Microprocessor Fundamentals
Topic 13
Square Waves, Frequencies, &
Duty Cycles
Objectives
• Gain a better understanding of the ATmega128
and assembly language programming
• Developing output waveforms (square waves)
– Specific frequency
– Specific duty cycle
– Multiple square waves
1/4/2010
2
Task
• Suppose:
– We want to output a square wave
• With a specific frequency
• With a specific duty cycle
– What would we have to do?
– How would you write the program?
1/4/2010
3
Task
• Suppose:
– We want to output a square wave
• With a specific frequency 10 kHz
• With a specific duty cycle 50% duty cycle
– What would we have to do?
– How would you write the program?
We’ll consider applications soon
1/4/2010
4
Square Wave
tH
T
f =
tL
1
T
tH
tH
DutyCycle =
* 100% = * 100%
tH + tL
T
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5
Square Wave
tH
T
tL
1
1
T= =
= 100µs
f 10k
tH =
1/4/2010
DutyCycle* T
100%
50% * 100µs
=
= 50µs
100%
6
The Plan
1. Initialize ports
2. Set the output bit
1. Delay for 50µs
3. Clear the output bit
1. Delay for 50µs
4. Go back to step 2
1/4/2010
7
Square Wave
tH
T
tL
1
1
T= =
= 100µs
f 10k
tH =
DutyCycle* T
100%
50% * 100µs
=
= 50µs
100%
Exercise: What would be the time high and time low
if you needed a 5kHz, 25% duty cycle square wave?
1/4/2010
8
Square Wave
tH
T
tL
1
1
T= =
= 200µs
f 5k
tH =
DutyCycle* T
100%
25% * 200µs
=
= 50µs
100%
t L = T − t H = 150µs
1/4/2010
9
Task
• Suppose:
– We want to output 2 square waves simultaneously
• Each with a specific frequency
• Each with a specific duty cycle
– What would we have to do?
– How would you write the program?
1/4/2010
10
Square Wave
f = 1kHz
f = 2kHz
This is what we need. How would you approach it?
1/4/2010
11
Square Wave
f = 1kHz
f = 2kHz
250µs
250µs
250µs
250µs
I would divide the waveforms into segments at transitions in the waveforms. In
this example it works out very well.
1/4/2010
12
Square Wave
f = 1kHz
f = 2kHz
Waveform 1 on bit0
Waveform 2 on bit1
250µs
250µs
250µs
250µs
1. Output 1 on b0 and 1 on b1 for 250 µs
2. Output 1 on b0 and 0 on b1 for 250 µs
3. Output 0 on b0 and 1 on b1 for 250 µs
4. Output 0 on b0 and 0 on b1 for 250 µs
5. Go back to step 1
1/4/2010
13
Task
• Suppose:
– We want to output 2 square waves simultaneously
• Each with the same frequency
• Each with the same duty cycle
• 90° out of phase
– What would we have to do?
– How would you write the program?
1/4/2010
14
Square Wave
W1
W2
This is what we need. How would you approach it?
1/4/2010
15
Square Wave
W1
W2
50µs
50µs
50µs
50µs
I would take the same approach as before and divide the waveforms into segments
at the transitions.
If both waveforms were 5kHz then we would need 50μs
1/4/2010
16
Task
• Suppose:
– We want to output 3 square waves simultaneously
• Each with different frequencies
• Each with different duty cycles
• All out of phase with the others
– What would we have to do?
– How would you write the program?
1/4/2010
17
Task
• Suppose:
– We want to output 3 square waves simultaneously
• Each with different frequencies
• Each with different duty cycles
• All out of phase with the others
– What would we have to do?
– How would you write the program?
Now it’s getting a little complicated! At this point we
may want to consider using multiple timers and
interrupts.
1/4/2010
18
Summary
• In this topic we:
– Gained a better understanding of the ATmega128 and
assembly language programming
– Developed output waveforms (square waves)
• Specific frequency
• Specific duty cycle
• Multiple square waves
1/4/2010
19