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Hardware > Strange Electroincs Question (not GBA related)

#154108 - thegamefreak0134 - Fri Apr 11, 2008 7:08 am

I'm working on a project for my school and I have a very very strange issue. I'm using a micro controller to drive a small car, and as an added feature, we've set up a turret on the top that can fire BBs. We've decided, in honor or retro, to control this turret using an NES controller, with the only real drawback being that we have to run around behind the car because we can't find a wireless NES controller, on account of they never made one.

</rant>

Anywho, I've got the microcontroller communicating with the NES contoller fairly well. the problem is, it only works when I have the data line for the NES controller hooked up to an oscilloscope. If I take that out and try to run it, I get no signal from the controller. To my knowledge, an oscilloscope is not supposed to interfere with the connections, only monitor them. I will admit that I do not know enough to really say that with any certainty. The end result is that when we try to run the car without looking at the lines, it behaves as though we aren't pushing any buttons.

Has anyone ever run into a problem like this before? It's so odd, because we put a lot of work into getting this far. Is there perhaps something that the oscilloscope is providing to my circuit that I am failing to reproduce?

I can send pictures if necessary, but I'm afraid it will be a smidgeon difficult to get a schematic up without help, I don't pretend to know how to draw one.

Thanks a bunch, this is veeery strange.

-thegamefreak
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#154112 - Dwedit - Fri Apr 11, 2008 8:12 am

Have you tried crossposting this on http://nesdev.parodius.com/bbs/ ? More NES guys there.
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#154113 - bluesceada - Fri Apr 11, 2008 8:12 am

The oscilloscope adds a BIG resistor in between the two lines you have it connected to.

But that's really strange, did you check the voltage levels the NES Controller needs and your Microcontroller uses?

I guess you connect the Oscilloscope to GND and some Data(Output from NES-pad) line. That would add a very small pull down to the Data line, seems to be good enough to put the NES-Pad to the needed level.

But somehow also strange, as the Microcontroller should also recognize a higher level as wanted as HIGH, otherwise it would probably be damaged...

Otherwise maybe by "getting no signal" the input line(s) to the controller are all HIGH (because the osci doesn't produce a pull down..)

It's all strange because it would be just a very small pull down...
(An oscilloscope might have 10 Meg Ohm or something like that)


But I can't really help you without more detail of your circuit and how you connect the oscilloscope.

//hint: schematics can be easily made with "eagle" freely (non-profit) available at http://www.cadsoft.de/

#164434 - Pukys - Sat Nov 01, 2008 11:31 am

thegamefreak0134 wrote:

Has anyone ever run into a problem like this before? It's so odd, because we put a lot of work into getting this far. Is there perhaps something that the oscilloscope is providing to my circuit that I am failing to reproduce?

I can send pictures if necessary, but I'm afraid it will be a smidgeon difficult to get a schematic up without help, I don't pretend to know how to draw one.


Maybe the oscilloscope acts as pulldown resistor? It could be useful to add pullup- oder pulldown-resistors to inputs, especially if the controller is initially not driving the signal (in case of INOUT), or the signal can be tristate (Z).

#167375 - bullboyshoes - Tue Mar 10, 2009 1:51 pm

the scope probe will be adding some capacitance to your circuit, try adding a 1nF to the signal to grond to see if that clears it up