1. Robocon 2007 Electronics Quickstart!
Session 3
Switch Reading & State
Prepared by
KI Chi Keung [chikeung＠ust.hk]
WONG Long Sing (Sam) [sam＠hellosam.net]

2. Today’s Timeline Reading a button State Debouncing
Level / Edge Detection
State
Edge Detection
Finite State Machine

3. Bouncing Switch takes a while to settle down 5V 0V ~1ms Point of time
when user press the button

4. Hardware Debouncing By using a capacitor 5V 0V ~1ms Point of time
when user press the button

5. Software Debouncing By using delay 5V 0V
Ignore the pin reading for some time after being triggered 5V 0V
~1ms
Point of time
when user press the button

6. Software Debouncing
Human cannot press switch at 1kHz anyway (Or the switch will probably break)
Read less frequently, 20Hz may be a good choice.
Time of sampling 5V 0V
~1ms
Point of time
when user press the button

7. Level Detection With simple if-then code, you get level detection.
Events
Clock 5V
High Level 0V
Low Level

8. Edge Detection But most of the time you want to do edge detection.
Falling Edge Events
Clock 5V 0V
Rising　Edge
Falling Edge

9. Task Description Change the LED status once when the button is pushed.
Can be solved in two ways
Simple Edge Detection
Finite State Machine

10. Simple Edge Detection Save the previous state
Compare with current state
Profit!

11. Finite State Machine
To facilitate edge detection, you need state machine.

12. Finite State Machine
Less state is good. Let’s simplify it.