Display Multiplexing

[cne]

Hello,

I am designing a scoreboard that uses seven segment led displays. I am using a PIC18f2220 chip.
My question is, what is the best hardware setup to multiplex 15 displays?

Thanks in advance,

Cameron

 

[Mosaic]

15 digits?

At what current per segment (continuous rated)?

 

[be80be]

15 of these
**broken link removed**

 

[cne]

I am prototyping with small displays (.5"), but when I build the final version it will have 5" high digits.

Prototype
.5" Green Displays
30mA forward current

Final Version
5 led's in series per segment
25mA each
Higher Supply Voltage (<=12v)

 

[be80be]

You need a bigger chip or use shift registers I would just use big brother

PIC18F4220

https://engknowledge.com/microcontroller_interfacing_multyplexing_7_segment.aspx

 

[cne]

In my current setup, I am using a 4543 BCD driver chip to reduce I/O. The problem is, the displays are very dim.

 

[be80be]

The HCF4543 can't handle the load If you look at the data sheet it has limited output of 40 ma max for the whole chip. You need some thing more like a 74hc595
which would give you 35ma a pin

 

[blueroomelectronics]

Yep a 1/15 duty is going to be dim. Your 5" digits might need more than a 5V supply too.
If you can use HE red you might require less current.

 

[Mike - K8LH]

If your 5 inch displays are common cathode then you can use high current MIC5891 serial-to-parallel "sourcing" drivers which have a "VBB" pin specifically for displays with multiple LEDs per segment that need higher voltage drive. With my MacMux designs you would need 6 pins (you would only need three of the four 5-digit display modules in the drawing below). Advantages are full-brightness display with PWM brightness control. A relatively high refresh rate and 20% duty cycle per digit.

Cheerful regards, Mike

 

[Mosaic]

Love that cct mike!

For a CA display u can use this one
https://www.electro-tech-online.com/custompdfs/2010/11/tpic6b595.pdf

 

[wkrug]

I've built a DCF77 Clock with 6 Digits 10cm 7Segement Displays with common Anode.

As Driver for the segments I've used an ULN 2003.

To drive the Anodes of the Digits there is a P-Channel Mos Fet, driven By an N-Channel Mos Fet ( BS 170 ).
Thats needed by using 12V for the Display Voltage.
The Controller works only with 5V.
Alternative it could be possible to use as Anode driver one from the UDN... Series.

To generate the Characters there is a Look Up Table into the Controller.
At Place 0 are the activated Digigts for "0".
At Place 1 are the digits for "1" and so on.

The Multiplexing of the Anodes is done in an Timer Interrupt Routine.
So all the Digits have the same Brigtness.

The Multiplexing goes from right to left. The opposite Direction is possible also of course.

The Lookup Table:

// Code Table for 7 Segment Display
//0,1,2,3,4,5,6,7,8,9,A,b,c,d,E,F,h,J,L,n,o,P,r,u,[]
volatile unsigned char uc_chartable[25]={
0b00111111,/*0*/
0b00001100,
0b01011011,
0b01011110,
0b01101100,
0b01110110,
0b01110111,
0b00011100,
0b01111111,
0b01111110,/*9*/
0b01111101,/*A*/
0b01100111,/*b*/
0b01000011,/*c*/
0b01001111,/*d*/
0b01110011,/*E*/
0b01110001,/*F*/
0b01101101,/*H*/
0b00011110,/*J*/
0b00100011,/*L*/
0b01000101,/*n*/
0b01000111,/*o*/
0b01111001,/*p*/
0b01000001,/*r*/
0b00000111,/*u*/
0b00000000};/*Leerzeichen*/

The Multiplexing Routine:

