Writing To the Ports
In the last tutorial, we showed you how to set up the IO port pins on the PIC
to be either input or output. In this tutorial, We are going to show you
how to send data to the ports. In the next tutorial, we will finish off by
flashing an LED on and off which will include a full program listing and a
simple circuit diagram so that you can see the PIC doing exactly what we expect
it to. Don’t try and compile and program your PIC with the listings here,
as they are examples only.
First, let us set up Port A bit 2 as an output:
bsf
03h,5
;Go to Bank 1
movlw
00h
;Put 00000 into W
movwf
85h
;Move 00000 onto TRISA – all pins set to output
bcf
03h,5
;Come back to Bank 0
This should be familiar from the last tutorial. The only difference is
that we have set all of the pins on Port A as output, by sending 0h to the
tri-state register.
Now what he have to do is turn an LED on. We do this by making one of
the pins (the one with the LED connected to it) high. In other words, we
send a ‘1’ to the pin. This is how it’s done (note the comments for an
explanation of each line):
movlw
02h ;Write 02h to the W register. In binary this is 00010, which
;puts a ‘1’ on bit
2 (pin 18) while keeping the other pins to ‘0’
movwf
05h ;Now move the contents of W (02h) onto the PortA, whose
;address is 05h
So, now our LED is on, we now need to turn it off:
movlw
00h ;Write 00h to the W register. This puts
a ‘0’ on all pins.
movwf
05h ;Now move the contents of W (0h) onto the Port
A, whose
;address is 05h
So, what we have done is turn the LED on then off once.
What we want is for the LED to turn on then off continuously. We do this by
getting the program to go back to the beginning. We do this by first
defining a label at the start of our program, and then telling the program to
keep going back there.
We define a label very simply. We type a name, say START, then type the
code:
Start movlw
02h ;Write 02h to the W register. In binary this is
;00010, which puts a ‘1’
on pin 2 while keeping
;the other pins to ‘0’
movwf
05h ;Now move the contents of W (02h) onto the
;PortA, whose address is 05h
movlw
00h ;Write 00h to the W register. This puts
a ‘0’ on
;all pins.
movwf
05h ;Now move the contents of W (0h) onto the Port
;A, whose address is 05h
goto
Start
;Goto where we say Start
As you can see, we first said the word ‘Start’ right at the beginning of the
program. Then, right at the very end of the program we simply said ‘goto
Start’. The ‘goto’ instruction does exactly what it says.
This program will continuously turn the LED on and off as soon as we power up
the circuit, and will stop when we remove power.
We think we should look at our program again:
bsf
03h,5
movlw
00h
movwf
85h
bcf
03h,5
Start movlw
02h
movwf
05h
movlw
00h
movwf
05h
goto
Start
OK, We know we have left the comments off. But, do you notice that all
we can see are instructions and numbers? This can be a little confusing if
you are trying to debug the program later, and also when you write the code you
have to remember all of the addresses. Even with the comments in place, it
can get a bit messy. What we need is to give these numbers names.
This is accomplished by another instruction: ‘equ’.
The ‘equ’ instruction simply means something equals something else. It
is not an instruction for the PIC, but for the assembler. With this
instruction we can assign a name to a register address location, or in
programming terms assign a constant. Let us set up some constants for our
program, then you will see how much easier to read the program is.
STATUS equ 03h
;this assigns the word STATUS to the value of 03h,
;which
is the address of the STATUS register.
TRISA
equ 85h ;This assigns the word TRISA to the value of
85h,
;which is the address of the Tri-State register for PortA
PORTA equ
05h ;This assigns the word PORTA to 05h which is the
;address of Port A.
So, now we have set up our constant values, let us put these into our
program. The constant values must be defined before we can use them, so to
be sure always put them at the start of the program. We will re-write the
program without comments again, so that you can compare the previous listing to
the new one:
STATUS equ 03h
TRISA equ 85h
PORTA equ 05h
bsf
STATUS,5
movlw
00h
movwf
TRISA
bcf
STATUS,5
Start
movlw 02h
movwf
PORTA
movlw 00h
movwf PORTA
goto Start
Hopefully, you can see that the constants make following the program a little
easier, even though we still have not put the comments in.
However, we are not quite finished.
Click here >>>>
Tutorial 4 (Delay Loops) |
