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'[PIC:] Is it better to use Shift Registers or lots'
2004\04\12@195429 by Jason S

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I'm working on a project that will need a couple of inputs and around 16
outputs.  The outputs will change at a few hertz at the fastest.  Is it a
better design to use an 8-pin PIC and send the outputs serially to a couple
of shift registers or a 28 pin PIC and use the outputs directly.

It seems like the shift register solution is better design because it just
feels wrong to waste so many IO pins on something that simple, but the
larger pic means less components, less soldering,  and simpler software.

Jason

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2004\04\12@202154 by Shawn Wilton

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I would go with the shift register so if something happens, you're not sinking all that current through the µController, but rather through a cheap chip.

-Shawn

Jason S wrote:

{Quote hidden}

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2004\04\12@202403 by Jan-Erik Soderholm

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Jason S wrote :

> I'm working on a project that will need a couple of inputs
> and around 16 outputs.  The outputs will change at a few
> hertz at the fastest.  Is it a better design to use an 8-pin
> PIC and send the outputs serially to a couple of shift
> registers or a 28 pin PIC and use the outputs directly.

Small series (like 1->10->100) : use simplest programming.

Large series (maybe from about 1.000 units ??) : focus more on
component cost...

It sounds as you are doing a single unit project ?
If so, use a 28-pin PIC.

>
> It seems like the shift register solution is better design
> because it just feels wrong to waste so many IO pins...

And instead wasting time on more complicated programming ?

But anyway, without knowing the number of units planned,
it's close to impossible to answer I'd guess...

Jan-Erik.

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2004\04\12@203232 by Floret Morgenstern

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Do what you prefer...
Do you like soldering more than programming, then use a single PIC...
Do you get off on a programming challenge, like multiple serial
communitacion in software, use the small PIC...

{Original Message removed}

2004\04\12@220324 by David Duffy

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Jason S wrote:

>I'm working on a project that will need a couple of inputs and around 16
>outputs.  The outputs will change at a few hertz at the fastest.  Is it a
>better design to use an 8-pin PIC and send the outputs serially to a couple
>of shift registers or a 28 pin PIC and use the outputs directly.
>
>It seems like the shift register solution is better design because it just
>feels wrong to waste so many IO pins on something that simple, but the
>larger pic means less components, less soldering,  and simpler software.
>
>

I often use a smaller PIC and a shift register.
For driving lamps and relays I use a UCN5821.
Also provides a degree of isolation from PIC <-> load.
The code to drive the S/R is simple and well worth it.
David...

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2004\04\12@230543 by Roy J. Gromlich

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OK - if the cost of the different PIC parts actually tracked the number of
pins, there might be some reason to use the S-R.  AS it is, the cost of a
PIC chip doesn't change that much from 8 to 18 to 28 to 40 pins.  And,
as you said, lots fewer parts and an easier programming job.

I usually use a 40-pin PIC for anything more complicated than a logic
chip replacement - it is surprising (not!) how often those extra pins find
uses on future projects,

Roy J. Gromlich

{Original Message removed}

2004\04\12@232251 by William Chops Westfield

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On Monday, Apr 12, 2004, at 17:24 US/Pacific, Jan-Erik Soderholm wrote:

>> It seems like the shift register solution is better design
>> because it just feels wrong to waste so many IO pins...
>
> And instead wasting time on more complicated programming ?
>
Ah, but you should only have to do the 'complicated programming' ONCE,
and then you can put as many shift registers as you want out there with
little additional effort.  And there are LOTS of types of shift
registers, including high current drivers and fancy SPI chips.

OTOH, more chips add a lot of complexity to a design when it comes to
actually getting it built.  And space issues, too.  I'd lean toward
using a bigger chip unless the rest of the circuit is already complex,
or unless the next-bigger chip gets expensive (relatively speaking.)

On the third hand, I have my "stock" of PICs and I'd rather use one of
those (which tend to be SMALL pics) than have to order something new.

BillW

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2004\04\13@020833 by Robert Ussery

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I'll have to be a fence-straddler here. Oftentimes there's no substitute for
more I/O pins in terms of hardware complexity, and often the larger chips
are the only ones that offer the other peripherals I need. OTOH, I'm
frequently doing stuff with remote displays, etc, where I want to minimize
the number of wires running from one board to another. In this case, shift
registers are invaluable. For instance, right the moment, I'm running an
LCD, several LEDs, and several buttons at the end of ~4 feet of ribbon
cable, using only 8 conductors. If I dedicated a wire to each I/O, I would
end up with ~20 wires... yuk!
As with most things in EE, it looks like the answer to this question depends
on your individual problems and design goals.

- Robert

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2004\04\13@140137 by Byron A Jeff

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On Mon, Apr 12, 2004 at 04:57:55PM -0700, Jason S wrote:
> I'm working on a project that will need a couple of inputs and around 16
> outputs.  The outputs will change at a few hertz at the fastest.  Is it a
> better design to use an 8-pin PIC and send the outputs serially to a couple
> of shift registers or a 28 pin PIC and use the outputs directly.
>
> It seems like the shift register solution is better design because it just
> feels wrong to waste so many IO pins on something that simple, but the
> larger pic means less components, less soldering,  and simpler software.

KISJ,

Your last statement says it all:

... the larger pic means less components, less soldering, and simpler software.

It would be the winner in my book.

BAJ

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2004\04\13@140550 by Byron A Jeff

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On Tue, Apr 13, 2004 at 02:00:41PM -0400, Byron A Jeff wrote:
> On Mon, Apr 12, 2004 at 04:57:55PM -0700, Jason S wrote:
> > I'm working on a project that will need a couple of inputs and around 16
> > outputs.  The outputs will change at a few hertz at the fastest.  Is it a
> > better design to use an 8-pin PIC and send the outputs serially to a couple
> > of shift registers or a 28 pin PIC and use the outputs directly.
> >
> > It seems like the shift register solution is better design because it just
> > feels wrong to waste so many IO pins on something that simple, but the
> > larger pic means less components, less soldering,  and simpler software.
>
> KISJ,
>
> Your last statement says it all:
>
> ... the larger pic means less components, less soldering, and simpler software.
>
> It would be the winner in my book.

I need to thrown in the caveat of the small production run. I always tend to
see questions like this from the hobbyist perspective. So to rephrase:

If you're only going to build a handful, save yourself the hassle and use the
larger chip. However if you designing for 100,000 widgets, then the cost
contraints will determine the best route to take.

BAJ

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2004\04\13@143141 by Eisermann, Phil [Ridg/CO]

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pic microcontroller discussion list wrote:
> As with most things in EE, it looks like the answer to this question
> depends on your individual problems and design goals.

I think this about sums it up. It's always a tradeoff. Cost has been
mentioned already, but I feel like jumping in on this thread

Cost and complexity are almost always interrelated. Cost is more
than just component cost. I personally think that component cost
only becomes relevant when you start talking about 10's or 100's of
thousands of boards. I look at it in terms of design time e.g.
engineering salary. In this case, DigiKey price for 100 16F872-ISO
is $2.81. 100 12F629-ISO plus 74HC164 is 1.32. Difference is $1.49
times 100 units = $149 dollars. How much more it costs to design in
a shift register depends on many factors, but I doubt its more than
30 minutes to an hour even if you include PCB layout. As long as this
isn't your first PIC project, clocking out serial data is trivial. So
the cost difference shrinks to maybe $100. Its pretty much a negligible
difference at low volumes in any case. But at higher volumes, the
component costs start to make a difference.

In general, the simpler the design, the less can go wrong, the less
time it takes to design and develop, and the cheaper it becomes.
Unless you are doing it as a hobbyist, time is not free.

So, there is no right answer to the original question, just guidelines :)


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2004\04\13@155904 by Jason S

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Thanks for all the replies.  It seems like it was a better question than I
thought.

This is a one-off hobby project, so component cost isn't much of issue.
I'll go with the bigger pic because it is easier.

Jason




{Original Message removed}

2004\04\14@043947 by Alan B. Pearce

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>Thanks for all the replies.  It seems like it was a better
>question than I thought.

Well that is one of the beauties of this list, there is a large amount of
people experienced in a wide field of expertise who are only too willing to
make their knowledge available.

>This is a one-off hobby project, so component cost isn't much
>of issue. I'll go with the bigger pic because it is easier.

For this sort of use the advise given pretty well holds. The only thought
that I had was you may still need to think about how big a chip you need in
terms of pins available, as once you go beyond the 40 pin chips they are all
surface mount, which can be a fright if your not used to them. I don't think
you are using that many pins yet though.

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2004\04\15@120837 by Dwayne Reid

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At 12:31 PM 4/13/2004, Eisermann, Phil [Ridg/CO] wrote:

>Cost and complexity are almost always interrelated. Cost is more
>than just component cost. I personally think that component cost
>only becomes relevant when you start talking about 10's or 100's of
>thousands of boards. I look at it in terms of design time e.g.
>engineering salary. In this case, DigiKey price for 100 16F872-ISO
>is $2.81. 100 12F629-ISO plus 74HC164 is 1.32. Difference is $1.49
>times 100 units = $149 dollars.

Another consideration to add to this discussion: power handling.  If you
need to drive large loads, it can be easier to use a SR with built-in high
current output stages.  I'm thinking of the TPIC6595 family but there are
others.

Once you have SPI code working, it becomes just another tool in the
arsenal.  Use it if it makes sense to do so, use a larger package PIC if not.

I use a LOT of 12c508 parts and using shift registers to provide the extra
I/O that I need seems completely natural.  But, then again, I've been using
shift registers since long before I even heard of PICs.

That said - for a one-off project being built from scratch (not able to use
an existing board design), I'd probably use one of the larger PICs *if* all
the I/O fits into one part.

dwayne


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