// Timer 0 overflow interrupt service routine
interrupt [TIM0_OVF] void timer0_ovf_isr(void)
{
#asm("SEI");      /* Interrupts für Stoppuhr freigeben !*/
switch (uc_segmentcounter)
    {
    case 0:
    PORTC=0;
    PORTA.5=0;
    delay_us(20);
    PORTA.0=1;
    PORTC=uc_segment[uc_segmentcounter];
    if(PINB.2>0)
        {
        PORTC.7=1;
        };
    break;
    case 1:
    PORTC=0;
    PORTA.0=0;
    delay_us(20);
    PORTA.1=1;
    PORTC=uc_segment[uc_segmentcounter];
    break;
    case 2:
    PORTC=0;
    PORTA.1=0;
    delay_us(20);
    PORTA.2=1;
    PORTC=uc_segment[uc_segmentcounter];
    break;
    case 3:
    PORTC=0;
    PORTA.2=0;
    delay_us(20);
    PORTA.3=1;
    PORTC=uc_segment[uc_segmentcounter];
    break;
    case 4:
    PORTC=0;
    PORTA.3=0;
    delay_us(20);
    PORTA.4=1;
    PORTC=uc_segment[uc_segmentcounter];
    break;
    case 5:
    PORTC=0;
    PORTA.4=0;
    delay_us(20);
    PORTA.5=1;
    PORTC=uc_segment[uc_segmentcounter];
    break;
        
    };
uc_segmentcounter++;    
if(uc_segmentcounter>5){uc_segmentcounter=0;};
}

The Variable Segmentcounter points the actual Digit.
When all Digits are processed it starts again.
At the variable array uc_segment ist stored the actual active segments for each digit.
The Timer Overflow Routine should be called minimum each 2ms for 6 Digits to avoid flicker.

The Code is written with CodeVision AVR in "C" for an ATMEGA16 Controller.
The princip should work in an PIC Controller also.

 

[cne]

I am most interested in Mike's suggestion using the MIC5891 chip. This should work with both the small and large digits right?

This is my understanding:

Supply a constant clock
Shift one pin high and low for data
Shift Output Enable high at the end of data
Scan through the sets of displays using a Timer Interrupt
Refresh the values

A question:

How do you tie multiple chips together?

 

[Mike - K8LH]

This should work with both the small and large digits right?

Yes. On regular displays you would connect the VBB pin on the MIC5891's to the 5v VDD pin and on the large multi-led-per-segment displays you would connect VBB to whatever high voltage you needed.

How do you tie multiple chips together?

That's the tricky part.

The wiring is similar to that in the drawing below. There are five column/digit driver lines and one PWM line. The <dat> pin on each driver IC is also connected to a unique column/digit line and the <clk> pin on each driver IC is connected in parallel and connected to the remaining column/digit line. The <lat> and </oe> pins are all connected together and driven by the PIC PWM pin. The PWM signal provides complete fade-to-black brightness control and allows re-tasking the five column/digit driver lines for temporary use as <clk> and <dat> lines to load the shift registers in parallel while PWM is high (display "off") at the beginning of each column/digit update interval (by writing 8 bytes and clocks onto the column/digit driver buss in about 24 cycles).

I'll try to come up with a better drawing for you as time permits.

Cheerful regards, Mike

 

[Mosaic]

mike, what s'ware app do u use to create those drawings?

 

[Mike - K8LH]

mike, what s'ware app do u use to create those drawings?

Drawings are Excel spreadsheets...

 

[Gayan Soyza]

Dear Cne.

When designing outdoor display applications like scoreboards that traditional scanning methods are useless.

You must tie separate latches for individual segment & control them uniquely when dealing with direct sunlight.

If you are just making a model & show to the people in indoor then the traditional scanning methods like Mike mentioned would work.

 

[Mike - K8LH]

Look at Mikes last schematic it will meet your requirements for small & large displays.

That circuit is for common anode displays. You could use a single PNP or P-FET "source" driver on each column for small displays (5v) or you could use an NPN+PNP or N-FET+P-FET "source" driver pair on each column for the larger multi-led-per-segment displays (>5v)...

 

[Gayan Soyza]

Mike I changed my reply while you were quoting my 1st reply.

 

[Mosaic]

Could u attach one of those....sheets pls...I never noticed those part shapes.

 

[Mike - K8LH]

Could u attach one of those....sheets pls...I never noticed those part shapes.

Everything I use is in the "drawing" toolbar... Have a look... Those exotic part shapes are called "lines" and "boxes" (grin)...