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    reducing number of interconnections in a circuit

    I have a design which consists of 7 sets of push wheel switches, each set with 4 digit. The digital parallel outputs from these switches amounts to 16 X 7!
    Is there a way to minimize wiring requirements. I was thinking of a serial shift register. Any ideas?

    •   Alt16th May 2018, 16:26

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    Re: reducing number of interconnections in a circuit

    Not sure what your switches look like. Isn't each digit four outputs and one common? If so, you could tie the four outputs of each digit together (i.e., 1 to 1, 2 to 2) then you would just need 28 commons and 4 outputs. You could further reduce the wires by using a 5:32 decoder to drive the commons==>9 wires.



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    Re: reducing number of interconnections in a circuit

    Hi,

    Calculating the minimum lines with simple multiplexing technology:
    16 Ũ 7 = 112 lines
    Sqrt(112) = something with 10.x
    This means 11 + 11 = 22 signal lines.

    The absolute minimum is a 7 to 128 decoder --> 7 outputs + 1 input = 8 lines.
    Or a 6 to 64 with 2 inputs.
    In either case you need 112 diodes to combine the signals.

    ***
    For sure one can think of some more intelligent solutions:
    * dedicated multiplexing IC similar to those in PC keyboards.
    * or a solution with resistors and ADC...

    In the end Barry's solution (but with the diodes) maybe is the most useful.

    Klaus
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    •   Alt16th May 2018, 23:40

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    Re: reducing number of interconnections in a circuit

    Diodes? What diodes? All you need is to drive each of 28 commons individually and wire-or the four BCD outputs of each digit. (This presumes the undriven commons float.)
    But a 112-bit shift register would be the absolute minimum wire count: Latch clock, data. You could do this in a CPLD.



    •   Alt17th May 2018, 14:46

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    Re: reducing number of interconnections in a circuit

    Hi,

    I assume without diodes it wonīt work:

    Two BCD switches. Each with 4 outputs
    Now assume the left one shows "0x1" and the right one "0xF".
    Click image for larger version. 

Name:	BCD.gif 
Views:	3 
Size:	2.7 KB 
ID:	146697
    Obviously the right one shorts all the BCD lines independent if itīs COM is selected or not.
    --> if COM1 is selected, all BCD outputs are acitve, instead of only line "1".

    Klaus
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    Re: reducing number of interconnections in a circuit

    Quote Originally Posted by KlausST View Post
    Hi,

    I assume without diodes it wonīt work:

    Two BCD switches. Each with 4 outputs
    Now assume the left one shows "0x1" and the right one "0xF".
    Click image for larger version. 

Name:	BCD.gif 
Views:	3 
Size:	2.7 KB 
ID:	146697
    Obviously the right one shorts all the BCD lines independent if itīs COM is selected or not.
    --> if COM1 is selected, all BCD outputs are acitve, instead of only line "1".

    Klaus
    Ahhhhh, you're right. I shouldn't post so early or so late in the day... I was thinking more in terms of how we multiplex LED displays. Not the same.

    This is making the shift-register approach a little more appealing. Could probably do this in a US$15 CPLD (rather than seven 16-bit SRs.)



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    Re: reducing number of interconnections in a circuit

    This is making the shift-register approach a little more appealing. Could probably do this in a US$15 CPLD (rather than seven 16-bit SRs.)
    Surely possible but uncomfortable pin count. I would prefer 14*74HC165 as a first approach, about 3 $.



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    Re: reducing number of interconnections in a circuit

    Quote Originally Posted by FvM View Post
    Surely possible but uncomfortable pin count. I would prefer 14*74HC165 as a first approach, about 3 $.
    144 pin CPLD has 80 fewer pins than fourteen 16-pin devices. And less board space. But this is all kind of academic without knowing what the OPs intent is. Is this a high-production design? A hobby thing?



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    Re: reducing number of interconnections in a circuit

    The question at all does not provide enough information to evaluate the suited solution. Depending on the distance, the wirings, the Vdd power level, and the change rate of the switches status, this can limit the implementation of some solutions. In particular, whenever possible I prefer the solution based on parallel input type 74xx373, once with this approach it is possible to repeat the reading in only one clock cycle, just to confirm the correctness of the previous reading.
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    •   Alt17th May 2018, 17:48

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    Re: reducing number of interconnections in a circuit

    Hi

    Some ideas:
    one could use multiple diodes in one package: like 8 diodes with common cathode: "D92C" from BI technologies.

    One could ask: why that much BCD switches. When we have microcontrollers and rotary encoder switches.

    One could make an IC per set (4 BCD switches). Then one has 7 identical devices connected vie I2C. Maybe something like PCF8547..

    Klaus
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    Re: reducing number of interconnections in a circuit

    Hi
    Thanks to everyone for giving excellent insight into this problem. Push wheel switches are passive and non volatile . In my application, I am prohibited to use memory devices. Now I have to decide between 373 vs shift register approach. 373 would add 16 outputs+ 7 enable signals. Shift register if cascaded as long shift register would take only 1 output (SO)+1 clk+1shift/ld ( common to 7 SR's)



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