Reading from the I/O ports.
Up to now, we have been
writing to Port A so that we can turn an LED on and off. Now, we are
going to look at how we can read the I/O pins on the ports. This is
so that we can connect an external circuit, and act on any outputs it
gives.
If you recall from our
previous tutorials, in order to set up the I/O ports, we had to switch
from Bank 0 to Bank 1. Let us do that first:
STATUS equ
03h ;Address of the STATUS register
TRISA equ
85h ;Address of the tristate register
for port A
PORTA equ
05h ;Address of Port A
bsf STATUS,5
;Switch to Bank 1
Now, to set up the port to
be an output, we sent a 0 to the TrisA register. To set a pin on a
port to be an input, we send a 1 to the TisA register.
movlw
01h ;Set the Port A pins
movwf TRISA ;to input.
bcf STATUS,5 ;Switch back to Bank 0
Now we have set bit 0 of
Port A to input. What we need to do now is to check if the pin is
high or low. For this, we can use one of two instructions: BTFSC and
BTFSS.
The BTFSC instruction means
‘Do a bit test on the register and bit we specify. If it is a 0, then
we skip the next instruction’. BTFSS means ‘Do a bit test in the
register and bit we specify. If it is set to a 1, then we skip the
next instruction.’
Which one we use, depends
on how we want our program to react when we read the input. For
example, if we are simply waiting for the input to be a 1, then we
could use the BTFSS instruction like this:
Code here
:
BTFSS PortA,0
Goto start
Carry on here
:
:
The program will only move
onto ‘Carry on here’ only if bit 0 on PortA is set to a 1.
Let us now write a program
which will flash an LED at one speed, but if a switch is closed it
will flash the LED twice as slow. You can probably work this program
out for yourself, but I have included the listing anyway. You could
try and write the whole program, just to see if you have grasped the
concepts. We are using the same circuit as before, with the addition
of a switch connected RA0 of the PIC and the positive rail of our
supply.
;*****Set up the
Constants****
STATUS equ
03h ;Address of the STATUS register
TRISA equ
85h ;Address of the tristate register for
port A
PORTA equ
05h ;Address of Port A
COUNT1 equ
08h ;First counter for our delay loops
COUNT2 equ 09h
;Second counter for
our delay loops
;****Set up the port****
bsf
STATUS,5 ;Switch to Bank 1
movlw 01h
;Set the Port A pins:
movwf TRISA
;bit 1to output, bit 0 to input.
bcf STATUS,5
;Switch back to Bank 0
;****Turn the LED on****
Start movlw 02h ;Turn
the LED on by first putting it
movwf PORTA
;into the w register and then on the port
;****Check if the switch is
closed
BTFSC PORTA,0
;Get the value from PORT A
;BIT 0. If it is a zero
call Delay
;a zero, carry on as normal.
;If is is a 1, then add an
;extra delay routine
;****Add a delay
call Delay
;****Delay finished, now
turn the LED off****
movlw 00h ;Turn the LED off by first
putting it
movwf PORTA ;into the w register and then
on the port
;****Check if the switch is
still closed
BTFSC
PORTA,0 ;Get the value from PORT A
;BIT 0. If it is a zero,
call
Delay ;carry on as
normal.
;If is a 1, then add an
;extra delay routine
;****Add another delay****
call Delay
;****Now go back to the
start of the program
goto
Start ;go back to Start and turn LED on
again
;****Here is our Subroutine
Delay
Loop1 decfsz
COUNT1,1 ;This second loop keeps the LED
goto
Loop1 ;turned off long
enough for us to
decfsz
COUNT2,1 ;see it turned off
goto
Loop1 ;
return
;****End of the
program****
end
;Needed by some compilers, and also
;just in case we miss the goto instruction.
What I have done here is to
turn the LED on. I then check to see if the switch is closed. If it
is closed, then I make a call to our delay subroutine. This gives us
the same delay as before, but we are now calling it twice. The same
goes for when the LED is off. If the switch is not closed, then we
have our old on and off times.
You can compile and run
this program. However a word of warning. The final circuit and code
will look un-impressive to someone who is not interested in
programming microcontrollers. So, don’t be upset if, when you show
your family and friends how you can change the speed of a flashing LED
with a switch, they show very little interest – I am talking from
personal experience, here!
If you have been following
these tutorials from the start, then you may be interested to know
that you have now learnt 10 of the 35 instructions for the PIC 16F84!
And all of these have been learnt just by simply turning an LED on and
off.
