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

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Discussion Overview

The discussion centers around the purpose of 4.7kΩ resistors in a PIC microcontroller circuit, specifically regarding their role as pulldown resistors and the implications of their presence or absence in the circuit. Participants explore concepts related to circuit behavior, input states, and current flow in the context of microcontroller inputs.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the necessity of the 4.7kΩ resistors, suggesting they may only create parallel loops and increase energy consumption.
  • Another participant explains that the pulldown resistors provide a logic low (0V) to the PIC inputs when pushbuttons are open, preventing floating states.
  • A participant seeks clarification on current flow in the circuit, questioning whether the circuit would function without the pulldown resistors.
  • It is noted that without pulldown resistors, the inputs would be floating, which could lead to unreliable behavior.
  • A participant expresses interest in understanding whether current flows through the input port when pulldown resistors are present and requests resources for learning about microcontroller logic and circuit calculations.

Areas of Agreement / Disagreement

Participants generally agree on the importance of pulldown resistors in preventing floating inputs, but there is some uncertainty regarding the specifics of current flow and circuit behavior without them. The discussion remains unresolved regarding the necessity of the resistors in all scenarios.

Contextual Notes

Participants have not fully explored the implications of different circuit configurations or the specific conditions under which the circuit may or may not function as intended. The discussion lacks detailed analysis of the electrical characteristics of the components involved.

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.
 
Last edited:
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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