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EE ME

FR LE

EPE PIC

TUTORIAL V2

JOHN BECKER

PART TWO

Quite simply the easiest

low-cost way to learn

about using PIC

Microcontrollers!

In this part we play with switches, make

noises, count times, and generally have

fun with some more PIC16F84 commands!

TUTORIAL 7

CONCEPTS EXAMINED

Switch monitoring

Command ANDLW

Command ANDWF

Command ADDWF

Command ADDLW

Nibbles

STATUS bit 1

Digit Carry flag

Bit code DC

CONNECTIONS NEEDED

All Port B to all l.e.d.s.

Port A RA0-RA3 to switches SW0-

SW3 (via CP19-CP16)

CP21 to +5V OUT

CP20 to 0V OUT

Capacitor C7 as 1mF

Preset VR1 set to minimum resistance

(fully clockwise)

From hereon we shall usually omit the

program initialisation commands that have

up to now been shown at the top of each

listing. Some will be included where they

help to clarify the program. Otherwise,

assume that any name used in the listing

extracts shown will have been defined or

equated in the headings. The commands

are included in full on the disk file pro-

gram listings (source code).

We now turn to looking at how data is

input via switches and shall continue to

show the results on individual l.e.d.s. In

Tutorials 21 and 22 we shall look at 7-seg-

ment l.e.d.s and alphanumeric l.c.d.s as the

output displays.

First connect TK3’s pushbutton switch-

es SW0, SW1, SW2, SW3 (via CP19,

CP18, CP17, CP16) to PORTA pins RA0,

RA1, RA2, RA3 respectively. Connect the

switch power pin CP21 to the +5V OUT

pin, and switch power pin CP20 to the 0V

OUT pin. Port pins RA0 to RA3 are now

EPE PIC

TUTORIAL

LISTING 8 PROGRAM

TK3TUT8

BEGIN

LOOP

CLRF COUNT

MOVF PORTA,W

ANDLW B’00000001’

ADDWF COUNT,F

MOVF COUNT,W

MOVWF PORTB

GOTO LOOP

connected so that they are normally biased

low (to 0V) but will go high (+5V) when

their respective switches are pressed.

Run TK3TUT8.HEX. Pushing switch

SW0 on and off, PORTB’s l.e.d.s will be

seen to go on and off in a binary sequence

when the switch is on (pressed), but will

remain in the last condition when the

switch is off (released). In this example,

the program tests whether the status of

switch SW0, which is connected to

PORTA RA0 (bit 0), is on or off. If the

switch is on then the counter variable,

COUNT, is repeatedly added to (by 1 in

this example). A value of zero is added to

the count if the switch is off. The count

value is output to PORTB.

First let’s look at two of the commands

introduced here, ANDLW and ADDWF.

Their counterparts ANDWF and ADDLW

will also be examined.

This technique is widely used in elec-

tronics and computing, the final answer

determining the subsequent action to be

taken by a circuit or software routine.

There are two ANDing commands

available with PICs, ANDLW (AND

Literal to W), and ANDWF (AND W with

File value). Suppose that the first number

in the foregoing examples (01110010) is

already contained within W, we then wish

to AND it with a fixed number as stated in

a program command. Assuming that the

fixed number is the second number quoted,

the command is:

ANDLW B’01011001’

The PIC ANDs the second (literal) num-

ber with that already held in W. The

answer (01010000) is retained by W and is

available to be further manipulated or

copied into any file as specified by the

command which follows ANDLW. You

could, for example, use the command

MOVWF PORTB which will turn on

l.e.d.s LD6 and LD4 (01010000).

Any of the three numerical formats may

be used with ANDLW, e.g. B’00011111’

(binary), H’1F’ (hexadecimal), 31 (deci-

mal), are all legitimate and equal. It is also

legitimate to use a name that has been

equated with a value, e.g. ANDLW

PORTB (which would AND 6 with W

since we have previously specified that the

name PORTB represents the value 6).

The command ANDWF is used to AND

an existing value within W to a value with-

in a named file, either retaining the answer

in W (ANDWF FILENAME,W) or putting

back in the named file (ANDWF

FILENAME,F).

