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Algorithms and Flowcharts
ECET 209 – Lecture 7Introduction to Microcontrollers
ECET 209 Purdue University 2
Overview
• Example Algorithms• Flowcharts and C Code
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Formal Problem Solving Steps
• Understand the Problem• Develop an Algorithm • Refine the Algorithm• Create an Action Plan• Generate a Solution• Test the Solution
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Algorithms
• Step by step method to solve a problem• Must include ALL required information
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Flowcharts
• Graphical representations of algorithms• Few basic symbols
– Terminal– Processes– Decisions
• Tool to translate algorithms into software– Flowcharts of C structures– Rules for Structured Flowcharting
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Flowcharting
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Decisions
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Decisions
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Decisions
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Decisions
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For Loop vs. While Loop
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Decision
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So Many Decisions,How do I choose?
• Flexibility – For Example
• Any For Loop can be expressed as a While Loop• Any Case Switch structure can be replaced by a
series of IF/ELSE structures
• The Flowchart is your guide!
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Follow the Flowchart
• May have to redraw the flowchart several times to get it into a form that fits one of the C structures.
• Rules to Flowcharting– A complex task can be shown as a single block– Start with the simplest flowchart possible– Any process can be replaced by a sequence– Any process can be replaced by a control
structure
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Flowchart Rules
Rule 1Rule 3
Rule 2
Rule 3
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Flowchart Rules
Configure PortC for Output
Configure PortA for Input
Turn on PortA Pull-ups
Configure I/O PORTS
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Averaging Example
Averaging a set ofnumbers
Calculate theAverage of the 6
Numbers
End
Averaging a set ofnumbers
End
sum = 0
i = 0
i = i + 1
sum = sum + ni
Does i = 6?
No
Yes
Average = sum / 6
Calculate the Sum ofthe Numbers
Divide the Sum by theNumber of Entries
Average a set ofNumbers
End
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Averaging a set ofnumbers
End
sum = 0
i = 0
i = i + 1
sum = sum + ni
Does i = 6?
No
Yes
Average = sum / 6
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Relational Operators
• Determine how one value relates to another– Equal to ==– Not equal to !=– Less than <– Greater than >– Less than or Equal to <=– Greater than or Equal to >=
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True vs. False
• The result of any relational operation is a True or a False indication
– False is defined as a Zero value
– True is defined as Not False( if it anything other than zero, it is true!! )
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More Likely Examples
( number_of_dogs > 3 )
( value != 0 )
( counter > 10 )
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Relational Operators are Typically used with Decisions
• For example…
Problem:Light the upper nibble of the LEDswhen the counter is above 10.
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
What is this??
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
if
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
ifWhat??
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
if ( counter > 10 )
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
if ( counter > 10 )
{
}
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
if ( counter > 10 )
{
}
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
if ( counter > 10 )
{
}
PORTC = 0xF0;
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
if ( counter > 10 )
{
}
PORTC = 0xF0;
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How do we get from the Flowchart to the C Code??
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
Yes
No
if ( counter > 10 )
{
}
PORTC = ~0xF0;
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Translated to C
if ( counter > 10 ){
PORTC = ~ 0xF0; // turn on LEDs}
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What if the condition isn’t true??
• We may or may not want to do something if the expression evaluates as False
– The False leg is known as the Else
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Problem Revisited
Problem:Light the upper nibble of the LEDswhen the counter is above 10. When the counter is equal to or below 10, light only the least significant bit of the LEDs.
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Updated Flowchart
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
YesNo
Turn on the LSBof the LEDs
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Updated Flowchart
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
YesNo
Turn on the LSBof the LEDs
{
}
else
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Updated Flowchart
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
YesNo
Turn on the LSBof the LEDs
{
}
else
PORTC = ~0x01;
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Transformation to C Code
Is the countervariable greater
than 10 ?
Turn on the upperfour LEDs
YesNo
Turn on the LSBof the LEDs
else
if (counter > 10)
{{
} }PORTC = ~0x01; PORTC = ~0xF0;
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Updated C Code
if ( counter > 10 ){
PORTC = ~ 0xF0; // turn on LEDs}else{
PORTC = ~ 0x01; // turn on LSB}
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Updated C Code
if ( counter > 10 ){
PORTC = ~ 0xF0; // turn on LEDs}else{
PORTC = ~ 0x01; // turn on LSB}
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Updated C Code
if ( counter > 10 ){
PORTC = ~ 0xF0; // turn on LEDs}else{
PORTC = ~ 0x01; // turn on LSB}
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