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1. Experiment 2 – Read the status of a switch
We have seen how LED’s can be used as output devices displaying output statuses. What
about reading information into the PICAXE to make decisions on?
1.1 The goal of Experiment 2
The goal of experiment 2 is to read the status of a switch as a digital input and make
decisions based on the status. To illustrate this let’s switch on a LED for 2 seconds every
time a button is pressed.
1.2 The Circuit Diagram of Experiment 2
The circuit diagram of experiment 2 is shown below, for more information on how read a
circuit diagram see Appendix 2 – “How to read a Circuit Diagram?”. Power is
supplied to the PICAXE-18M2 via pin 5 and pin 14. Pin 5 is connected to ground and
pin 14 is connected to the +5V rail.
Note: The J1 Header is a polarised header that plugs into the breadboard as shown in the
sketch of the header beneath the J1 header in the diagram.
The J1 header and resistors R1 and R2 forms the download circuit used to program the
PICAXE circuit. The serial programming cable plugs into the J1 header for
programming. Pin 1 of J1 goes to the Serial Output (C.3) pin and Pin 2 of J1 goes to the
Serial In (C.4) pin. Pin 3 of the J1 header goes to ground. This part of the circuit diagram
will be used in every experiment.
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Pin C.1 will be used as a digital input pin and it is connected to R4 and the Pushbutton
(Non-Latching Switch). The other side of R4 is connected to the 0V rail. The other side
of the Pushbutton is connected to the +5V rail. The resistor R3 and LED1 serves as a
digital output.
IMPORTANT
A digital signal can only have two states, ON (1 or high) and OFF (0 or low). A digital
output like the LED on C.0 can only be ON or OFF. The same applies for a digital input
and the input signal can also only have two values a 1 (high) or 0 (low).
By combining a non-latching switch (pushbutton) and a resister we can create a clever
digital input. When the pushbutton is not pressed the switch is open and the voltage on
input C.1 is at 0V which is the OFF or 0 state. If the button is pushed, the contact closes
and current flows thru resistor R4 and the voltage on input C.1 jumps to +5V which is the
ON or 1 state. By using R4 as a pull down resistor we ensure that the voltage on input pin
doesn’t float around between 0V and +5V which can give unstable and false readings.
When the switch is open the input pin is connected to ground and when the switch is
closed it is connected to +5V.
1.3 The Breadboard layout of Experiment 2
The breadboard layout is shown in the picture below and it is a physical representation of
the wiring diagram on the breadboard. See Appendix 3 – “How to Wire a
Breadboard?” for more information on wiring breadboards.
In the diagram red wires are used for the +5V supply and blue wires for the 0V ground
reference. It is a good idea to try and stick to this rule as far as possible.
Note: The header connector shown is just a representation of the polarised header in your
kit.
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Following the breadboard wiring schedule the circuit diagram can very easily being
constructed on the breadboard. The Breadboard Wiring Diagram is also a handy tool that
can be used for fault finding.
Wiring items from 1 to 16 is the same as in experiment 1 and items 17 to 23 is new to
wire the digital input as shown in the breadboard layout.
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Breadboard Wiring Schedule for Experiment 2
Component
From
To
Comment
PICAXE
PICAXE – pin 1
20e
Placing the
PICAXE
PICAXE
PICAXE – pin 18
20f
Red Wire
24j
+5V rail
+5V supply for IC
Blue Wire
24a
0V rail
0V supply for IC
Resistor – R1
18c
22c
22k Ohm
Resistor – R2
18b
0V rail
10k Ohm
Header – J1
Pin 1
10c
3 pin Polarised Male
Header connector
Header – J1
Pin 2
11c
Header – J1
Pin 3
12c
Wire
10d
21d
J1 – Pin1 to C.3
Wire
11e
18e
J1 – Pin2
Wire
12a
0V rail
J1 – Pin3
Resistor – R3
17h
21h
330 Ohm
LED1 - Red
Anode of LED
17j
Longest Leg
LED1 - Red
Cathode of LED
16j
Flat Side
Blue Wire
16f
0V rail
OV – LED1(Red)
Pushbutton – S1
Top Left Pin
31f
Placement of the
Pushbutton S1
Pushbutton – S1
Bottom Left Pin
31e
Pushbutton – S1
Top Right Pin
33f
Pushbutton – S1
Bottom Right Pin
33e
Resistor – R4
31b
0V rail
10k Ohm
Red Wire
33j
+5V rail
Connect S1 to +5V
Wire
20g
31d
Input on pin C.1
1.4 New Programming Commands used in Experiment 2
Let’s look at a few new commands and elements that we haven’t used before.
