Experiment No. 5: Multiplexed Seven Segment Displays

In this experiment, we are going to learn how to interface more than one 7-segment LED display to a PIC Port using multiplexing technique. We are going to interface a 4-digit common cathode seven segment display to our PIC board. The multiplexing circuit is already built up in the board using 4 transistors and few resistors (Read Make Your Own PIC Development Board). The basic idea of multiplexing is that all seven segment displays are connected to the microcontroller in parallel and the microcontroller alternately prints ones, tens, hundreds, and thousands digits, selecting one at a time. The switching among the digits is so fast that it gives an impression of simultaneous light emission.

Experimental Setup:

1. Connect RA0 through RA3 to 7-Segment Digit Select headers DG1, DG2, DG3, and DG4 using jumper wires.
2. Insert 7FR5641AS 4-Digit Seven Segment module in to its place on the board.

Software:
/*
 * Project name:
     4-Digit UP Counter with Four 7-Segment Display Multiplexing
 * Copyright:
     (c) Rajendra Bhatt, 2009.
 * Description:
     This code is an example of multiplexing 4 Seven Segment Displays.

 * Test configuration:
     MCU:             PIC16F628A
     The common cathode of four seven segment dispalys are
     connected to RA0, RA1, RA2 and RA3
*/
//-------------- Function to Return mask for common cathode 7-seg. display
unsigned short mask(unsigned short num) {
  switch (num) {
    case 0 : return 0x3F;
    case 1 : return 0x06;
    case 2 : return 0x5B;
    case 3 : return 0x4F;
    case 4 : return 0x66;
    case 5 : return 0x6D;
    case 6 : return 0x7D;
    case 7 : return 0x07;
    case 8 : return 0x7F;
    case 9 : return 0x6F;
  } //case end
}
unsigned short i, DD0, DD1, DD2, DD3, NUM  ;
void main() {
  CMCON  |= 7;      // Disable Comparators
  TRISB = 0x00;    // Set PORTB direction to be output
  PORTB = 0xff;    // Turn OFF LEDs on PORTB
  TRISA0_bit = 0;  // RA.0 to RA3 Output
  TRISA1_bit = 0;
  TRISA2_bit = 0;
  TRISA3_bit = 0;
  TRISA4_bit = 1;      // RA.4 is Input only

  NUM   =    0;  // Initial Value of Counter

  do {
  DD0 = NUM%10;  // Extract Ones Digit
  DD0 = mask(DD0);
  DD1 = (NUM/10)%10; // Extract Tens Digit
  DD1 = mask(DD1);
  DD2 = (NUM/100)%10; // Extract Hundreds Digit
  DD2 = mask(DD2);
  DD3 = (NUM/1000);  // Extract Thousands Digit
  DD3 = mask(DD3);

  for (i = 0; i<=50; i++) {
      PORTB = DD0;
      RA0_bit = 1;          // Select Ones Digit
      RA1_bit = 0;
      RA2_bit = 0;
      RA3_bit = 0;
      delay_ms(5);
      PORTB = DD1;
      RA0_bit = 0;
      RA1_bit = 1;        // Select Tens Digit
      RA2_bit = 0;
      RA3_bit = 0;
      delay_ms(5);
      PORTB = DD2;
      RA0_bit = 0;
      RA1_bit = 0;
      RA2_bit = 1;         // Select Hundreds Digit
      RA3_bit = 0;
      delay_ms(5);
      PORTB   = DD3;
      RA0_bit = 0;
      RA1_bit = 0;
      RA2_bit = 0;
      RA3_bit = 1;         // Select Thousands Digit
      delay_ms(5);
      }
      NUM = NUM + 1 ;
  } while(1);                             // endless loop
}

Experimental Output Video: