What is the purpose of the 4.7kΩ resistances in this PIC circuit?

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The 4.7kΩ resistors in the PIC circuit serve as pulldown resistors, ensuring that the inputs are at a defined logic low (0V) when the pushbuttons are not pressed. Without these resistors, the inputs would be "floating," leading to unreliable behavior and undefined states. When a pushbutton is pressed, it pulls the input to a logic high (5V), while the resistor limits current flow and stabilizes the voltage. The discussion also highlights the importance of maintaining a clear logic state on microcontroller inputs to avoid unpredictable outcomes. Resources for understanding microcontroller logic and circuit calculations were requested for further learning.
Zalajbeg
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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.

http://www.biltek.tubitak.gov.tr/gelisim/elektronik/dosyalar/29/devre.jpg

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?
 
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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...
 
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?
 
Zalajbeg said:
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?

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.
 
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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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