Constants
What is a constant used in PICAXE basic programming? A constant is a ‘fixed’
number or string. A number constant can have a value between 0 and 65535,
represented by a word integer. Constants can be declared in four ways: decimal,
hex, binary, and ASCII.
Decimal
Hexadecimal (hex)
numbers are typed directly without any prefix.
numbers are preceded with a dollar-sign ($) or (0x).
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Binary
ASCII text strings
numbers are preceded by a percent-sign (%).
are enclosed in quotes (“).
Examples:
100
; 100 decimal
$64
; 64 hex
0x64
; 64 hex
%01100100
; 01100100 binary
“A”
; “A” ascii (65)
“Hello”
; “Hello” - equivalent to “H”,”e”,”l”,”l”,”o”
If..then
If we want to evaluate variables and then make a decision based on the outcome
the If..then statement is ideal for this application. I am only going to cover the
basic version of the If..then statement there are a lot of variations and
combinations.
Syntax
IF variable ?? value THEN
{code1}
ELSE
{code2}
End IF
The ?? can be any of the following conditions:
=
equal to
is
equal to
<>
not equal to
!=
not equal to
>
greater than
>=
greater than or equal to
<
less than
<=
less than or equal to
Function
The If..then statement compare the variable to the value using one of the above
conditions. If the comparison is true code1 is executed and if the comparison is
false it skips over code1 and executes code2. Remember the statuses of a pin can
also being used by using the input pin variable (PinC.1).
Examples:
IF PinC.1 = 1 then
High C.0
ELSE
Low C.0
END IF
‘{code1}
‘{code2}
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If the value of pin C.1 is equal to 1 (high) then pin C.0 is made high and the
{code2} in the ELSE statement is skipped. If the value of pin C.1 is not equal to 1
(high) then the {code1} is skipped and pin C.0 is made low.
The ELSE part of the statement can be left out and {code1} is only executed if the
condition is true. If the condition is false {code1} is skipped.
IF PinC.1 = 1 then
High C.0
End IF
‘{code1}
This explains the If..then statement in essence there are a number of more
complicated variations, see PICAXE manual 1 for more detail.
1.5 The Program and Code for Experiment 2
The best way to analyse a task or problem is talk yourself through it in normal everyday
language before you start writing code to do the task. By doing that you will understand
the task better and you would have put the events in the correct sequence. All that is left
is to translate your task or problem description in basic programming code. Let’s try that
with the goal of Experiment 2, our talk through would look something like this:
1.
2.
3.
4.
5.
We need to read the status of a pushbutton (digital input)
Evaluate the status of the switch
If the switch was pressed we switch on a LED for 2 seconds
If the switch wasn’t pressed we don’t have to do anything
Repeat the process to make sure every time the switch is pressed the LED is
switched on
The above description explains what must happen and in what sequence. Let’s put that in
code.
Step 1 – How do we read the status of an input pin (pushbutton is a digital input on pin
C.1)? The values of the input pins on port C are stored in the pinsC variable and the
value of a specific pin like C.1 is available in variable pinC.1.
