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'[PIC:] multiplex 7seg LED with drivers'
2004\02\09@155740 by Vlad

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Hello folks,

I'm in the process of designing a toy circuit to keep track of life
counters for various games.  It should support 3 players.  Each player
will have two 7 segment LED displays that will display a number between
0-99, and also 2 switch buttons that will be used to increase and
decrease the number displayed on these LED displays.

I have a handful of 16f84 PIC's and currently learning how to program
them.  With only a limited number of input/output pins on this chip
(13 I believe) what would be a good way to allocate these pins for the
switch buttons and displays?

I have read various articles in the Usenet archives which suggest
multiplexing displays, this requires 7 pins for each segment and 1
pin for every other display.  I think this will eat up in to all my
pins though.  Can I use the same multiplexing technique with a single
display/driver chip?  Any suggestions?  Also any thoughts on the switches?

This is my first electronics project and PIC programming project
(ambitious, yes)

Thanks
-v

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2004\02\09@160614 by Colombain Nicolas

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Hi Vlad,

I have an example of this kind of circuit in my simulator. It works with a
16C71 but the pin number is the same and the 16F84 is also supported.
You can take a look on this link: http://perso.wanadoo.fr/udev
Regards,

Nicolas


{Original Message removed}

2004\02\09@163034 by Vlad

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On Mon, Feb 09, 2004 at 10:33:30PM +0100, Colombain Nicolas wrote:
> I have an example of this kind of circuit in my simulator. It works with a
> 16C71 but the pin number is the same and the 16F84 is also supported.
> You can take a look on this link: http://perso.wanadoo.fr/udev
> Regards,
>

Hi Nicolas,

Thanks for the link.  Oh do you have separate schematics or code I can
look over?

I noticed your software is for Win32 platforms, which I don't have access
to :(  My development environment is under GNU/Linux using the gpasm compiler
and the gpsim simulator

Take care
-v

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2004\02\09@172437 by Steve Smith

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F84 has got a full port B and RA0-3
Seven bits for the display and one spare (port B)
RA0 digit 0
RA1 digit 1
Seven buttons on port B
That leaves RA2,3 and RB7 spare you can do an LCD with three wires as
well

Leaves space for three spare pins and up to seven buttons
Crappy ascii sketch


                       E---+5v
RA0,1----R1-----B
                       C
                       |
                       com anode-------internal to other segments
                       |
                       led cathode
                       |
                       R2
RB(x)-------------|
                       R3
                       |
                       sw1
                       |
                       0V

R1 22k base resistor
R2 470R digit resistor
R3 4k7 switch resistor
Transistor PNP any to take led current

Principle
1. set up the digit pattern on port B digit 0
2. make RA0 low
3. make RA0 high
4. make port b inputs
5. read buttons (active low)
6. make port B outputs again
7. set up pattern for digit 1
8. make RA1 low
9. make RA1 high
10. short wait or do sumfing else
11. repeat


Its an idea and works on paper
Regards Steve...

{Original Message removed}

2004\02\09@173058 by Pedro Drummond

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Vlad, have you seen the multiplexing scheme that MAX6950 does ?
8 seven-segment displays with only 9 lines.
Clever design, and maybe just what you need.


{Original Message removed}

2004\02\09@173515 by Bob Blick

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Vlad said:
> I'm in the process of designing a toy circuit to keep track of life
> counters for various games.  It should support 3 players.  Each player
> will have two 7 segment LED displays that will display a number between
> 0-99, and also 2 switch buttons that will be used to increase and
> decrease the number displayed on these LED displays.

6 displays and 6 switches? You can do it with almost no extra parts on a
16F84.

Use common cathode displays.

Use all of port A and one pin (RB7) from port B to drive 6 NPN
transistors, one for each digit, common emitter. With base resistors. RA4
will also need a pullup resistor.

The 7 segments are driven from the first 7 pins of port B through resistors.

Build that first and get it working.

Then add your switches to port B, on the segment side of the resistors.
The switches other side to ground.

