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enter image description here

Why are pins 1,3,7 missing from the schematic? Also why are wires from most pins like 4,5,8 going to ground. I am unable to understand the circuit.

Mahendra Gunawardena
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Commander Creatives
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    Your question suggests very limited understanding. Did you find a datasheet? That is always a good way to learn about a component. Where did the circuit diagram come from, and what is its puurpose? You realise that this is a programmable device, I assume? – Sean Houlihane Oct 01 '16 at 08:37

2 Answers2

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The picture is a quick example of some functionality of the MCU, which is hardly to be used without context, and you can only guess what is should do.

It seems to use 8-pin package (e.g. PDIP), pins which are not drawn are simply not connected anywhere, OR they are assumed to be connected to somewhere and you are expected to know it :)

Pin 1 "RESET": I did not find guideline what to do with it in datasheet. Probably it has internal pull-up,or it should be connected to weak pull-up resistor. This is gray area which is not detailed in this picture, but I consider relatively important.

Pins 4 and 8 are just power supply for the circuit.

Pin 5 is digital input. Circuit assumes that chip has internal pull-up activated on this pin, otherwise when button is not pressed logical level on the wire will be undefined.

Pins 6 and 3 are digital outputs, lighting LEDs according to some pre-programmed algorithm.

Pins 2 and 7 are just not used, and left floating (not connected anywhere) as they are GPIO.

Anonymous
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  • This answer does little to help the OP learn how to investigate these trivial questions for himself. I think really the question needs to be deleted - unless its significantly improved. – Sean Houlihane Oct 01 '16 at 09:00
  • The question was to make individual understand the circuit, why things are connected as they are. I tried answering this part as much as possible with given information. Regarding deletion... any person has right to ask question. While you/we may not like it, giving our point of view on asked question may save asker's life. – Anonymous Oct 01 '16 at 09:11
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enter image description here

When powering a device, how it interfaces to the outside world and what is common determines the criteria for selecting the choices of switched circuits.

If I were to guess.

A grounded metal frame saves on a ground wire going to a switch.

So a switched ground was used to power the circuit in isolation to anything else..

Tony Stewart EE75
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