Multiplexing: will this work?

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odometer
 
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Joined: Sun Aug 21, 2011 11:01 pm

Multiplexing: will this work?

Post by odometer »

Please correct me if I am wrong.
I believe that:
  • the 74HC595 shift register sources current, and
  • the TPIC6B595 shift register sinks current.
I also know that diodes, by definition, are a one-way street for current.

So: if I want to multiplex some common-anode LED displays, I connect:
  • each of the cathode pins of the display units to the corresponding cathode pins of the other display units;
  • the current source to the common anode pins of the displays; and
  • the current sink to resistors which are in turn connected to the (previously connected-together) cathode pins of the displays.
Is this right?

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easternstargeek
 
Posts: 347
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Re: Multiplexing: will this work?

Post by easternstargeek »

Yes, that pretty much will work as you described. A little nitpick, however- a 74HC595 is an equal-opportunity source or sink to the tune of up to 35mA (absoolute maximum), because it's output stage is a symmetrical totem-pole type.
The TPIC6B595, OTOH, as you correctly state, will only sink current because it's output stage is of an open-drain design. (The CMOS equivalent of a TTL Open-Collector output, which is nothing more than a switch that connects the output pin to ground).
There are many ways to do this kind of multiplexing. If you are interested in the software implications of multiplexing using different schemes, please have a look at my blog entries:
http://easternstargeek.blogspot.com/201 ... ensor.html
and
http://easternstargeek.blogspot.com/201 ... splay.html
Both describe real projects that use multiplexed 7-segment displays, and give schematics and (Arduino) software examples.

Hope this helps- Good luck with your projects!

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