It is not possible to directly AND the

contents of two files together, the value of

one or other file must have already been

moved into W before the ANDing can take

place. With both commands ANDLW and

ANDWF, if the answer is zero, the Zero

As no doubt most of you are aware, if

one binary number is ANDed with anoth-

er, then only if the same bits of both num-

bers are set (1) will the answer also have a

1 in that position. Any zeros on either or

both sides for any bit will automatically

produce a result of 0, e.g.:

First number:

Second number:

ANDed answer:

01110010

01011001

01010000

COMMANDS ANDLW

AND ANDWF

Everyday Practical Electronics, May 2003

PIC Tutorial V2 Supplement

–

flag of STATUS is set. If the answer is

greater than zero, the Zero flag is cleared.

Zero is the only flag affected by an AND

command.

COMMANDS ADDLW

AND ADDWF

There are two ADDing commands avail-

able with PICs, ADDLW (ADD Literal to

W), and ADDWF (ADD W to a File value).

Command ADDLW is used where a fixed

number (literal) within a program is to be

added to an existing value within W and

which has been obtained by a previous

operation. Suppose that W holds the

answer produced in the previous ANDing

example, 01010000 (decimal 80), and you

wish to add a fixed value to it, 53 decimal

(00110101), for instance. The command

would be:

ADDLW 53 (or ADDLW H’35’ hex-

adecimal, or ADDLW B’00110101’ binary).

The answer in this instance is 10000101

(decimal 133) and is retained in W for fur-

ther use or copying into a file, e.g.

MOVWF PORTB.

Command ADDWF adds the contents of

W to the value within a stated file. The

answer can be held in W (ADDWF

PORTB,W) or put back into the named file

(ADDWF PORTB,F).

Three flags within STATUS are affected

by any ADD command, Carry, Zero and

Digit Carry. If the answer to an addition is

greater than 255, the Carry flag is set, oth-

erwise it is cleared. If the answer equals

zero, the Zero flag is set, otherwise it is

cleared. The third flag, Digit Carry, you

have not encountered yet. Although the

concept is not illustrated until later

(Tutorial 19), it is appropriate to describe it

now.

If you imagine that an 8-bit binary num-

ber (e.g. 10110110) is split into two halves

(known as “nibbles”), 1011 and 0110, the

righthand nibble is monitored by the PIC

as a separate digit and it is served by its

own flag, the Digit Carry flag. If an addi-

tion takes place which produces a result

greater than 15 (binary 1111) for that nib-

ble, the Digit Carry flag is set, otherwise it

is cleared.

count value. With the command GOTO

LOOP, the sequence is repeated.

It will be seen that there is only an

increase in the count value if PORTA bit 0

holds a 1, therefore the count will only

change if the switch is on (pressed).

Pressing any other switch connected to

PORTA has no effect. When the counter

passes 255, its value rolls over to zero and

starts counting upwards again.

8.2. Could one of the Zero flag testing

commands be used instead of BTFSS? If

so, how, and would an AND command be

useful? (Remember that PORTA has more

bits than just bit 0).

TUTORIAL 9

CONCEPT EXAMINED

Responding to a switch press only at the

moment of pressing

CONNECTIONS NEEDED

All Port B to all l.e.d.s.

Port A RA0-RA3 to switches SW0-SW3

(via CP19-CP16)

CP21 to +5V OUT

CP20 to 0V OUT

Capacitor C7 as 1mF

Preset VR1 set to minimum resistance

(fully clockwise)

EXERCISE 7

7.1. Can you see another way of writing

the first two lines using MOVLW and

ANDWF?

7.2. Can you see how the BTFSS or

BTFSC commands might be used to

achieve the same output result; the use of

MOVLW 1 or ADDLW 1 could be useful

here.

7.3. There is also the opportunity to use

INCF in this type of situation. Try rewrit-

ing to include this command.

