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Shift Register Help

Former Member

Hi all,

I'm using 2 x 14 pin 74HC164Ns, cascaded together, to operate an LCD, 3 LEDs and a Piezo sounder. Controlled by a PIC16F874. Running at 4MHz.

My problem is: when I write to the shift registers, I get a clicking  sound on the Piezo, and flashing on the LEDs. The LCD is unaffected as  I'm driving the en input directly from the PIC.

The PCB has already been designed and manufactured, so I am a bit stuck.  Even toggling the clear input on the shift register doesn't solve the  problem.

Ideally I should have used a shift register with tri state outputs such  as the 595. However I am stuck with making what I have here work.

Possible solutions:

a) run the PIC at a higher speed?
b) fit a small filter to my LED and Piezo outputs (which are on flying leads) so they ignore these very short signals?

I'd appreciate your comments on the above - many thanks.

Phil

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  • 0

    Hi Phil.

     

    Well, if I'm right, you use these shiftregisters as an kind of port expander.

     

    If you are changing some values, you have to schift the whole 28 bits. And there's the problem.

    The other bits rattle over the output pins which drives the piezo and the leds. So if the speed is

    not high enought, you will see or hear this.

     

    So I have two possible solutions for this.:

     

    1)    try to increase the speed of the writing to the registers

     

    2)    solder a socket instead of the 74HC164Ns into your PCB. If you have good luck, you will find a pincompatible ic with an extra output enable pin, which you can connect to an free pin of the PIC, lcd enable pin or maybe to the last output bit of thes registers.

     

    I hope I helped you with this.

     

    yours woolf

  • 0 in reply to Former Member

    I think that woolfs error analyze is good, I think he's right.

     

    But I also think that the solution isn't the best.

     

    If you increase the speed you don't remove the fault you just hide it! I don't like to solve problems like this way.

     

    I think you have to edit your project basically to get a perfect result.

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