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'[EE] Display multiplexing anyone?'
2008\06\28@083528 by Tomás Ó hÉilidhe

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The largest display multiplexer I've ever done that actually looked good
(i.e. it was nice and bright and had no observable flicker) had 16
stages (i.e. 16 individual LED's).

Have you any idea what the "maximum" amount of stages is? Would it be
possible to make a multiplexer that has 42 stages? I'm using the
PIC16F887 to orchestrate the display multiplexer. If I want to pull this
off, should I definitely run the PIC at 20 MHz?

2008\06\28@175434 by Dennis Crawley

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On Saturday, June 28, 2008 9:34 AM [GMT-3=CET],
Tomás Ó hÉilidhe  wrote:

> The largest display multiplexer I've ever done that actually looked good
> (i.e. it was nice and bright and had no observable flicker) had 16
> stages (i.e. 16 individual LED's).
>
> Have you any idea what the "maximum" amount of stages is? Would it be
> possible to make a multiplexer that has 42 stages? I'm using the
> PIC16F887 to orchestrate the display multiplexer. If I want to pull this
> off, should I definitely run the PIC at 20 MHz?

Look at the bottom of this page,
http://www.geocities.com/proyectosenpic/

I did it several years ago with a 16F872 at 20Mhz
There were 40 columns.
I do not recomend this circuit neither the method (column by column)
Raw by raw is better and you can reduce the frequency of the oscillator.

Dennis

2008\06\28@191609 by David Meiklejohn

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Tomás Ó hÉilidhe wrote:
>
> The largest display multiplexer I've ever done that actually looked
> good
> (i.e. it was nice and bright and had no observable flicker) had 16
> stages (i.e. 16 individual LED's).
>
> Have you any idea what the "maximum" amount of stages is? Would it be
> possible to make a multiplexer that has 42 stages? I'm using the
> PIC16F887 to orchestrate the display multiplexer. If I want to pull
> this
> off, should I definitely run the PIC at 20 MHz?

The limitation won't be the '887.

You want to multiplex the LEDs in your Connect4, right?  If you multiplex
the LEDs singly (but I'm not sure how you'd practically wire this, but
anyway...), each will only be lit for 1/42 of the time.  So - will they
still be bright enough, with your current source?  That's the limitation...


David Meiklejohn
http://www.gooligum.com.au


2008\06\28@194233 by Tomás Ó hÉilidhe

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David Meiklejohn wrote:
> The limitation won't be the '887.
>
> You want to multiplex the LEDs in your Connect4, right?  If you multiplex
> the LEDs singly (but I'm not sure how you'd practically wire this, but
> anyway...), each will only be lit for 1/42 of the time.

Exactly, I'll multiplex them one LED at a time. This way, I'll have no
need for drivers at all. Plus I can work off two AA batteries (i.e. 3
volts).

Are you definitely sure that the 887 running at 8 MHz will be good
enough? Would it be noticeably better if I ran it at 20 MHz (but of
course then I'd need 5 V from somewhere...)

>   So - will they
> still be bright enough, with your current source?  That's the limitation...

I'm still looking for decent 3-pin LED's. My supplier is Farnell and I'm
having a hard time finding good LED's on their site.

2008\06\28@210633 by Jinx

face picon face
> Exactly, I'll multiplex them one LED at a time

The 'normal' way would be to strobe whole columns, as in the
4-digit schematic I sent a link to

There are two things to consider - light output and persistence
of vision

Essentially you have a 14 x 6 matrix of LEDs. For a static display
I don't think that is much of a problem, timing-wise at least

To slightly digress, the moving sign I built is 64 columns refreshed
at 200Hz and scrolling on that looks quite smooth. Admitedly it is
smaller, using 3mm LEDs, than all-purpose commercial units, but
the principle is the same. You will often see poorly-designed units
where a too-low refresh rate causes the display to 'lean'. This will
happen especially when the display is too long for one micro and
the time delay to get from column1 to columnX causes spatial
effects. For mine, each of 64 columns is refreshed 3 times per sec
at an overall 200Hz trobe rate, which at the moment is adequate

If you choose to make the columns 14 LEDs high, gettting their
'high-side' data and current straight from the PIC, then you need
only 6 column drivers, which simplifies strobing considerably

Your unit does not have a need for scrolling, although there's no
reason why you can't add special effects like star-bursts or all
the tiles falling down at the end of a game. Once this driver issue
has been sorted out that's all in s/w anyway

Another time strobing is seen is when an LED is driven by AC,
perhaps from the front end of a PSU. Moving your head with the
LED in your peripheral vision will make the mains frequency, for
example, apparent. However, looked at front on you would not
be able to tell. It does indicate though that 50Hz is too slow for
steady illumination