TUTORIAL 8

LISTING 10

PROGRAM TK3TUT10

BEGIN

TESTIT

CLRF COUNT

CLRF SWITCH

BTFSC PORTA,0

GOTO TSTPRV

BCF SWITCH,0

GOTO TESTIT

BTFSC SWITCH,0

GOTO TESTIT

INCF COUNT,F

MOVF COUNT,W

MOVWF PORTB

BSF SWITCH,0

GOTO TESTIT

CONCEPTS EXAMINED

Increasing speed of TK3TUT8

Bit testing for switch status

CONNECTIONS NEEDED

All Port B to all l.e.d.s.

Port A RA0-RA3 to switches SW0-SW3

(via CP19-CP16)

CP21 to +5V OUT

CP20 to 0V OUT

Capacitor C7 as 1mF

Preset VR1 set to minimum resistance

(fully clockwise)

TSTPRV

LISTING 9 PROGRAM

TK3TUT9

LOOP

BTFSS PORTA,0

GOTO LOOP

INCF COUNT,F

MOVF COUNT,W

MOVWF PORTB

GOTO LOOP

LISTING 8 FLOW

Having described the new terms, we

shall now detail what happens in Listing 8.

As said at the start of Tutorial 7, switches

SW0 to SW3 are biassed so that their

respective PORTA pins are normally at 0V

(low) but go high when pressed. In this

example program, at the label LOOP the

contents of PORTA are copied into W

(MOVF PORTA,W), which then holds the

status of all five usable bits of that port. We

are only interested, though, in the status of

the switch on PORTA bit 0, switch SW0.

Therefore, in the next command (ANDLW

B’00000001’) bit 0 is ANDed with 1 to

isolate its value, the other seven bits in W

being cleared by the respective zeros of the

ANDed value.

The answer in W is then added to the

contents of the counter (ADDWF

COUNT,F). Next, the contents of the

counter are brought back into W (MOVF

COUNT,W) and then copied into PORTB

(MOVWF PORTB), whose l.e.d.s are

turned on or off depending on the binary

In TK3TUT8 we saw that the count

adding commands etc. were performed

even if the count value was zero. This is a

waste of processing speed, why bother to

add zero to a count? The program in

Listing 9 shows a faster alternative. Run

TK3TUT9.HEX.

By using the command BTFSS to check

the status of a switch (in this case still SW0

on PORTA bit 0), if the switch is not

pressed we can avoid the count increment-

ing procedure, jumping immediately to a

further switch status test. Alternatively, in

another program, by substituting another

destination instead of LOOP, we could

jump to a totally different routine and per-

form some other procedure.

Another choice is to use the command

RETURN instead of GOTO LOOP to

return to another routine which had called

this one. Commands CALL and RETURN

will be covered in Tutorial 13.

It is expected that you will recognise

from Listing 9 what the program does and

how it does it. If you don’t, re-read Tutorial

4 and the section on BTFSS.

EXERCISE 8

8.1. What happens if you use BTFSC

instead of BTFSS?

In the switch press examples of

Listings 8 and 9, we saw that the counter

was incremented for the entire duration

of the switch being on. Often, only a sin-

gle response to a change of switch status

might be required. This entails testing

the switch status and comparing it with a

previous test. Only if the switch is on and

if that on condition has not yet been

responded to will the next action be

performed.

Load TK3TUT10.HEX. You are still

monitoring PORTA bit 0 for the switch

press (SW0), responding to it via the l.e.d.s

on PORTB. Observe the l.e.d.s while press-

ing SW0 on and off. For each pressing,

only one change of the l.e.d. count will

occur (but note that low-cost switches may

cause switch-bounce, resulting in the count

increasing for each bounce – a matter cov-

ered later).

Study Listing 10: the entry to the routine

is at BEGIN where two variables, COUNT

and SWITCH are cleared. At the label

TESTIT, the command is BTFSC

PORTA,0, testing the status of PORTA bit

0 (is it clear?). Remember that we are only

interested in the bit being set. If it is false

that bit 0 is clear (i.e. that it is set – the

switch is pressed) the command GOTO

TSTPRV is performed and then the status

of SWITCH bit 0 is tested, BTFSC

SWITCH,0. This bit serves as the flag to

keep track of the previous status of the

switch. At this moment, the bit will be

clear because the whole byte was cleared

on entry to the routine. Consequently, the

GOTO TESTIT command is skipped, the

count is incremented and its value output to

PORTB.

