74HC595 Shift Register
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Does anyone know for sure how many 595's I can daisy-chain together ?
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Comments
limit BUT the term "dozens" was used in an app note on the Parallax web site
using three I/O lines. Being able to use "dozens" makes it unlikely you will
run out of capability any timje soon.
Mike B.
Original Message
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Sent: Saturday, June 08, 2002 2:58 PM
Subject: [noparse][[/noparse]basicstamps] 74HC595 Shift Register
> Does anyone know for sure how many 595's I can daisy-chain together ?
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>Does anyone know for sure how many 595's I can daisy-chain together ?
As many as you want! The limiting factor is how long it takes to update
all of the shift registers. Only you can answer that - how fast can you
update 1 shift register and how much time do you have available to update
them all. Divide the 1st number into the 2nd and that tells you how many
you can drive for your application.
After finding out how many you can SRs you can drive in the time you have
available, then take a look at the total load you are going to present to
the clock driver. Figure that each '595 is going to look like a small
capacitor - Fairchild data sheet says max 10 pF per chip. If you are
driving LOTS of shift registers, you may need to split the load among
several clock drivers.
I usually limit my projects to 4 or 8 shift registers per chain but that's
what works for me. You should have no problems with a dozen or two.
A real world example of a product implemented with long SR chains is the
old ?Farfisa? electronic accordions - they use one long chain of shift
registers to encode all the keys on the accordion. I can't remember how
many keys that was but I think there was 11 or 12 shift registers in the
chain.
Another example that I was a part of is the scrolling message sign that was
part of Epcot Center in Florida - this was a very long message sign that
was part of the handrail that went down a long spiral ramp. There might
have been more than a hundred SRs per chain times 16 chains (I think) - I
can't recall exactly how many but it was a LOT.
dwayne
--
Dwayne Reid <dwayner@p...>
Trinity Electronics Systems Ltd Edmonton, AB, CANADA
(780) 489-3199 voice (780) 487-6397 fax
Celebrating 18 years of Engineering Innovation (1984 - 2002)
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