Sunday, December 21, 2008

6 Digits LED 7-Segment Multiplexing

In the post 'LED 7-Segment Multiplexing', I have explained about the concept and benefits of multiplexing. Multiplexing implementation is very similar to driving Led Dot Matrix. I use Timer0 interrupt for switching through each digit. Timer0 or TMR0 is an 8-bit timer which overflows every 256 (0xFF) counts. It's known that the refresh rate above 50Hz would be enough for human's eyes to see the display without recognizing the flickering. If I set TMR0 with 1:8 Prescaler, the multiplexing frequency will be

4MHz(internal OSC.)/4(working OSC)/8(prescaler)/256(max counts of TMR0)/6(number of digits) = 81.3 Hz which is good for a display.

Just an example, I have implemented (in Proteus) a 999999-second counter by using 6 Digits LED 7-Segment Multiplexing technique. There are 2 main components in the project, PIC16F627A or PIC16F628 and 6 x LED7-segment display. The schematic shows below. The crystal is 32.768KHz as usual. There is a 10KOhm pull up resistor at RA4 pin as this pin is an open-drain pin as I described in "Open-Drain RA4 pin on PIC Microcontroller".

7-Segment LED Multiplexing PIC16F627A or PIC16F628
The source code in MikroC is listed below: (.hex is also available, please feel free to contact me)
//PIC16F627A
//4MHz Internal OSC
//MUX by the MUC itself with Interrupt
//TMR0 .. check the prescelar+delay in scan routine as they are related
//punkky@gmail.com
unsigned short number [10] = {
    0x5F0x060x9b0x8f0xC60xCd, 0xDD0x07,
    0xDf, 0xCf
};
unsigned short digit [6];
unsigned short counter;
unsigned short shift_register;
unsigned short x1;
unsigned short x2;
unsigned short x3;
unsigned short x4;
unsigned short x5;
unsigned short x6;
unsigned short tick;
void interrupt ()
{
    if (INTCON.T0IF)
    {
        //Scan digits with TMR0
        INTCON.T0IF = 0;
        if (counter == 5)
        {
            PORTA = number [digit [counter]];
            Delay_us (500);
            shift_register = 0x01;
            PORTB = ~shift_register;
            PORTA = 0x00;
            counter = 0;
        } else
        {
            PORTA = number [digit [counter]];
            Delay_us (500);
            shift_register = shift_register << 1;
            PORTB = ~shift_register;
            PORTA = 0x00;
            counter ++;
        }
    }
    if (PIR1.TMR1IF)
    {
        TMR1H = 0x80;
        PIR1.TMR1IF = 0;
        tick = 1;
        //update current time
        x6 ++;
        if (x6 > 9)
        {
            x6 = 0;
            x5 ++;
            if (x5 > 9)
            {
                x5 = 0;
                x4 ++;
                if (x4 > 9)
                {
                    x4 = 0;
                    x3 ++;
                    if (x3 > 9)
                    {
                        x3 = 0;
                        x2 ++;
                        if (x2 > 9)
                        {
                            x2 = 0;
                            x1 ++;
                            if (x1 > 9)
                            {
                                x1 = 0;
                            }
                        }
                    }
                }
            }
        }
    }
}
void main ()
{
    //Digital I/O for PORTA
    CMCON = 0x07;
    TRISA = 0x00;
    PORTA = 0x00;
    TRISB = 0x00;
    PORTB = 0x00;
    //Internal Clock 4MHz
    PCON.OSCF = 1;
    counter = 0;
    // Enable TMR0
    OPTION_REG.T0CS = 0;
    // Enable Prescaler
    OPTION_REG.PSA = 0;
    // PS0,1,2 = 010 = 3
    // 3 means 1:8 prescaler
    // 1:2, 1:4, 1:8, 1:16, 1:32, 1:64, 1:128, 1:256
    OPTION_REG.PS2 = 0;
    OPTION_REG.PS1 = 1;
    OPTION_REG.PS0 = 0;
    INTCON.T0IF = 0;
    INTCON.T0IE = 1;
    INTCON.GIE = 1;
    INTCON.PEIE = 1;
    T1CON = 0x0F;
    TMR1H = 0x80;
    TMR1L = 0x00;
    // Enable TMR1 interrupt
    PIE1.TMR1IE = 1;
    shift_register = 0x01;
    x1 = 0;
    x2 = 0;
    x3 = 0;
    x4 = 0;
    x5 = 0;
    x6 = 0;
    while (1)
    {
        if (tick)
        {
            tick = 0;
            //update digits
            digit [0] = x1;
            digit [1] = x2;
            digit [2] = x3;
            digit [3] = x4;
            digit [4] = x5;
            digit [5] = x6;
        }
    }
}

3 comments:

PaulWoody said...

Could you also save several pins by selecting the appropriate CD4543 using a Decade Counter that reset after the sixth output?

You would only need one output from the PIC which clocked the decade counter.

I am a newbie and have often wondered if that would work - I keep meaning to try it.

tony said...

Sorry, my English is not good, hope you will understand !

Will this circuit on the possibility of changing the up counters, repeatedly push switch, display will be 000001 until 999999

Please guide, thanks you

Petru said...

i need to do an electronic speedometer using pic16f876. my big problem is when i try to do the counter. Could u tell me if i can adapt this code to my need?
thx in advance.