– PIC Tutorial V2 Supplement

Everyday Practical Electronics, May 2003

Now SWITCH bit 0 is set (BSF

SWITCH,0) to indicate that the count has

been incremented for this switch press (i.e.

the flag is set), and the program jumps

back to TESTIT. If the switch is still

pressed, then at TSTPRV the BTFSC

SWITCH,0 command will produce a false

answer and the command GOTO TESTIT

will be performed, thus preventing the

counter from being further incremented at

this time.

What is now needed is for the switch to

be released so that the two commands BCF

SWITCH,0 (clear the flag) and GOTO

TESTIT can occur. The stage is then once

again set for the next switch press to be

responded to by the counter.

LISTING 11

PROGRAM TK3TUT11

BEGIN

TEST1

CLRF COUNT

CLRF SWITCH

BTFSC PORTA,0

GOTO TSTPR1

BCF SWITCH,0

GOTO TEST2

BTFSC SWITCH,0

GOTO TEST2

BSF SWITCH,0

INCF COUNT,F

GOTO OUTPUT

BTFSC PORTA,2

GOTO TSTPR2

BCF SWITCH,2

GOTO TEST1

BTFSC SWITCH,2

GOTO TEST1

BSF SWITCH,2

DECF COUNT,F

MOVF COUNT,W

MOVWF PORTB

GOTO TEST1

switches on PORTA (SW0 to SW3) while

observing the l.e.d.s on PORTB. This rou-

tine should need no further comment.

Another way of expressing the first two

commands is:

LOOP

MOVLW B’00001111’

ANDWF PORTA,W

TSTPR1

Now load TUT13.HEX and run it, again

experimenting with pressing any combina-

tion of the switches on PORTA (SW0 to

SW3) and observing the l.e.d.s on PORTB.

TEST2

LISTING 13

PROGRAM TK3TUT13

LOOP

SWAPF PORTA,W

ANDLW B’11110000’

MOVWF PORTB

EXERCISE 9

9.1. In Listing 10, AND and MOV com-

mands could have been used instead of

BTFSC and BCF. How, and with what

other command?

9.2. Would using BTFSS instead of

BTFSC involve more commands and

labels having to be used as well?

9.3. Because low cost switches have

probably been used, there is the danger that

mechanical switch bounce might occur,

causing the count to be incremented unde-

sirably. Another counter could be used to

cause a delay in the rate of switch testing to

eliminate the effects of switch bounce.

How would you implement the delay, and

where would you put the commands

required. Hint, another label will be need-

ed as well.

TSTPR2

OUTPUT

EXERCISE 10

TUTORIAL 10

CONCEPTS EXAMINED

Performing different functions depend-

ing upon which of two switches is

pressed

The use of a common sub-routine serv-

ing two other routines

CONNECTIONS NEEDED

All Port B to all l.e.d.s.

Port A RA0-RA3 to switches SW0-SW3

(via CP19-CP16)

CP21 to +5V OUT

CP20 to 0V OUT

Capacitor C7 as 1mF

Preset VR1 set to minimum resistance

(fully clockwise)

Run TK3TUT11.HEX and experiment

with the switches on PORTA bits 0 and 2

(SW0 and SW2). You will discover that

switch SW0 causes the count displayed on

the l.e.d.s to be increased, and that switch

SW2 decreases the count. The basic logic

flow is the same as that in Listing 10,

except that two switches are used and each

switch is responsible for a different

routine.

Note that whilst each switch could have

had its own routine to output to PORTB,

the two routines would be the same.

Consequently, each switch routine is rout-

ed into a common output sub-routine

(OUTPUT). At the end of SW0’s routine,

the command GOTO OUTPUT needs to be

given, but at the end of SW2’s routine, no

GOTO OUTPUT command is needed

because OUTPUT follows immediately

after it. It is said to reach OUTPUT by

default

because it does not need to be told

to go there.

10.1. How do you think a single test for

neither

of the switches being pressed could

be introduced, shortening the testing time?

Could an AND be used with a STATUS

check, or can a STATUS check be used on

its own without an AND? (Think carefully

about the latter.)

