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Topic: How many output 595 shift register I can use in series ? (Read 1 time) previous topic - next topic

MarkT

You have to have a low impedance path from power and ground pins to the decoupling
capacitor for it to work properly.  That means short wide traces (ideally groundplane).  Its
not the resistance that matters at all (never an issue with logic chips), its the inductance
that matters at these speeds, and shorter wider traces have less inductance.  The timescales
involved are a few ns.

Every logic chip needs decoupling, its not optional.
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thebadtall

#16
Sep 18, 2013, 01:21 am Last Edit: Sep 18, 2013, 01:27 am by thebadtall Reason: 1
okey, considering your information and knowlege
i have now made this , putting 0,1uF ceramic capacitor near each IC





Thank you very much for your info and help.
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SirNickity

In general, you should consider using ground (and power) planes or fills instead of traces.  Try to avoid right-angle traces as well, especially for high(er)-speed signals.

You're using vias on a board you will be making with toner transfer?  Brave.  ;)

The decoupling thing comes up almost daily around here.  If you haven't had enough finger-wagging, just browse back a few pages to read plenty of other threads about why you need them.

Docedison

#18
Sep 18, 2013, 07:34 am Last Edit: Sep 18, 2013, 07:58 am by Docedison Reason: 1
A Rule of thumb for me when I was designing boards was that as suggested use a copper pour for Vcc on one side of the board and Gnd on the other... If that proves difficult... then pre route the two planes It looks like the router wasn't assisted by pre routing and locking power and ground. using a star topology. Power and ground rails should be about 2 to 3 X the width of the signal (control) 'wires' when a good ground plane isn't possible. Even thicker doesn't hurt. Neither would buffering the data every second or third chip. This is done to break up the loading on the Arduino and thus increase it's speed capabilities. The other end is the necessity of the 100 nF caps.
There is a LOT of talk about missing caps but you will never hear about too many bypasses..
The other thing is that holes are nearly free, SMD pads are free and it's most expensive to add a hole or 'scrape a pad' later to add missing bypasses but costs nothing to not add the part.
Smaller 'distributed' caps work better than big ones in one place. The reason is that each trace or wire also represents an inductance which becomes a loss of decoupling and a noisy and unstable board.
By-passes, in production are cheap and when hand made they are soldered on the pcb first.
There are a great many concerns involved in even a low speed (< 20 MHz) PCB besides parts placement and being able to get the autorouter to run to completion...
In my experience wires (PCB traces) are either transmission lines or just wires and can be routed at pretty much any angle depending on the rules for wires (constant width) and transmission lines [striplines] (constant impedance). Transmission line effects don't usually come in to play until the frequency approaches 50 MHz and above and there is becomes most important for many things... including 16, 32, 64 bit buss's and is the reason why many high speed mother boards have what looks like weird bus wiring... where some 'wires are actually 'squiggled' about. this done is to equalize bus line length so that signals reach the same point at the same time with minimal reflections from impedance mismatches... Generally a PCB is as complex as the job it has to do.

Doc
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CrossRoads

I have a board with 12 TPIC6B595s on it, 74HC595 is similar, but with much lower drive capacity.
I have daisy chained 2 of these boards for 20 shift registers total, 4 MHz SPI control signals.
Decoupling caps are key as GM says.
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