The PIC can appear to be as busy as you choose it to be, but it's
basically doing very little. A timer or WDT interrupt to check for
buttons, format new data, strobe the column drivers. That's about
it. In between it can be asleep. Data and column selection is on
the port pins during sleep, so there's no need to be awake

Now, no matter what frequency you use, each LED is going to
get only a fixed time slice - (100/6)%. In that time, you have to
supply enough power to the LED for it to light effectively. If you
used the same resistor as you would for static display, the bright-
ness at 16% duty-cycle is obviously going to go down. So the
resistor is reduced accordingly. The determining parameter is the
LED wattage/peak repetitve current rating. And you'll have to
find that in the datasheet of the LED you choose

2008\06\28@222557 by Jinx

face picon face
> each will only be lit for 1/42 of the time.
>
> Exactly, I'll multiplex them one LED at a time

I'm pretty sure 2.38% duty cycle is not going to look very good. LEDs
will be on, but quite dim. You need only one LED and some s/w to test
that

(1) the refresh rate would have to be fairly high to cover all the LEDs
without flicker

(2) increasing the scan rate reduces the time each LED is serviced

(3) reducing the service time increases the current needed to make the
LED "brighter than normal" for persistence of vision to kick in for any
appreciable time. 97.6% 'off' time in fact

(4) PIC pins on their own cannot supply that much burst current

2008\06\29@082144 by olin piclist

face picon face
Tomás Ó hÉilidhe wrote:
> Exactly, I'll multiplex them one LED at a time. This way, I'll have no
> need for drivers at all.

Only if you can tolerate very dim LEDs.  If each LED will only be on for a
short time, the current during that short time will need to be large to get
the same average current.  For example, if you have 50 LEDs with never more
than one on at a time and you want 10mA average for a LED that is lit, then
you'd need 1/2 amp thru the LED during the brief time it is on.  You have to
look carefully at the maximum instantaneous current spec for the LED.  You
would probably exceed it with your propsed scheme.

> Plus I can work off two AA batteries (i.e. 3 volts).

Without a boost supply for the LEDs at least, the LED brightness will vary
greatly over the battery life and the batteries won't be usable even when
there is still some life left.

> Are you definitely sure that the 887 running at 8 MHz will be good
> enough?

There is no way to tell because you haven't explained what exactly it has to
do beyond multiplexing the LEDs.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\06\29@092626 by Dennis Crawley

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On Sunday, June 29, 2008 9:23 AM [GMT-3=CET],
Olin Lathrop  wrote:

> Tomás Ó hÉilidhe wrote:
>> Exactly, I'll multiplex them one LED at a time. This way, I'll have no
>> need for drivers at all.
>
> Only if you can tolerate very dim LEDs.  If each LED will only be on for a
> short time, the current during that short time will need to be large to
> get the same average current.  For example, if you have 50 LEDs with
> never more than one on at a time and you want 10mA average for a LED that
> is lit, then you'd need 1/2 amp thru the LED during the brief time it is
> on.  You have to look carefully at the maximum instantaneous current spec
> for the LED.  You would probably exceed it with your propsed scheme.
>

...One tip.
 During the developing you would like to use a current limited power
supply.
I just forgot a flag in my sw and, after switched on, the micro hanged
(hung?) up in a column.
The green LEDs have turned into red and one of them puff-ed up.
When you are really sure your LEDs sequence is all rigth you can feed up the
circuit with more current.
An LM317 can do the job.

;)
Dennis

2008\06\29@104519 by PicDude

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I've also multiplexed 16, but that was 15 years ago using very generic (ie:
dim) bulk LED's.  With brighter LED's, you'd probably be able to do 42, but
you'd need to experiment with the specific LED's you have.  You just need to
write some code that toggles an output at 1/42 duty-cycle and test various
LEDs to see how bright they look.  Not that enclosure and filter can improve
visibility (by improving contrast).

If too dim, perhaps you can do 2 rows of 21.  And you'd need much less lines
running to the LED's.  With 1 of 42 multiplexing, you'd probably need some
additional I/O chip, such as a shift register or demux.

Minimum PIC oscillator speed will be a function of what else is happening in
the code -- I've been able to multiplex well with 1Mhz with no visible
flicker, but changing duty-cycle (to get dimming functionality) required
that I up the PIC to 4 Mhz.  Later, adding more functionality in the PIC,
especially floating-point math, forced me to up to 8Mhz.  These are all 16F
PICs, btw.

Cheers,
-Neil.



Tomás Ó hÉilidhe-2 wrote:
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