10.2. How would you increase the count

by more than one, say two, at each press of

switch SW0? With the knowledge you’ve

gained so far, three ways should come to

mind, one of them including the use of a

new named variable.

10.3. If you want to add 255 each time a

switch SW0 press occurs, do you need an

ADD command, or is there another com-

mand which will do the same job? (Think

rollover.)

TUTORIAL 11

CONCEPTS EXAMINED

The ease of reflecting PORTA’s switches

on PORTB’s l.e.d.s!

Command COMF

Command SWAPF

Inverting a byte’s bit logic

Swapping a byte’s nibbles

CONNECTIONS NEEDED

All Port B to all l.e.d.s.

Port A RA0-RA3 to switches SW0-SW3

(via CP19-CP16)

CP21 to +5V OUT

CP20 to 0V OUT

Capacitor C7 as 1mF

Preset VR1 set to minimum resistance

(fully clockwise)

Load TUT12.HEX. Experiment with

pressing any combination of the four

You will see while you press PORTA’s

four switches, that they are having their

status displayed on PORTB’s four lefthand

l.e.d.s (LD7 to LD4), even though you have

not changed the wiring to PORTB and the

l.e.d.s. Had there been a fifth switch, on

PORTA RA4, it would be affecting the first

l.e.d. on the right (LD0) – if a different

AND value were used (what value?).

What is happening is that the software

has been told to swap and move into W

(SWAPF PORTA,W) the left and righthand

four bits of PORTA (its nibbles, as intro-

duced in Tutorial 7). The answer is then

ANDed with bits that reflect the swapped

status in order to remove any possibility of

influence by the unused bits of PORTA’s

The SWAPF command is especially use-

ful if the values of the two nibbles are

required separately as values of up to 15

(00001111). A good example of its use will

be seen in Tutorial 21. It is illustrated now

because of its programming similarity to

TK3TUT12 and TK3TUT14.

The F suffix can be used with SWAPF

instead of W, as with other files discussed.

There is no command which allows nibbles

to be swapped once the byte is in W. If a

byte within W needs swapping, it must be

put out to a file, and then the SWAPF

(FILENAME),W command given to bring

it back into W.

Let’s look now at another command

which uses a similar demonstration routine

to TK3TUT12 and TK3TUT13. Run

TK3TUT14.HEX. Once more, experiment

with pressing any combination of switches

SW0 to SW3 while watching PORTB’s

l.e.d.s.

LISTING 14

PROGRAM TK3TUT14

LOOP

COMF PORTA,W

ANDLW B’00001111’

MOVWF PORTB

GOTO LOOP

LISTING 12

PROGRAM TK3TUT12

LOOP

MOVF PORTA,W

ANDLW B’00001111’

MOVWF PORTB

GOTO LOOP

You will now discover that instead of

l.e.d.s being turned on when a switch is

pressed, they are turned off, and vice versa.

This is due to the command COMF, which

automatically inverts each bit of a byte, 1s

becoming 0s, 0s becoming 1s, i.e. it per-

forms a task known as

complementing,

hence COMF, which means COMplement

File.

Everyday Practical Electronics, May 2003

PIC Tutorial V2 Supplement

–

There are several uses for this command,

one of which is the situation when all the

switches are biased to the +5V line instead

of 0V. In that instance, and using the switch

testing techniques shown earlier, pressing

the switches would produce the wrong bit

levels for the commands shown: switches

would need to be held pressed for off,

releasing them for on. Not an easy thing to

do with push-switches!

Swap over the 0V and +5V connections

to pins CP21 and CP20 so that PORTA

pins RA0 to RA3 are biassed to +VE,

going low when switches SW0 to SW3

are pressed. Run the program again. You

will find that the l.e.d.s respond as they

did for TK3TUT12. Now run TK3TUT12

again and confirm that the l.e.d. results

are the inverse of that previously seen

with it.

Another use for COMF is in subtraction.

This is a concept for experienced program-

mers and will not be demonstrated here. In

a nutshell, the use of COMF allows addi-

tion to be used instead of subtraction while

still achieving the desired objective. This

technique can be easier in some instances

than using the available subtraction

commands.

