1. CSCI1600: Embedded and Real Time Software
Lecture 16: Input/Output II
Steven Reiss, Fall 2016

2. Simon Game Tasks Detect buttons Initialize Reset
Periodically (every 10 ms) check 3 sets of buttons
Set flags indicating on for > 20 ms
Initialize
Based on selected game flag
Set up play array play[32], play_length
Set up state time, play time
Reset
If start is ever pressed, go back to initialization state
Lecture 16: Input/Output II
12/6/2018

3. Simon Game Tasks Game Task
Alternate between play and read
More sophisticated for 2 player
Play: incr ctr, play current sequence through counter
Handle illegal inputs
Set light and sound variable accordingly
Increment counter every k ms
Reset counter when done, enter read
Read: incr ctr, read input for current sequence element
Time out (> k sec in state) go to error
Input matches  loop back
Input doesn’t match  go to lose state
ctr == length, go to play state with next element or to win state
Lecture 16: Input/Output II
12/6/2018

4. Simon Game Tasks Win/Lose task Play sound task (run every 1/8 ms)
Choose sequence based on win/lose
Play that sequence
Afterwards go to start state
Play sound task (run every 1/8 ms)
Based on sound variable
Set output at proper frequency
Lights task
Set light based on light variable
Lecture 16: Input/Output II
12/6/2018

5. Simon Tasks What do the models for these tasks look like
What will the code look like
The code should reflect the models
The models should reflect the code
Lecture 16: Input/Output II
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6. Input Management We can attach inputs to the Arduino
Directly or indirectly
What are the problems
Sampling
Latency
Conditioning
Range, sensitivity, noise
Lecture 16: Input/Output II
12/6/2018

7. A simple switch You want to read the switch
Can’t do it continually – you need to sample
How fast should you sample?
What does this depend upon?
Minimum On time
Minimum Off time
Bouncing
Lecture 16: Input/Output II
12/6/2018

8. Switch Input Ideal Switch Minimum inter-event sampling time
Assume min on-time is 2 units
Assume min off-time is 1 units
What is the minimum sampling rate
What is a safe sampling rate
Minimum inter-event sampling time
Might need to compute these values
How to check min on time given sampling rate
Lecture 16: Input/Output II
12/6/2018

9. Switch Input Real switches bounce
Input takes some time to stablize
Possibly ms
What happens if you sample faster than that
Debouncing
Sample > 50 ms
Condition the inputs
Check it is on/off for at least k samples
Lecture 16: Input/Output II
12/6/2018

10. Input Issues Pull-Up Set Arduino switch mode
Lecture 16: Input/Output II
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11. Sampling Actual Signals
Actual signals are continuous
Digital inputs are discrete
Certain number of values
This determines the accuracy of your input
Sensors have different response curves
Lecture 16: Input/Output II
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12. Actual Signals Microphone Thermistor Light Sensitive Diode
Lecture 16: Input/Output II
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13. Response Curves Different devices react differently
Can be linear, log, …
Can saturate
Lecture 16: Input/Output II
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14. Response Curves May be other Specific to the device
Lecture 16: Input/Output II
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15. Sampling Changing Inputs
Suppose we sample audio
How fast should we sample?
Need to avoid aliasing
Need to be > 2 times the maximum input frequency
What is audio range?
What is a CD range?
Lecture 16: Input/Output II
12/6/2018

16. Aliasing Suppose there is a high-frequency component to the input
What would this show up as in low-frequency sampling?
Need for a low-pass (anti-aliasing) filter
Can be done in software (if you sample fast enough)
Very easy to do in hardware (capacitor and resistor)
Lecture 16: Input/Output II
12/6/2018

17. Low Pass Filter
Lecture 16: Input/Output II
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18. Low Pass Filter
Lecture 16: Input/Output II
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19. Analog Signals Include Noise Need to know S/N ratio
Input should take this into account
Lecture 16: Input/Output II
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20. Input Issues: Analog to Digital
Analog signals are continuous, digital discrete
Digital signal
Set of bits (8, 12, …)
Binary value represents the voltage level
Lecture 16: Input/Output II
12/6/2018

21. Analog-Digital Conversion
Range
Highest and lowest possible values (0 .. 7, )
Precision
Number of bits (0..255, , …)
This is all you can tell apart
Sampling Rate
How fast the ADC can sample (it’s not immediate)
Samples per second
Lecture 16: Input/Output II
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22. How a ADC works Successive approximation Controller guesses next value
DAC converts guess to analog value
Comparator sees compares input and reference value
Controller takes result and makes next guess
Lecture 16: Input/Output II
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23. Analog to Digital
Lecture 16: Input/Output II
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24. Input Arrays Does this work? What can go wrong
Lecture 16: Input/Output II
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25. Input Array Issues
First note that if no switch is pressed, noting is connected to the input ports – might get random values. Hence diagram 2
Second note what happends if switches 3,5,6 are pressed – it will look like 2,3,5,6 are all on. Hence the use of diodes as in second figure
Lecture 16: Input/Output II
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26. Input Array Issues
In the second case the pins serve double duty depending on their polarity. 4 pins used for 12 switches
What happens with multiple switches on the first example. R1*R1/(R1+R2).
What type of precision would you need on the analog input for 8 switches. What about pair-wise?
Lecture 16: Input/Output II
12/6/2018

27. Latency Difference in time between input and corresponding output
What if user pushes switch for his TTT move
The machine does computation to determine its move
Then it turns on both lights
Would this work?
Difference between setting output and actual output
Motor won’t react immediately
Lecture 16: Input/Output II
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28. Acceptable Latency Reaction should be << 100 ms
100 ms is noticeable
How is latency affected by conditioning
Filtering, waiting for stability, eliminating noise
How does this affect the sampling rate
When latency is important
Processing sound, other multimedia
Detecting events (button presses, mouse movement, …)
Lecture 16: Input/Output II
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29. Other I/O Issues To be covered later Communications between components
Serial I/O and timing
Handshaking Protocols
Lecture 16: Input/Output II
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30. Homework Read Chapters 8.2 and 9 Exercises 8.3, 9.2, 9.3
Other parts of 8 might be interesting (not covered)
Exercises 8.3, 9.2, 9.3
Lecture 16: Input/Output II
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31. Output Array This lecture might be short. Lecture 16: Input/Output II
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32. Output Array Alternatives
First is shift register with serial input
Second is latch register with parallel input
Lecture 16: Input/Output II
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33. Output Issues: Glitches
Intermediate values
Output glitches
Arduino library minimizes these
Lecture 16: Input/Output II
12/6/2018