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Resistances in a PIC circuit

  1. Apr 17, 2013 #1
    Hi everyone,

    I wanted to make some tries on PIC programming and needed some circuits. When I was searching I found some circuits but they seem confusing to me.


    In the circuit picture attached I wonder why are there the 4.7kΩ ressistances on the left hand side? I think they only form parallel loops and don't provide anything apart from more energy consumptation. What do I miss?

    I also cannot understand the lines in the microcontroller. Can someone show the simplified streams?
  2. jcsd
  3. Apr 17, 2013 #2


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    Staff: Mentor

    The pulldown resistors on the left are meant to present a logic low (0V) to those RC[3:0] inputs to the PIC. When one of the pushbuttons on the left are closed, that pulls that input up to a logic high (5V). The pushbutton is a low impedance (less than 1 Ohm), so there is very little voltage drop across them when closed, and there is a 5V drop across the associated 4.7k resistor when the switch is closed.

    I'm afraid that I don't understand your last question about the lines and streams for the PIC...
  4. Apr 18, 2013 #3
    Thanks for your reply.

    I wanted to mean that which ways the current follows in the PIC. If there is current on the wires which go to the inputs then the ressistances just form parallel circuits. If they were not there wouldn't the circuit still work?
  5. Apr 18, 2013 #4


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    Gold Member

    No, without the pulldown resistors the inputs would be "floating" when the push-buttons are open and "floating" can lead to undefined/unreliable/unrepeatable consequences that you do not want. You need to have a logical one or zero on the input at all times and "floating" is not either one, reliably.
    Last edited: Apr 18, 2013
  6. Apr 18, 2013 #5
    Thank you very much for your nice explanation.

    I think now I need to know this logic. If the pull down resistances doesn't exist there would be no current flow through the input port. I think you call this condition "floating". Then when there are pull down resistances will there be current through the input port (may be in the opposite direction)?

    Also is there any source you can advise, which shows the logic of micro controllers, and calculation methods for elements of circuits?
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