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'[OT] what chip to use?'
1999\03\25@084519 by Ken Johnson

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Hi, I would like suggestions on what to use to decode some bcd to 7-seg. I
have done it with a 4511 and 74hc237, it works ok but I can't get rid of the
multiplexing noise (the signals are multiplexed). I would like to use a
single-chip solution such as programmable logic of some sort so I can put it
all in a small shielded box (it lives in a very sensitive 137MHz weather
satellite receiver). I though of an eprom but I need 14 output lines (7
segments, decimal point and 6 digits). I don't want to use a pic, then I
would have to get rid of the pic's clock as well.
Any (well, almost any!) suggestions welcome.

Cheers, Ken, vk7krj
spam_OUTvk7krjTakeThisOuTspamsouthcom.com.au

1999\03\25@121223 by Wagner Lipnharski

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Why don't you use two eproms, one for each 7-seg display? It would cost
less than any other no-noise solution... no clocks, no processing, no
multiplexing, no nothing. See, eproms are being used for several strange
solutions where you need 8 output pins with up to 16 inputs... it is a
nice programmable logic device... by low cost.  You can feedback some
outputs to be part of the addressing, and automatic tables can be
created, just a whole use of imagination. :)
Wagner

1999\03\25@124953 by Harold Hallikainen

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On Fri, 26 Mar 1999 00:23:12 +1100 Ken Johnson <.....vk7krjKILLspamspam@spam@SOUTHCOM.COM.AU>
writes:
>Hi, I would like suggestions on what to use to decode some bcd to
>7-seg. I
>have done it with a 4511 and 74hc237, it works ok but I can't get rid
>of the
>multiplexing noise (the signals are multiplexed). I would like to use
>a
>single-chip solution such as programmable logic of some sort so I can
>put it
>all in a small shielded box (it lives in a very sensitive 137MHz
>weather
>satellite receiver). I though of an eprom but I need 14 output lines
>(7
>segments, decimal point and 6 digits). I don't want to use a pic, then
>I
>would have to get rid of the pic's clock as well.
>Any (well, almost any!) suggestions welcome.
>

       The 4511 does not have a multiplexed output.  It's a BCD input
with a latch and decoder driver.  There could indeed be noise radiated
off the multiplexed BCD driving the 4511.
       I just did a product having a PIC drive an Allegro UCN5832.  This
is a serial in, open collector parallel out shift register.  It has 32
open collector outputs, each of which can sink over 100mA.  Your 6 digits
will take more than 32 outputs, but you can cascade the 5832s to get as
many outputs as you want, still only using 3 pic pins (data, clock, and
latch).  Driving LEDs without multiplexing takes a LOT of resistors!
       I've seen some LED drivers that multiplex the LEDs, though the
current is relatively low (average under 10mA per segment).  These chips
have a current control pin so you don't need all the extra resistors
outside.  This, of course, moves the power dissipation from the resistors
to the chip, which could become a problem.
       For me, the 5832 worked well!


Harold



Harold Hallikainen
haroldspamKILLspamhallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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1999\03\25@130646 by Sibert, Steve

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Hi again Ken,

I've got some LR1707 decoder/driver chips; these include all
the circuitry necessary to decode and display a 7-segment digit
with both left and right decimals.  They're fairly expensive
(Digikey had something like that, but I couldn't find it in
the latest catalog).

How many do you need?  I might be able to part with some...

Steve Sibert

{Original Message removed}

1999\03\25@154711 by Marc

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> it works ok but I can't get rid of the multiplexing noise (the signals are mul
tiplexed).

If you can't -avoid- the noise, you should -control- it.  Choose a frequency tha
t
affects your weather sat receiver least, and/or is easy to filter with a few
components.  Let the offending circuit operate at that frequency.

R-Cs in the output lines help to suppress high frequency components at the signa
l
edges.

Compared to discrete logic, usually a PIC makes it easier for you to control
these aspects, and how many & which gates switch at the same time, and so on.

1999\03\25@180911 by Tony Nixon

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Ken Johnson wrote:
>
> Hi, I would like suggestions on what to use to decode some bcd to 7-seg. I

How can you multiplex displays without a clock source anyway?

--
Best regards

Tony

PicNPoke - Multimedia 16F84 Beginners PIC Tools.
*** FREE PCB OFFER ***

http://www.picnpoke.com
Email .....picnpokeKILLspamspam.....cdi.com.au

1999\03\26@092203 by Ken Johnson

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---Thanks to all those who replied to my request,  Wagners suggestion of
using a couple of eproms fits the bill nicely. I will probably use a couple
of bipolar proms for it, small and cheap (assuming they are still available
here in sleepy hollow!)
Steve, thanks for the offer on the chips but as I said above, I will go prom
way.
Marc, just to explain a little further, I didn't explain it all that well
first time, the signals are bcd, multipled off of the synthesiser board, all
I am doing is decoding them and feeding them (still multiplexed) to the
displays.

Cheers, Ken
EraseMEvk7krjspam_OUTspamTakeThisOuTsouthcom.com.au

1999\03\26@092452 by paulb

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Ken Johnson wrote:

> I have done it with a 4511 and 74hc237, it works ok but I can't get
> rid of the multiplexing noise (the signals are multiplexed).

 You pose an interesting conundrum.  You have of course left out the
*important* detail - where does the data come from to be displayed.  If
the data itself is multiplexed as you sort-of imply, you may have a
problem, because you will have hash at that point if not all the time.

 For a frequency display of course, it need only change when you are
changing the frequency (tuning) and since you can't receive as the
frequency is actually changing, no problem.

 Since the 4511 contains a latch, just use six 4511s (and an HC138 or
the 237, whatever...).

 If you are using a synthesiser and *need* to use a MPU, use the
reference oscillator (which you have to shield anyway) to clock the MPU.

> I would like to use a single-chip solution such as programmable logic
> of some sort so I can put it all in a small shielded box (it lives in
> a very sensitive 137MHz weather satellite receiver).

 The Allegro chip suggested sounds good.  If it doesn't multiplex at
all (whilst receiving), you don't need shielding.

> I though of an eprom but I need 14 output lines (7 segments, decimal
> point and 6 digits).

 Yes, hardly "single chip".

> I don't want to use a pic, then I would have to get rid of the pic's
> clock as well.

 Definitely [OT] then!

 The most interesting approach to multiplexing is that used by bedside
clocks.  They use a two-phase multiplex generated by a centre-tapped
power transformer winding (about 3V per side).  Half the LEDs go to each
end, the centre tap being ground.

 The clock has an input from the transformer which serves the dual
purpose of keeping time and gating the alternate LED sets.  The beauty
of this is that the sinewave applied to the LEDs generates virtually no
harmonic component beyond a few hundred Hertz.  Switching of the drivers
occurs at zero voltage.
--
 Cheers,
       Paul B.

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