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Author Topic: How many output 595 shift register I can use in series ?  (Read 1026 times)
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Hellas
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Hello, I intend to use an atmega with arduino bootloader, and im willing to use 12 shift registers.

Is it possible to conect 12  595's   in series ? (using digital pins)

Thank you
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In theory there's no limit realistic to the number of shift registers you can put in series. Obviously you have to get the wiring right but that's not too hard and there are several examples around.

In practice you''ll need to watch the load time. If you're using shiftOut then it's “bit banging” under the hood and that can be quite slow. If you use the SPI interface to blast your bits down the wire then it can easily go at then times the speed!

The importance of this depends of how fast you want to update your output values. If, like me, you're doing analogue simulation using PWM (old school) or BAM (bit angle modulation, lots of fun!) then you'll need something a little faster!

Hope this helps!
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There is no limit as to how many you can daisy chain, other than the memory requirements and time needed to set all the outputs (more outputs require more time).
That being said 12 shift registers are no big deal. It can easily be done.
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Hellas
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Im not interested about the speed (i just hope it has less than 1sec lag) , as all the outputs will be led indicators.
ive made some eagle pcb here
so far so good i hope

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There is no limit as to how many you can daisy chain,
Yes there is. Both the clock and the latch pins of all the shift registers need to be connected to one arduino pin each. The number of logic inputs one arduino output can drive is determined by the technology of the shift register. If you exceed this you will have to buffer these two lines in batches of n, where n is less than the number of gates you can drive with one pin.
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Hellas
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in other words I can drive 12 595's with 3 uC  digital pins , right ?
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You might also want to add some decoupling caps to your design. One 0.1uF for every Vcc input on each IC.
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Hellas
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why should i complicate even more the board by adding one cap on each ic, instead of increasing the total capacitance of the one filter cap ?

just asking dont be insulted
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why should i complicate even more the board by adding one cap on each ic, instead of increasing the total capacitance of the one filter cap ?

just asking dont be insulted

Because you want it to work properly and don't want to come back here in a week or two saying 'I am using x shift registers to do xyz, but I'm getting intermittent behaviour, can anyone help' smiley-razz

Any idea what the first answer to such a question might be? smiley-wink
« Last Edit: September 16, 2013, 12:53:21 pm by tack » Logged

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why should i complicate even more the board by adding one cap on each ic, instead of increasing the total capacitance of the one filter cap ?

It doesn't work that way. You want a capacitor near each chip so it can supply immediate extra power. Think of it like the tank on your toilet -- each one in your house has its own reservoir; you don't have a big tank on the roof for all your bathrooms.

Good luck!
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why should i complicate even more the board by adding one cap on each ic, instead of increasing the total capacitance of the one filter cap ?

just asking dont be insulted

Because that is the most effective means to cut down the digital switching noise from the source of the noise, at the chips, using bypass filter caps of the appropriate size, not too small or larger then needed.

Lefty
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in other words I can drive 12 595's with 3 uC  digital pins , right ?
What sort of 595s are you talking about? TTL, LS HC  or what?
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Hellas
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well allright but instead of one on each ic cant I use one 0,22 on 4? also that cap should be electrolytic or unpolarised ?
its not about cost , just I want to have it with as less parts as possible because I will
make the pcb's using toner transfer and want to have thick trace for upper and down layer

about the 595's im talking about the 74HC595 ic  by the way that its used on the arduino playground example

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No. As has been pointed out, the decoupling cap needs to go as close to the power pins of each IC as possible. It is small and fast, to keep the IC supply rail propped up to cope with very short transients.

Just 0.1uF ceramic cap at each IC.
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Read this
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
For best results decouple every chip with a 0.1uF ceramic capacitor, other types will not work as well.
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