Re: Driving 16 7-Segment Displays

joe.denehan_at_oceanfree.net
Date: 09/02/04 

Date: 2 Sep 2004 08:12:01 -0700

Thank Guys for all the suggestions.

I very much like the idea of using an Interrupt Service Routine to
control to displays (eliminating more IC's).

I've been trying to figure out the timing necessary to do this duty
cycle. Can someone tell me is the diagram below will work. I would use
8 i/o's to control the 7-segment displays (including DotPoint).I
propose an ISR every 6ms to enable/disable the digit and use a lower
resistor to overdrive the Segments.

This means that the segment is only driver for 6ms then off for 90ms.
Will this work?

        6ms
         |
         _
Digit1 _| |_____________________________________________________________
             _
Digit2 _____| |_________________________________________________________
                 _
Digit3 _________| |_____________________________________________________
                     _
Digit4 _____________|
|_________________________________________________
                         _
Digit5 _________________|
|_____________________________________________
                             _
Digit6 _____________________|
|_________________________________________
                                 _
Digit7 _________________________|
|_____________________________________
                                     _
Digit8 _____________________________|
|_________________________________
                                         _
Digit9 _________________________________|
|_____________________________
                                             _
Digit10_____________________________________|
|_________________________
                                                 _
Digit11_________________________________________|
|_____________________
                                                     _
Digit12_____________________________________________|
|_________________
                                                         _
Digit13_________________________________________________|
|_____________
                                                             _
Digit14_____________________________________________________|
|_________
                                                                 _
Digit15_________________________________________________________|
|_____
                                                                     _
Digit16_____________________________________________________________|
|_

       |------------------------------ 96 ms
---------------------------|

         Common Segment Inputs(x16)

         |||||||| ||||||||
         //////// ////////
   33ohm \\\\\\\\ 33ohm \\\\\\\\
         //////// ////////
         \\\\\\\\ \\\\\\\\
         |||||||| ||||||||
         -------- --------
        | | | |
        | | | |
        | | | |
        | ------ | | ------ |
        | | | |
        | | | |
        | | | |
         -------- --------
            | |
            | |
           / /
   Dig1---| TR1 Dig2---| TR2 etc
           \ \
            | |
            | |
           --- ---
            - -

Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote in message news:<0hrbj0l423vb7bknclfjtgum6gfdkdniqo@4ax.com>...
> On 1 Sep 2004 05:12:20 -0700, the renowned joe.denehan@oceanfree.net
> wrote:
>
> >Hi,
> > I'm just completed a progject where it's necessary to drive 16
> >displays (7-segment). Because I didn't want to tie up the
> >microcontroller and have maximum brightness, I used Maxim's MAX7219.
> >This works perfectly but I need to have 2 of them as they only drive 8
> >digits each.
> >
> >Problem is they are expensive devices. I wondered if there are other
> >similar devices or methods I could use without tying up the
> >microcontroller. (uPD78F0034)
>
> What do you mean by "tying up the microcontroller"? You should be able
> to multiplex 16 digits without using significant bandwidth.
> Personally, I try to stay away from expensive single-sourced chips,
> even if it increases the parts count. You have to be able to service a
> short periodic ISR without jitter of more than 5-10% at 1-3 kHz. That
> is not a very limiting requirement for most micros.
>
>
> >Thanks,
> > Joe
> >
> >PS. If I didn't use any display drivers. Would an 8Mhz micro be able
> >to get around all the displays (not diming too much) and monitor a 4x4
> >keypad input also???
>
> Yes, of course. You can probably share some or all of the port pins as
> well. The brightness is basically a function of the average drive
> current, not the multiplex frequency.
>
> Best regards,
> Spehro Pefhany

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