Hey guys, got a very simple question about this method:
Is there any downside I should be aware of?
I'm planning to use about 15x 4-digits 7s displays, each one with its own 595....would like to save as many pins as possible.
Thanks in advance.
You should use decoupling capacitors on each chip.
For 14 shift registers there is no need to do any buffering of the clock or latch signals.
However be aware of the maximum current for a shift register chip, not only per pin but the total per package. You might not want to go with the HC variant but some other technology.
Thank you!
I don't really understand the datasheets
The chip seems to support 20ma per output and 70ma total?
And the display one...
http://www.paralight.us/uploads/pdf/A-564SR%20GW.pdf
It says 30ma per segment.
I'm confused 
I'm confused
Why?
It says 30ma per segment.
That is the maximum current you can put through a segment not what you have to put through it. The more you put through the brighter it will be.
The chip seems to support 20ma per output and 70ma total?
Yes. So suppose you have one display per chip that is 7 outputs being used. That is 70mA to share between 7 outputs, so that means that you should design the current limiting resistor on each segment to give 10mA per segment.
Thanks for clarify it Mike.
They are common anode displays, and I'm using a 220 resistor on the anode.
I=V/R
5/220 = 0.22ma
Does this have any sense?
And how this would be affected if I connect other chips in a chain?
Should I really care about current?
I mean, even if I chain them I'm still connecting a single display per chip.
No if they are common anode you need a resistor in each cathode. Then. Each cathode goes to a shift output.
Resistor is 5 - forward volt drop of led / 0.01
For a 10 mA segment current.
Yes you cair about current a lot.
Alright but it's the same if I have 1 or many chained chips, isn't it?
Their outputs will use the same current, right?
Sounds like you'll be sending 15 bytes out at a time, and selecting 1 digit of 4 out of each group for your multiplexing, leaving each one on for 1/30/4 S = 8.3mS tops. 4 PNP transistors, or P-channel MOSFETs to source current to the one digit in each group that will be on.
You should be using TPIC6B595. Each output can sink 150mA continuously, so you can have 20-30mA per segment & max brightness. 74HC595 is not a good part for this application.
Jay98:
Alright but it's the same if I have 1 or many chained chips, isn't it?
Their outputs will use the same current, right?
Not at all sure what you mean. Once you have got the design of one shift register and display correct you simply duplicate it.
I got the impression you were not doing multiplexing with your design, is that right?
Here's how I see 15 shift registers daisy chained to drive 4x7 segment displays.
Use SPI.transfer to send data to the 15 registers, then turn on 1 of 4 common anodes in each display to light up a digit.
Leave on for a few mS, turn off, send out data for the next set of digits.
74HC595 not the best choice here, I would use TPIC6B595 myself, and have daisy chained up to 20 like this so far.
I have another design for 45, with SCK & SS buffered for every 9 devices. Wired up 27 so far, shifting data thru 8 MHz.
Can you guys please help me understanding this...
Segment take 30ma each, that's 240ma total as I didn't use any resistor at the 595 outputs.
If the IC can only handle 80ma total, how is that has been working without problems after several hours?
If the IC can only handle 80ma total, how is that has been working without problems after several hours?
Because you have been killing the chip slowly.
It is like getting a dose of radiation, if the dose is above the critical dose then you might not die immediately, but you will die much sooner than you would otherwise.
The LEDs are also over current and so they will fade in brightness a lot sooner than they would otherwise do.
as I didn't use any resistor at the 595 outputs.
After all we have said, I am sorry but that is just stupid.
Time for a bit of reading:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html
Got it, ty.
I wired it before I noticed the issue.