB mustbe cleared (set |o 0 or pulled down to 0) to negate the pull up on this pin
to allow programning to take place. See pages 42 and 142 in the datasheet fbr more
infomation regarding thc 93 pin. This is wry inportant.
Pins 84 to B? will cause an intefupt 10 occur when lhcir state changcs if they are
configured as inputs and the appropriate irltelTupts are configured. Any of these four pins
that are conligured as outputs will be excluded fiom the intelrupt leature Thc intenxpts
are controlled by the INTCON (interupt control) register. This PORTB interupt capa
bility has the special feature oflelting it be used to awalen a sleeping MCU.
Pin B0 has spccjal inteffupi functions that are controlled tbrough the INTEDG bit'
which is bit 6 of the OPTION-REG. See datashect for more intbrmation rrt?/dl
interrup arc nuted tu the PIC through this pin.Extemal inteffupts are those that are
created bv events outside the PIC MCU.