Multiplexing LED 7 Segment Displays

• Physical Connection -
  • note error in graphic 2N2700 should read 2N7000
  • note; only use two seven segment displays
  • Check resistor size, 220 may be too low, 1k will probably not be bright enough, remember each display is only going 1/2 duty cycle.

• Explanation
  • It is possible to drive multiple LED displays by connecting the displays in parallel. In this case the displays are common cathode. The number that you want to display on the particular LED display is outputted to port B at the same time as the transistor for that display is turned on. The number for the next LED display is outputted to port B at the same time as the transistor for that display is turned on and the other display transistors are turned off.
• Timing
  • As you can see each display is flashed off and on in sequence to display the desired character on the correct LED. The key to having all LED displays appear as if they are all on is to "flash the displays at a rate higher that the visual perception of the human eye. This rate falls between 15 to 50 Hz. depending on brightness and field of view. (some people report perception of flicker at 60 Hz. which is why many computer monitors now operate at a refresh frequency of 75 HZ.)
• Assignment
  • Create a system which will count from 0 to 99 and repeat. This will be displayed on both LED displays multiplex connected to a pic 16f84A.
• Note - Do not use pin RA4 as an output in this assignment. This output is of a type known a s 'open drain'. Go to section 4.1 in the spec sheet and discover the difference between the open drain pin RA4 and the rest of the output pins. What is the difference? What use could you envision for an open drain output?

• New Concepts
  • Timer
  • Interrupt Handling
    • Reference : http://www.mikroelektronika.co.yu/english/product/books/picbasicbook/06.htm#6.1
    • An interupt is a series of instructions whic will be executed whenever an interrupt occurs. When an interrupt occurs the main program execution is suspended until the processor services the interrupt routine. When the interrupt routine is completed program execution returns to the main program at the next instruction from where it was interrupted.
    • There are many types (causes) of interrupts
      • Types (on pic16F84A)
        • Timer
        • Counter
        • external on pin B(0)
        • Change to PortB
      • Other Types (available on models of microcontrollers
        • pwm (pulse width modulation)
        • analog to digital conversions
        • USART - universal serial asynchronous reciever transmitter
        • Capture
        • Compare
        • Comparitor
• Implementing the Timer Interrupt
  • Timer Config command - input the interrupt time
  • Timer Start command
  • (Global) Interrupt Enable must be implemented
• Structure of Program - some changes may be neede here, check Mikrobasic information.
  • setup
    • pic type, oscillator type and frequency
    • define variables
  • write interrupt routine
    • may be done only once in a program (for each type of interrupt)
    • must be placed before the main program
  • set port B and at least A(0),A(1) and A(2) as output
  • set up 7 segment array values (to display numbers)
    • note change to common cathode (1 is on, 0 is off)
  • configure timer
  • start timer
  • enable global interrupt
  • write main program