Turn on "weak pullup on Port B".  Tristate RB0 through RB6. Make all of
port A and also RB7 low.

Read Port B for the switch status.

Done. Almost no parts.

Cheers,

Bob

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2004\02\09@174136 by Pedro Drummond

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Yes, I have not read it right. You have enough lines to use a more
conventional approach.


----- Original Message -----
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Sent: Monday, February 09, 2004 8:32 PM
Subject: Re: [PIC:] multiplex 7seg LED with drivers


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switches?
{Quote hidden}

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2004\02\09@181837 by Dmitriy Fitisov

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Vlad,
I have similar project - timer.
4 7 segment LEDs.
You may see assembler with comments.

http://www.radier.ca/pic/pictimer.php

Dmitriy

{Original Message removed}

2004\02\09@194232 by Josh Koffman

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The options presented so far are all rather good. I know that using the
same pins for different things can be a bit intimidating, so allow me to
offer another idea. Serial shift registers. You could use a serial to
parallel to light your displays, then a parallel to serial to read your
switches. More components and thus a higher cost, however you are less
dependent on time.

Josh
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fools.
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Vlad wrote:
> I'm in the process of designing a toy circuit to keep track of life
> counters for various games.  It should support 3 players.  Each player
> will have two 7 segment LED displays that will display a number between
> 0-99, and also 2 switch buttons that will be used to increase and
> decrease the number displayed on these LED displays.

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2004\02\09@202935 by Bob Axtell

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Right on, Josh. The PICs work great when the LED's are solid (not
multiplexed) but when multiplexing is needed, there is usually not
enough current.

The 74HC595 is hefty BUS-driver part, and can pump a lot of current into
a multiplexed display. I used it for both the HIGH side AND the LOW
side. Only current limiting resistor needed.

--Bob

Josh Koffman wrote:

{Quote hidden}

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2004\02\09@204633 by Jinx

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> You could use a serial to parallel to light your displays

I have an example here, on my much-neglected ;-(( site

(where oh where does the time go ?)

http://home.clear.net.nz/pages/joecolquitt/2wireled.html

> then a parallel to serial to read your switches

I've used a 4051 to read opto-interrupters, he work good,
and a 74HC251 to serialise data from an EPROM, he work
good also too

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2004\02\10@003237 by Brooke Clarke

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Hi Vlad:

Another option, and the way I like to power 7-segment displays is using
the Allegro 6276.  This is a serial input driver chip that drives 16
pins with a constant current, not multiplex.  These chips can be
connected in series so you can drive any number of digits you want
without using more I/O pins and with great brightness.  You can drive
any color or size LED display very brightly with just a few pins.
See:http://www.pacificsites.com/~brooke/PClock.shtml  The first version
used multiplexing, but with 6 digits the display was not as bright as I
would have liked.  With the 6276 you might consider a brightness
adjustment so you could handle bright sun, normal room light or a dim room.

Have Fun,

Brooke Clarke, N6GCE
http://www.PRC68.com

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2004\02\10@020745 by Wouter van Ooijen

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> The 74HC595 is hefty BUS-driver part, and can pump a lot of

What is the maximum (peak and average if they differ) current for a
HC595 output pin? I recall reading the datasheet but not finding this
info.

Wouter van Ooijen

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2004\02\10@021159 by Vlad

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On Mon, Feb 09, 2004 at 03:46:02PM -0500, Vlad wrote:
> I'm in the process of designing a toy circuit to keep track of life
> counters for various games.  It should support 3 players.  Each player
> will have two 7 segment LED displays that will display a number between
> 0-99, and also 2 switch buttons that will be used to increase and
> decrease the number displayed on these LED displays.
>
> I have a handful of 16f84 PIC's and currently learning how to program
> them.  With only a limited number of input/output pins on this chip
> (13 I believe) what would be a good way to allocate these pins for the
> switch buttons and displays?

