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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

    I looked at the datasheet for the 164 and see that it has no latch for the outputs.

    I couldn't find a pin compatible shift register with an output latch.

     

    A latch would enable you to clock in all the bits, without them rippling over the outputs, then when you are ready latch the data to the outputs.

    Such a chip would be something like the 74hc595 - but it's not pin compatible.

     

    Running the pic at a higher rate might mitigate the problem, but it's a bit of a cludge I'm afraid.

     

    Your best work around might be to fit a filter on the leads to the leds/piezo.

     

    In the long-run you need to redesign the device.

  • 0 in reply to KennyMillar

    Guys, thanks for your help - you are abolutely right. I have just raised the PIC speed from 4MHz to 8MHz, and the problem has reduced but not gone away.

     

    Could anyone suggest a simple filter circuit for the LEDs and sounder? Filters not my strong suit! The LEDs and sounders are running at 12v. There is already a 220R resistor in series with each of the 3 LEDs. The piezo sounder is running straight off the 12v. I am using a the shift registers to drive a ULN2003 darlington IC. Each of the LEDs and sounders gets 12v, then I pull the other side down to 0v via the darlington.

     

    Ideally I would like to run the PIC back at 4MHz so as to leave my other timings unaffected. It takes me approx 12 - 13 clock cycles to clock a bit down through the shift register.

     

    Many thanks,

     

    Phil

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  • 0 in reply to KennyMillar

    Guys, thanks for your help - you are abolutely right. I have just raised the PIC speed from 4MHz to 8MHz, and the problem has reduced but not gone away.

     

    Could anyone suggest a simple filter circuit for the LEDs and sounder? Filters not my strong suit! The LEDs and sounders are running at 12v. There is already a 220R resistor in series with each of the 3 LEDs. The piezo sounder is running straight off the 12v. I am using a the shift registers to drive a ULN2003 darlington IC. Each of the LEDs and sounders gets 12v, then I pull the other side down to 0v via the darlington.

     

    Ideally I would like to run the PIC back at 4MHz so as to leave my other timings unaffected. It takes me approx 12 - 13 clock cycles to clock a bit down through the shift register.

     

    Many thanks,

     

    Phil

Children
  • 0 in reply to Former Member

    It depends how big the pulse width of the "click" is - ie the time when the signal is in a changing state. Try to put a capacitor from the IC side of the 220R resistor to ground. Something like 100nF, and test results.

     

    Without the schematic I am not sure about this, but it may be possible to swap the position of LCD, buzzer and LED.

     

    Put the LCD connections at the lower bits of the shift register, as these change more frequently. The enable pin on the LCD means these changes will not effect the function. Put the LED and buzzer up on the higher bits of the shift register as these change less frequently. Maybe your circuit is already like this? Without a schematic I cannot say.

     

    Good luck.