The F suffix can be used with COMF

instead of W, as with other files discussed.

There is no command which allows the

inversion of a byte once it is in W. If a byte

within W needs inversion, it must be put

out to a file, and then the COMF (FILE-

NAME),W command given to bring it back

into W.

LISTING 15

PROGRAM TK3TUT15

SOUND

GETKEY

MOVLW 80

MOVWF NOTE

MOVWF FREQ

BTFSS PORTA,0

GOTO GETKEY

DECFSZ NOTE,F

GOTO GETKEY

MOVF FREQ,W

MOVWF NOTE

MOVLW B’00010000’

ADDWF PORTA,F

GOTO GETKEY

EXERCISE 11

So far we have been outputting data to

l.e.d.s, and at a comparatively slow rate.

We have also been using one port as a

switch input and the other port as the out-

put. Here we examine how the same port

can be used simultaneously for input and

output via different bits. In doing so, we

use sound as the medium by which we

indicate the status of a switch, generating

an audible frequency when it is pressed.

The 1mF capacitor used up till now for

C7 should be omitted from hereon.

Connect a 330W resistor between RA4

and the +5V connection at CP21. Pin RA4

is an open-collector pin and this resistor

biasses it so that an output can be generat-

ed on it. Connect a 1mF capacitor with its

positive lead on the junction of RA4 and

the resistor. Connect the negative lead of

the capacitor to the signal terminal of a

jack socket that suits your personal (high-

impedance) headphones (see Fig.3).

5V OUT

3.5mm STEREO

JACK SOCKET

(SEE TEXT)

With these exercises, reconnect the +5V

connection to CP21 and 0V to CP20.

11.1 If SWAPF was not available as a

command, how would you write a routine

which produced the same result (would

RLF or RRF be suitable commands)?

11.2

Rewrite

TK3TUT13

and

TK3TUT14, putting the contents of W out

to a file of any name (which you must

equate at the beginning of the program),

performing another COMF or SWAPF

action, and then bringing it back into W

for output to PORTB. Can PORTB be used

as the temporary file store in these

rewrites?

11.3. Write a routine that allows the nib-

bles of a byte to be put into separate files

and each having a value no greater than

H’0F’ (decimal 15); there are several ways

of doing it.

330Ω

1µ

HEADPHONES

When that occurs, file NOTE is decre-

mented and its zero status tested. If it is

not yet zero, the routine jumps back to the

switch test.

When the switch has been pressed for

long enough (mere thousandths of a sec-

ond), NOTE will eventually reach zero, at

which point the command MOVF

FREQ,W is reached, followed by the fixed

value of FREQ being reset into NOTE.

Next, the value in PORTA has 16 (binary

00010000) added to it to increment the

count at bit 4 (so alternating the bit

between 0 and 1), and then there is a jump

back to further switch testing.

For as long as switch SW0 is pressed,

PORTA bit 4 will be periodically incre-

mented. The speed at which the routine

runs causes this bit to change at the audio

frequency rate to which you are listening.

If you adjust the rate setting preset, VR1,

you will hear the change in the resulting

frequency.

In a real-life situation, of course, the

operating frequency of the system would

normally be fixed. One frequency correc-

tion choice then is to change the value of

FREQ.

There is, though, another factor that

will affect the resulting audio frequency:

the number of commands within the con-

trolling loops. To illustrate the point, let’s

change the number of commands

involved.

You may think that to add more com-

mands would be difficult, what would they

do which would not interfere in the com-

pletion of the loop? Well, there are several

options, such as repeating some of the

existing commands, MOVF FREQ,W for

example, or MOVWF NOTE. Neither of

these commands would actually change

anything, except for the rate of operation.

However, a tailor-made command is

already available in the PIC’s command

codes which is intended for use where

minor delay tactics are needed, command

NOP.

TO RA4

0V OUT

COMMAND NOP

Fig.3. Audio output connections.

Do not connect a loudspeaker directly to

this circuit as there is insufficient power to

drive it.

Load TK3TUT15.HEX and press switch

SW0 on and off. A frequency tone will be

heard when the switch is pressed.