I appreciate all the responses I have received thus far.  I am new
to electronics so I will have to research all these ideas to fully
understand them.  Perhaps then I will come back with more specific
questions after I have picked one of these methods... (although the
technique that Bob is describing sounds attractive, tristate the pins
to out/in so that they may share LEDs and switches)

Well if you have any other good suggestions, keep em coming. :) I will
summarize later if there is enough interest.

Thanks
-v

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2004\02\10@073031 by Bob Axtell

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Wouter van Ooijen wrote:

>>The 74HC595 is hefty BUS-driver part, and can pump a lot of
>
>
> What is the maximum (peak and average if they differ) current for a
> HC595 output pin? I recall reading the datasheet but not finding this
> info.
>
> Wouter van Ooijen

I don't think I've seen a spec, either, but I think I calculated about
40mA sink/source.

--Bob

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2004\02\10@092628 by William Bross

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From the data sheet:

http://focus.ti.com/lit/ds/symlink/sn74hc595.pdf

Up to 15 LSTTL loads
Continuous output current, IO (VO = 0 to VCC) ±35 mA
Continuous current through VCC or GND ±70 mA
±6-mA Output Drive at 5 V (This is the one I live by.)

I use these all the time with the low current AlGAs LEDs, about 2 or 3
mA each.  I'd be wary about using a lot of regular LEDs.  Go to the
TPIC6C595 or TPIC6595, NOT the TPIC6B595 as kicked around last week I
believe.

Bill

{Original Message removed}

2004\02\10@130753 by Spehro Pefhany

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At 05:28 AM 2/10/2004 -0700, you wrote:
>Wouter van Ooijen wrote:
>
>>>The 74HC595 is hefty BUS-driver part, and can pump a lot of
>>
>>
>>What is the maximum (peak and average if they differ) current for a
>>HC595 output pin? I recall reading the datasheet but not finding this
>>info.
>>
>>Wouter van Ooijen
>
>I don't think I've seen a spec, either, but I think I calculated about
>40mA sink/source.

focus.ti.com/lit/ds/symlink/sn74hc595.pdf
www.fairchildsemi.com/ds/MM/MM74HC595.pdf
http://www.fairchildsemi.com/ds/MM/MM74HC595.pdf

Abs. max. 70mA or 75mA through Vdd/Vss (sorry Vcc/GND, I guess nobody told
it that it was CMOS and not TTL) pins, +/-40mA or +/-35mA maximum per output.
That's Abs. max ratings, with all that implies.

The outputs are characterized and the drop is guaranteed at 6mA/5V.

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2004\02\10@132701 by Wouter van Ooijen

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> Abs. max. 70mA or 75mA through Vdd/Vss (sorry Vcc/GND, I guess nobody
told it that it was CMOS and not TTL) pins, +/-40mA or +/-35mA maximum
per output. That's Abs. max ratings, with all that implies. The outputs
are characterized and the drop is guaranteed at 6mA/5V.

I don't consider 'absolute maxima' (the chip-will-not-die figures) as
relevant inputs for a design, so I do not find the figures I'd like to
have (maximum allowd current). Without further info a feeble 6mA is the
design maximum.

Wouter van Ooijen

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2004\02\10@160507 by Spehro Pefhany

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At 07:26 PM 2/10/2004 +0100, you wrote:
> > Abs. max. 70mA or 75mA through Vdd/Vss (sorry Vcc/GND, I guess nobody
>told it that it was CMOS and not TTL) pins, +/-40mA or +/-35mA maximum
>per output. That's Abs. max ratings, with all that implies. The outputs
>are characterized and the drop is guaranteed at 6mA/5V.
>
>I don't consider 'absolute maxima' (the chip-will-not-die figures) as
>relevant inputs for a design, so I do not find the figures I'd like to
>have (maximum allowd current). Without further info a feeble 6mA is the
>design maximum.

Good advice, in general (though they are certainly relevant if they are
approached or exceeded). Unless one has additional information.

BTW, why not use the 7.8mA characterization at Vdd=6V? ;-)

Best regards,

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