Page 40 - Making PIC Microcontroller Instruments and Controllers

FORIA 29

Configuring and Controlling

the Properties of the Pofts

The PIC 16F877A provides 33 VO pins distributed across five porls. Each of the ports
has unique capabilities built into it. The following section of this book discusses the capa_
bilities of each of the pots with special attention to these propeties.
These descriptions are cufsory and are designed to provide a quick and ready refer-
ence. Refer to the actual datasheet for detailed infomalion on these polls. The dalasheet
provides information at a level lhat cannot be provided in a short introductory text like
this. Refenhg to the datasheets frequently will make you comfortable with them in a
short time.

PORTA

PORTA is a 6 bit wide bidirectional port with both analog and digital capability

Note The general rule is that ifa PIC de\)ice has any analog inputs built into it, it
will come up as an analog input.lerice on reset dntuor staftup.
The PIC 16F877A has analog capability on PORTA (and PORTE) so it comes up as
an analog device on statup and rcset. lf you are going to use it as a digital device, you
must set register ADCON as needed. The most common \alne is ADCONI to
q.0000A111 for vsirg the LCD display. This line of code will be seen in nany of the
programs in tlis book and is explained in Chapter 5- (7o00000111 all the analog
sets
pins to digital, but there are many otherchoices.)
The PIC 16F87?A supports external access |o only six of the eight pins on this port.
Each of the six pins may be set to function as an input or output by appropriately load-
"0"
ing the TRISA register A (zero) in a bit in this register sets the cofesponding pin
to function as an output, while a "1" (one) sets it to function as an input.
Thus, selting...

IRIsA=%o0U1000
(% mcans this is a binary numbel Wo wilt use this binary notation throughout this book because
bh, is on fte
it makes it easier to see what eacb bit is being seito. Bil7. the mosl significant
is on the right.)
ieftibit 0, the leasl significant.
Would make lines A0, A I , and A2 outputs and lines A3, A4, and A5 inputs. The most
sienifcant 2 bits are igtrr,'ed (and could be set to I s or 0s) because PORTA has only
six activelines. (However, the 2 ignoredbits are used by the processor and they can be
read when necessary. You should not set them. Again, see lhe datasheet for specific
details.)
As mentioned earlier, the specilic functions (the uses the pins are put to) of the pins
are controlled with the ADCONI (the first A-to-D CONtrcl) rcgister