Step 2 – How do we evaluate the value of pin C.1 in variable pinC.1? There are a few
options but the most suited one is the IF..THEN statement. Using the IF..THEN statement
we get:
IF pushbutton is pressed THEN
Do something
ELSE
Do nothing
END IF
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How do we test if the switch was pressed? We have to look at the circuit of the digital
input, in our case if the pushbutton is pressed we get a high (1) input on pin C.1, see page
1 for the circuit diagram. Substituting the pin variable and pin value our IF..THEN
statement will look like:
IF pinC.1 = 1 THEN
Do something
ELSE
Do nothing
END IF
If the pinC.1=1 is true then the ‘Do something’ code is executed and then the ‘Do
nothing’ code after the ELSE is skipped over without executing it. If the pinC.1=1 is
false then the ‘Do something’ code is skipped over without executing it and the ‘Do
nothing code’ is executed.
Step 3 – How do we switch on a LED for 2 seconds? We have already done this in
experiment 1. We use the High C.0 command to switch on the LED on pin C.0 followed
by a 2 second pause to keep it on for two seconds and then we switch it off with a Low
C.0 command. Let’s add that to our IF..THEN statement:
IF pinC.1 = 1 THEN
High C.0
‘Switch on LED
Pause 2000 ‘Pause for two seconds
Low C.0
‘Switch off LED
ELSE
Do nothing
END IF
Step 4 – If the pushbutton wasn’t pressed we don’t do anything. In our IF..THEN
statement the ‘Do nothing’ code is empty.
IF pinC.1 = 1 THEN
High C.0
‘Switch on LED
Pause 2000 ‘Pause for two seconds
Low C.0
‘Switch off LED
ELSE
‘Nothing to do
END IF
We can leave out the ELSE part of the IF..THEN statement to simplify
IF pinC.1 = 1 THEN
High C.0
‘Switch on LED
Pause 2000 ‘Pause for two seconds
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Low C.0
END IF
‘Switch off LED
Step 5 – The current code will only execute once and then the program will stop. How do
we repeat the process to make sure every time the pushbutton switch is pressed the LED
is switched on? We use a GOTO command with a LABEL address to the beginning of
the main code.
Main:
IF pinC.1 = 1 THEN
High C.0
‘Switch on LED
Pause 2000 ‘Pause for two seconds
Low C.0
‘Switch off LED
END IF
GOTO Main
Now the pushbutton will be evaluated continuously and if the button is pressed the LED
will be switched on.
The code does what it should but it is difficult to read it with all the variables and
constants included in the main code. Let’s improve the readability of the code:
We know the pinC.1 variable represents the status of the pushbutton, pin C.0 the LED
and the constant 1 the ‘Pressed or ON’ status of the pushbutton. We can use the Symbol
command to rename these to make more sense in our program in the Init part of our
program. When we loop back we don’t need to initialise the symbols again and we can
just start at the beginning of the program at label “Main”.
Init:
Symbol RED_LED = C.0
Symbol PushButton = PinC.1
Symbol Pressed = 1
‘Rename port C.0 to RED_LED
‘Rename PinC.1 variable to PushButton
‘Rename constant 1 to Pressed
Main:
IF PushButton = Pressed THEN
High RED_LED
Pause 2000
Low RED_LED
END IF
GOTO Main
End
‘Switch on LED
‘Pause for two seconds
‘Switch off LED
‘Loop back to beginning
‘End of Program
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The final code for experiment 2 is shown above.
1.6 Conclusion and application of knowledge learned in
Experiment 2
In Experiment 2 we have learned how to create a digital input with a switch and a
resistor. We have also learned how to read the value of the digital input and then make
decisions based on the value of the input and run conditional code.
So what is the application of what we have learned? The application is we can bring
information in digital format in from the outside world (outside the microcontroller), do
some processing on it and send out outputs to do something else based on the inputs.
There are thousands of applications in everyday life, take for instance a washing
machine. It has a water level sensor as an input and an inlet valve as output. Water is let
in until the level sensor indicates it is full, the output valve is then closed shutting off the
water.
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