In the initialising statements at the head

of the full TK3TUT15.ASM program,

PORTA has been set with bits 0 to 3 as

inputs and bit 4 as an output (MOVLW

B’00001111’, MOVWF TRISA). You

should now recognise all the commands

given in the heart of the program shown.

Only a general commentary on what hap-

pens is now given.

On entry into the routine headed

SOUND, a value of 80 is loaded into the

files named NOTE and FREQ. The value is

arbitrary as far as this demonstration is

concerned. You may choose any other from

1 up to 255; the lower the value, the higher

the frequency generated.

PORTA’s status is monitored at

GETKEY and the setting (logic 1) of

PORTA bit 0 by switch SW0 is being

looked for. Switch testing is repeated

until SW0 is pressed, setting RA0 high.

TUTORIAL 12

Command NOP simply stands for No

OPeration. Responding to this command

takes the PIC just as long as responding to

any other single-cycle command but its

response is to just do nothing!

This command, then, can be used here to

slow down the resulting note frequency.

Insert it immediately before DECFSZ

NOTE,F. When running the amended pro-

gram you will notice that a change in the

output frequency has occurred.

CONCEPTS EXAMINED

Generating an output frequency in

response to a switch press

The use of two port bits set to different

input/output modes

Command NOP

CONNECTIONS NEEDED

All Port B to all l.e.d.s.

Port A RA0-RA3 to switches SW0-SW3

(via CP19-CP16)

Port A RA4 connected as in Fig.3 (audio

connection)

CP21 to +5V OUT

CP20 to 0V OUT

1mF capacitor C7 omitted (from hereon)

Preset VR1 set to minimum resistance

(fully clockwise)

EXERCISE 12

12.1. Experiment with different values for

FREQ. What happens if you set FREQ to

zero – does it stop a note being generated?

Explain the result.

12.2. Experiment with more than one

NOP command in the loop.

12.3. At which other places can you

alternatively insert NOP, and is the fre-

quency change still noticeable?

12.4. Are there any places where you

cannot use NOP?

12.5. When the audio frequency is not

being generated there is the likelihood that

RA4 will be set low, so sinking current

through the 330W resistor. Can the pro-

gram be modified so that this cannot

occur.

– PIC Tutorial V2 Supplement

Everyday Practical Electronics, May 2003

TUTORIAL 13

CONCEPTS EXAMINED

Command CALL

Command RETURN

Command RETLW

CONNECTIONS NEEDED

All Port B to all l.e.d.s.

Port A RA0-RA3 to switches SW0-SW3

(via CP19-CP16)

CP21 to +5V OUT

CP20 to 0V OUT

Preset VR1 set to minimum resistance

(fully clockwise)

results could occur. For example, the fol-

lowing is “illegal”:

PROG1 CALL PROG2

GOTO PROG1

PROG2 GOTO PROG1

This is “legal”, though:

PROG1 CALL PROG2

GOTO PROG1

PROG2 RETURN

When the program returns from a CALL

following a RETURN command, the con-

tents of W are those which were put there

by the last command which used W.

Consequently, you can perform a complex

sub-routine, end up with an answer in W

and, using the RETURN command, return

to the main program with that result still

retained in W.

Command RETLW, though, returns to

the main program with W holding the

value which RETLW has acquired as part

of that command. A literal value is always

specified as part of the RETLW command,

e.g. RETLW 127 or RETLW 0. That value

replaces any other value within W and is

the one which is held in W on the return to

the calling point. The value may be

expressed in decimal, hexadecimal, binary

or as a “named” value equated during pro-

gram initialisation.

To explain Listing 16 then, at LOOP the

sub-routine at PROG1 is CALLed from

where the value held in PORTA is moved

into W. A return is made to the loop where

the next command to be performed is

MOVWF PORTB, after which the GOTO

LOOP command again takes us back to

CALL PROG1 again.

It is important to be aware that PICs

have a limit to the number of calls that can

be nested (calls being made from within

calls). This is due to the PIC’s Stack (the

LISTING 16

PROGRAM TK3TUT16

LOOP

CALL PROG1

MOVWF PORTB

GOTO LOOP

PROG1 MOVF PORTA,W

RETURN

Before looking further into sound gener-

ation, there are several commands that we

should examine. Three of those are associ-

ated with calling sub-routines: CALL,

RETURN and RETLW.

