CSCI1600: Embedded and Real Time Software Lecture 14: Input/Output II Steven Reiss, Fall 2015

Input Management  We can attach inputs to the Arduino  Directly or indirectly  What are the problems  Sampling  Latency  Conditioning  Range, sensitivity, noise

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

Switch Input  Ideal Switch  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

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

Input Issues  Pull-Up  Set Arduino switch mode

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

Response Curves  Different devices react differently  Can be linear, log, …  Can saturate

Response Curves  May be other  Specific to the device

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?

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)

Low Pass Filter

Analog Signals  Include Noise  Need to know S/N ratio  Input should take this into account

Input Issues: Analog to Digital  Analog signals are continuous, digital discrete  Digital signal  Set of bits (8, 12, …)  Binary value represents the voltage level

Analog-Digital Conversion  Range  Highest and lowest possible values (0..5, )  Precision  Number of bits (0..255, , …)  This is all you can tell apart  Sampling Rate  How fast the ADC can sample (its not immediate)  Samples per second

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

Analog to Digital

Input Arrays  Does this work?  What can go wrong

Input Array Issues

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

Acceptable Latency  Reaction should be << 100 ms  100 ms is noticable  How is latency affected by conditioning  How does this affect the sampling rate

Output Array

Output Array Alternatives

Output Issues: Glitches  Intermediate values  Output glitches  Arduino library minimizes these

Homework (for Wednesday)  Read Chapters 8 and 9  Hardware issues are for your enlightenment  We will concentrate on the programming issues