Video Game Power Limiter

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Video Game Power Limiter

• Group 20
• Zuhaib Sheikh Eric Sands
• Senior Design
• Fall 2005

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Introduction

• Designed to help parents control their childrens
  gaming hours.
• Can also be used for other household appliances
  like heaters, table lamps, and coffee makers.

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Inspiration

• A new parent expressed his concern in controlling
  his newborns future in this technologically
  advanced world.
• He suggested a device that would limit his
  childs game playing hours.

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Features

• weekly time-settings
• 7 programmable slots a day
• separate settings for each device
• a beeping system to warn the player about the
  imminent power cut off
• A casing to prevent the cords from being removed
  to bypass the settings of the device

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External View
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Internal Prototype View
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Circuit Schematic
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Main Circuit

• PIC18F452 Microprocessor
• 40 MHz Oscillator
• Voltage Regulator Circuit
• Keypad Encoder
• 1X20 LCD Character Display
• Buzzer

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PIC18F452

• Can sink/source up to 25mA
• Operating Voltage 4.2V 5.5V _at_ 40MHz
• Typical Power Useage 1W

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Voltage Regulator Circuit

• Output
• 50.2 VDC
• 8mA(max)

• Voltage Regulators
• Input
• 7-20VDC
• 5mA 1A

• 150mAh 9V rechargable battery
• 0.3mA Trickle charge current
• (12V 9V)/10kO 0.3mA
• 150mAh/0.3mA 500h charge time

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Voltage Regulator Circuit
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Keypad Encoder

• Encoder MM74C922N
• Debouncing Time 0.01s
• Used a 0.1uF Capacitor
• Keypad Scanning Rate 600 Hz
• Used a 1uF Capacitor

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Keypad and Encoder Circuit
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LCD

• One line with 20 characters
• 4-bit data interface
• Draws maximum current of 3mA
• Drawback
• Needs initialization every time power is turned
  on
• Solution Interrupt PIC and initiate the LCD when
  power is restored

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Solid State Relays

• Optically Coupled
• LED indicator
• Input
• 3-32VDC
• 16mA(maximum)
• Output
• 40-280VAC
• 25A rated

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Relay Circuit
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• Programming the PIC

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Choice of PIC18F452

• Large Program Memory
• Total 16384 instruction words
• Used 12200 instruction words (74)
• PIC16 series only provide a maximum of 8Kwords of
  Program memory
• Data Memory
• Total 1536
• Used 34-37

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Initial Concerns

• PIC18 series introduced the 32 Kbytes programming
  memory size
• How would the PIC18F452 handle a massive program?
• Stability of the PIC to unwanted interrupts and
  noise
• How to cope with the initial self-restarting
  issues?

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Objectives

• User-friendly interface
• Robustness in face of power_INT (power failure
  interrupt)
• Real time synchronization of Timer0_INT and
  keypad_INT
• Stability of the Program code

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Choice of Clock

• 40 MHz Clock
• Average execution time 25ns
• Can execute 20 million instructions before the
  next timer interrupt takes place
• Timer INT takes place every 500ms
• Worst case execution
• (approx. 10000 assembly insts.) 2.5ms ltlt
  500ms

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Timer Module
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Working

• Timer0 module
• External Clock / 4 pre-scaled by 256
• Increments every 25.6 us
• Count from 0x6769 till 0xFFFF every time Timer0
  INT occurs (every 500ms)
• INTERRUPT on OVERFLOW

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Power Interrupt

• Every time the mains power goes off the LCD and
  the Keypad are turned OFF
• So the PIC has to ensure program execution jumps
  to the Display mode from anywhere else

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Interrupt Priority Levels

• Timer0 interrupt gets the highest priority
• Timer0_INT gt Power_INT gt Keypad_INT
• PROBLEMS Couldnt handle re-entrancies for
  functions called outside the Interrupt Service
  Routines
• Small Stack Space in PIC18F452
• Time lost in function calls from within the
  Timer0 INT
• SOLUTION Separate declarations of functions used
  inside the Interrupt Service Routines

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Addressing the Initial Concerns

• PIC18F452 very effectively handled large program
  size
• Did not exhibit any unwanted behavior in view of
  the large program size
• Program size within the allocated 16Kwords
  without having to go for extended memory
  allocation

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The Master Clear Capacitor

• Initially, the PIC was restarting on its own - in
  addition to some really unwanted behavior
• A filter capacitor between the ground and the
  MASTER Clear Pin solved the problem
• Difference between SUCCESS and FAILURE

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Without the Capacitor
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With the Capacitor
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Clock pulse
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Testing and Debugging

• Looking for Corner cases that could de-stabilize
  the operation
• Reduce the minute length
• 1 Minute 1 second
• Long run time with a Short Minute
• Transition from one day to another

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Future Development

• Cost Reduction
• Smaller relays
• Possibly rated at 5A instead of 25A
• GFCI Receptacles for greater safety
• Logic controlled locking mechanism
• Internal Power supply to transform 120VAC to
  12VDC