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PICList Thread
'Binary to BCD Converter'
1998\05\11@124727 by n Midgley

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Just a quick note to thank everyone who helped out with this. It would
be invidious to single out individuals for praise (it's like an awards
ceremony, isn't it?), so generally, thanks; that bit of my app now works,
and I've got a handy routine tucked away in my library for future use.

So now if I could just push my luck a bit further, and ask for suggestions
for manufacturers/part numbers for:

1) A nice, straightforward BCD to 7-segment decoder chip. I'm doing this in
code at the moment, but that takes lots of pins.

2) (On a related note...) a serial in, 4 bit parallel out shift register.

I've browsed various manufacturers Web sites, and become thoroughly
confused - even a single manufacturer seems to produce a bewildering
number of variants that all seem to do the same thing.

So, suggestions anyone?

Regards

John M

1998\05\11@132041 by Mike Keitz

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On Mon, 11 May 1998 17:42:03 +0100 John Midgley
<spam_OUTJohn.MidgleyTakeThisOuTspamENORFOLK-HA.ANGLOX.NHS.UK> writes:

>1) A nice, straightforward BCD to 7-segment decoder chip. I'm doing
>this in
>code at the moment, but that takes lots of pins.
>
>2) (On a related note...) a serial in, 4 bit parallel out shift
>register.

If you are just going to connect the shift register to the decoder,
forget the decoder and shift out the data for all segments to a 7-bit
shift register.  The number of PIC pins is the same.  The PIC software
would convert BCD to 7-segment and shift it out.  Besides not needing a
decoder chip, the PIC would have individual control of all 7 segments so
patterns other than numbers could be displayed.

8-bit shift registers are common.  The 74HC164 is about as basic as they
get.  Its major limitation is that the data in the register flows
directly to the output pins.  While you are shifting in new data, the
output pins will assume unwanted states.  If you shift fast enough, they
won't be visible on the LED.  Or you could turn off the LED's power
supply while shifting.  More advanced shift registers have latched and/or
tri-stated outputs.

>I've browsed various manufacturers Web sites, and become thoroughly
>confused - even a single manufacturer seems to produce a bewildering
>number of variants that all seem to do the same thing.

There are a large number of variants of the 74HC164.  They all have the
same effective logic (an 8-bit shift register), but the internal
circuitry is different.  This causes variations in the performance such
as speed, acceptable input voltage, supply current, and output current
and voltage.  For this LED application, the output ratings are most
important.

For example--
74HC164 - silicon gate CMOS - similar to PIC chip
74HCT164 - similar to HC164 - different input voltage limits
74C164 - old version with metal gate CMOS - slow.
74LS164 - old version with low-power TTL logic - high Icc.
74164 - very old TTL logic version - even higher Icc.
and so on.  But all of them will plug into the same socket and work
approximately the same if the rest of the circuit can stand them.


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1998\05\11@132842 by David VanHorn

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>8-bit shift registers are common.  The 74HC164 is about as basic as they
>get.  Its major limitation is that the data in the register flows
>directly to the output pins.  While you are shifting in new data, the
>output pins will assume unwanted states.  If you shift fast enough, they
>won't be visible on the LED.  Or you could turn off the LED's power
>supply while shifting.  More advanced shift registers have latched and/or
>tri-stated outputs.
>

We use a pair of 164s to drive a single char alphanumeric. The load process
is not even close to observable, on a z8 running at <1 mips. Unless you get
hung in a long int or something during reload, it's not a problem.

remember to add current limiting resistors for each segment !

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