Wiring Schematic

singalls

Hello everyone,

I purchased a Whats a Microcontroller package to start playing around with electronics, etc.· I have been a corporate programmer most of my life, so I have no problem understanding the Basic language.· What I am having problems with is reading the wiring schematic.· It is really easy to follow the diagram for plugging in stuff, but I would really like to understand the schematic.· Does anyone have a link or the like to explain this more?· I appreciate any help.

Steve

Mike2545

What in particular don't you understand?

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Mike2545

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Franklin

Here is a whole slew of sites that can help. www.google.com/search?q=how+to+read+schematics&rls=com.microsoft:en-us&ie=UTF-8&oe=UTF-8&startIndex=&startPage=1

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- Stephen

singalls

Thanks for the quick responses. If you download·the manual (http://www.parallax.com/Portals/0/Downloads/docs/books/edu/wamv2_2.pdf) I am confused with the diagram on page·90 - Figure 3-11.· I understand the top one, but am confused about the bottom one.

Steve

Mike2545

Sorry, that link is no good. Please try again...

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Mike2545

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singalls

Sorry, http://www.parallax.com/Portals/0/Downloads/docs/books/edu/wamv2_2.pdf

Mike2545

Ok the vdd is + and the 10k resistor holds the pin low so that it does not float when you are not pressing the button. Floating pins sometimes give unpredictable conditions so you have to hold them one way o the other (+/-).

The 220ohm resistor just keeps the full vdd from going into the stamp when you press the button.

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Mike2545

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singalls

Thanks again for the quick reply.· From the schematic, what am I missing that the vdd is + ?· I think I understand the pin low information.· I am again confused about what you mean about the 220ohm.

Steve

Mike2545

Vdd = +
Vcc = -

The 220 ohm is not necessarily needed but makes good design practice. You can put 5v on the input but the resistor adds a level of safety. When you are dealing with a $50 chip a .02 resistor makes good economic sense.

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Mike2545

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Post Edited (Mike2545) : 4/3/2009 11:07:38 AM GMT

Mike Green

Actually, Vdd and Vcc are both terms for the "active" power terminal. In the case of most circuits you'll see nowadays, it's the positive terminal. With Vcc, the "c" comes from "collector" which is one terminal of a junction transistor (the others are the base and the emitter). Vss is usually the negative terminal and the "s" comes from the "substrate" which is the term for the actual chip itself rather than the transistors and diodes built onto it.

The 220 Ohm resistor is to protect against short circuits on the I/O pins of a microcontroller like the Stamps. If you connect the I/O pin to another device or a switch, that device or switch typically will connect the I/O pin to either Vss or Vdd since Vss is considered a logic low or zero and Vdd is considered a logic high or one. If, by mistake, the I/O pin is switched to output mode rather than input mode and the setting of the output pin happens to be opposite from that of the switch or other device, you have a short circuit between Vss and Vdd and it doesn't take long for the internals of the I/O pin circuitry on the Stamp to melt. It's designed for a current of 20mA to 25mA. The 220 Ohm resistor limits the short circuit current to about 25mA which the Stamp can withstand indefinitely and usually doesn't affect other uses of the I/O pin.