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'[PIC] 74HCT259 Latch Use'
2005\06\22@215034 by Steven W

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Hi.

I'm very new to this and it's just a hobby (I'm a full time software
developer but I've never gotten into hardware) so my question may be pretty
dumb... but here goes anyways:

I want to control two 7-segment LEDs using a 12F675. Obviously I don't have
enough pins to do this straight up, so I figured I could use a 74HCT259
8-bit addressable latch as a 3-to-8 demultiplexer and hook up the common
cathodes on the seven-segments to pins on the PIC that can act as sinks.

(I'm not concerned about whether or not this is a feasible solution to the
problem - I'm more concerned with just getting to at least try out my idea)

What I'm confused about is the operation of the latch. It seems like I would
have to set the 3 inputs to the latch (not a problem), set the associated
cathode pin on the PIC to act as a sink (also not a problem), and then
toggle the MR pin on the latch to reset the current output to 0, then toggle
the LE pin to remember my selection. That means I would need a total of 5
pins to operate the latch... which doesn't make sense to me because then you
are really only saving three pins on the PIC - and then I still don't have
enough pins left to control the cathodes.

Am I missing something here?

(http://rocky.digikey.com/WebLib/Texas%20Instruments/Web%20data/CD74HC(T)259
.pdf)

Thanks in advance!

-Steve W

2005\06\22@233412 by phil B

picon face
Look at the 74HC595 shift register.  use 3 pins to
control it (2 if you can live with some flickering
while updating).  daisy chain them together, one per 7
seg display, you can make the chain pretty long if you
want.  No multiplexing needed.  I'd limit the current
per segment to 10mA but that's still pretty bright.
If you have SPI HW (675 doesn't) you can drive from
that but bit bang SPI is way easy.  There was a really
good tutorial on this in CCI from a few years ago but
a quick search didn't turn it up.


--- Steven W <spam_OUTPICListTakeThisOuTspamripplingcreek.com> wrote:

{Quote hidden}

(http://rocky.digikey.com/WebLib/Texas%20Instruments/Web%20data/CD74HC(T)259
> .pdf)
>
> Thanks in advance!
>
> -Steve W
>
> --

2005\06\23@120957 by Dwayne Reid

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At 07:52 PM 6/22/2005, Steven W wrote:

>I want to control two 7-segment LEDs using a 12F675. Obviously I don't have
>enough pins to do this straight up, so I figured I could use a 74HCT259
>8-bit addressable latch as a 3-to-8 demultiplexer and hook up the common
>cathodes on the seven-segments to pins on the PIC that can act as sinks.

Can I suggest you look at a pair of shift register such as 74hc595 or
TPIC6595?  One shift register is used for each display.  You need 3 lines
from the PIC to control these in the simplest configuration (clk, data,
latch).  The data out of the 1st SR feeds the data in of the next for as
many stages as you might need (you need 2 but could have many more).

All 3 lines can also be used as inputs for other purposes with a little
attention to how and when you read those inputs.

You can combine two of the lines onto a single PIC pin if you use a RC
delay network on either the data or latch lines.  I normally delay the
latch line and combine it with the clock line.  Use short (narrow) clock
pulses while shifting data into the SR string and allow the clock to remain
HI after the last data pulse in the entire packet has been sent.

I should also mention that this works well with the TPIC6595 because its
inputs all have schmitt trigger stages.  It will probably NOT work with
74hc595s since those do NOT include schmitt trigger stages on their inputs
and the RC delay violates the minimum rise and fall times required for the
chip to work.

Hope this helps!

dwayne

--
Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2005\06\23@133713 by Anno, Jeff

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Steve,

I couldn't agree with Dwayne more about the TPIC6A595!  I've had Excellent
luck with them
driving large amounts of LEDs (http://home.fuse.net/theannos/snrdes.bmp) and
even stepper
motors.  I've found that they are reliable and easy to use.

-Good luck,
Jeff Anno

<Snip from Dwayne's e-mail>

Can I suggest you look at a pair of shift register such as 74hc595 or
TPIC6595?

I should also mention that this works well with the TPIC6595 because its
inputs all have schmitt trigger stages.
<Snip>

2005\06\23@135649 by David P Harris

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Anno, Jeff wrote:

{Quote hidden}

Hey, nice guitar!

David

2005\06\23@174314 by Denny Esterline

picon face
(http://home.fuse.net/theannos/snrdes.bmp) and
> even stepper
> motors.  I've found that they are reliable and easy to use.
>
> -Good luck,
> Jeff Anno

Hey, wasn't that guitar a senior design project at SVSU?

-Denny

2005\06\25@142152 by Byron A Jeff

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On Wed, Jun 22, 2005 at 09:52:19PM -0400, Steven W wrote:
> Hi.

Hello. And welcome.

I'm late to the party. I've been out of town for the week.

> I'm very new to this and it's just a hobby (I'm a full time software
> developer but I've never gotten into hardware) so my question may be pretty
> dumb... but here goes anyways:

There are two types of dumb questions:

1. Unasked ones.
2. Unresearched ones.

> I want to control two 7-segment LEDs using a 12F675. Obviously I don't have
> enough pins to do this straight up, so I figured I could use a 74HCT259
> 8-bit addressable latch as a 3-to-8 demultiplexer and hook up the common
> cathodes on the seven-segments to pins on the PIC that can act as sinks.

A 74HC595 or variant would be a better choice. Also you don't need an HCT
family part since the PIC is fully CMOS compliant. An HC part will do just
fine.

Also you need at least 4 lines to drive the HCT259 and most likely 5:
3 selects, data and latch.

{Quote hidden}

Bingo. That's why the HC595 is a better part for the job. It truly uses 3
pins (data,clock,latch) to drive 8 outputs. Also they are stackable so that
you can drive lots of output pins with just the three inputs.

>
> Am I missing something here?

Nope.

BAJ

2005\06\25@144356 by Harold Hallikainen

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You might also want to look at the http://www.allegromicro.com/sf/6276/ 16
bit LED driver with serial input. It will also take only 3 PIC lines and
do the whole job. No current limit resistors required, as the current on
all 16 outputs is set by one resistor. I use this part a lot...

Harold
"The ideal design has zero parts."

--
FCC Rules Updated Daily at http://www.hallikainen.com

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