Load TK3TUT16.HEX and experiment

with pressing different combinations of

switches SW0 to SW3 while observing

PORTB’s l.e.d.s.

area that monitors the return addresses

when calls are completed) being limited to

only eight address values. If the Stack

receives more than eight addresses it will

over-write the earlier ones, causing a pro-

gram crash.

There is no way to read or write to the

Stack or to determine how full it is. It is

therefore imperative that if you are using

nested calls then you must keep very care-

ful track of how many you are using. In

such cases consider whether you could

achieve the same result by using GOTO

commands for some of the calls, or by

returning to the previous calling routine

before making the next call.

Run program TK3TUT16 and confirm

that l.e.d.s LD0 to LD3 respond as expect-

ed to the pressing of the four switches.

EXERCISE 13

13.1. Rearrange TK3TUT16 so that

reading PORTA is in the main loop and

outputting data to PORTB is in the called

routine.

13.2. Try adding other commands in the

subroutine, such as AND or ADD.

13.3. Use RETLW as the final statement

in the subroutine, using any literal value of

your choice, verifying its operation!

TUTORIAL 14

Tables

CONCEPTS EXAMINED

COMMANDS CALL,

RETURN AND RETLW

Programs can be written as a series of

sub-routines which can be reached in one

of three ways, directly by default (without

being told to go there), via a GOTO com-

mand, or by a CALL command. (Routing

following automatic detection of an inter-

rupt event is another matter and is dis-

cussed later.)

We have shown several examples of the

first two. Program TK3TUT4 (Tutorial 4)

uses them both: the sub-routine LOOP1 is

entered directly following the initialisation

routine. LOOP2 is also entered directly

from the end of LOOP1. Both LOOP1 and

LOOP2 are then further accessed by

GOTO commands.

However, a CALL command can be

used if one routine needs to make use of

another and then once that has been com-

pleted, for the program to jump back to

continue from the command that follows

the call. The use of sub-routines allows the

same routine to be accessed from many

other areas within the overall program, so

saving on program space.

A second command always has to be

used before the program returns to the call-

ing origin. That command takes one of two

forms, RETURN (which is an obvious

command – return to where you came

from) or RETLW (RETurn to where you

came from with a Literal value held in W).

There is a third return command, RETFIE,

which we shall meet later in connection

with interrupts.

A GOTO command can never be used to

end a sub-routine call – the PIC will con-

tinue to expect a return command and, if

repeated calls to a sub-routine are made

without a RETURN or RETLW command,

it will become confused and unpredictable

CONNECTIONS NEEDED

All Port B to all l.e.d.s.

Port A RA0-RA3 to switches SW0-SW3

(via CP19-CP16)

CP21 to +5V OUT

CP20 to 0V OUT

Preset VR1 set to minimum resistance

(fully clockwise)

LISTING 17 PROGRAM TK3TUT17

TABLE

BANK0

GOTO LOOP

ANDLW B’00001111’

ADDWF PCL,F

RETLW 255

RETLW 1

RETLW ’5’

RETLW 0

RETLW 31

RETLW 193

GOTO OTHER

RETURN

RETLW B’10101010’

RETLW H’C7’

RETLW ’A’

RETLW 65

RETLW ’B’

RETLW ’x’

GOTO OTHER1

MOVF STORE,W

RETURN

MOVF PORTA,W

CALL TABLE

MOVWF PORTB

GOTO LOOP

RETLW STORE

MOVLW 128

ADDWF PORTA,W

RETURN

; AND W with 15

; ADD to PCL

; 0 11111111

; 1 00000001

; 2 00110101

; 3 00000000

; 4 00011111

; 5 11000001

; 6 00100000

; 7 00000111

; 8 10101010

; 9 11000111

; 10 01000001

; 11 01000001

; 12 01000010

; 13 01111000

; 14 10001110

; or 10011110

; 15 00000000

LOOP

OTHER

OTHER1

Everyday Practical Electronics, May 2003

PIC Tutorial V2 Supplement

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