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On one board I have a microcontroller with one power supply and on another I have a Flip Flop with a separate power supply. I want to connect a pin of the microcontroller to the reset pin of the flip flop. Can I just put a wire accross or does it need more than that? I was going to do that but now I'm not sure that would work because it wouldn't be a complete circuit and the two boards may not have quite the same ground levels. The two boards have to have separate power supplies because the board with the DSP is premade and the power comes from the mains. Thanks

Ayeayeron
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4 Answers4

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A single wire may or may not work - check to see if your boards have a common-enough ground. You may get lucky and it will be just fine (since both boards will be connected to the mains ground, at least in some way). At the very least, you can probably run two wires - one to tie the two grounds together and another for the data. If for some reason you can't have a common ground, you can look into optoisolators.

Carl Norum
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If the supplies on both the boards are isolated, e.g., batteries or most wall-warts, you just need to bridge the grounds together, which provides the return path for the signal you want to use.

If you're at all worried that the grounds might not be isolated, you can temporarily connect them through a 100K resistor and measure the voltage drop across the resistor. If the drop across the resistor is nearly zero, then you should not have a problem connecting the two grounds.

The reason for the resistor is that a DVM has high enough input impedance that just measuring between the grounds might show you some voltage that's more electrostatic than actually electromotive; the resistor dissipates the electrostatic aspect.

JustJeff
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  • Sorry, I know you wrote this awhile ago, but could you explain what the last part about electrostatic vs. electromotive means? – NickHalden Jun 21 '11 at 12:13
  • @JGord - np. I think I was getting carried away with 25-cent words. By 'electrostatic' I meant something with very high impedance, that would not be able to sustain the voltage if any current is drawn from it. By 'electromotive' I think I was trying to indicate the sort of voltage you *can* draw current from. Perhaps some editing is in order.. – JustJeff Jun 21 '11 at 23:05
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If on separate boards, use a differential signaling approach such as LVDS, with a driver on one board, and a receiver on the other. (This assumes the two boards have ground voltages that are reasonably close together -- if not then you need an isolation barrier.) Reset signals are an example of something that is sensitive to glitches -- a momentary glitch on a reset line has an effect on the future state of the chip going to it.

You can get away with less-careful approaches if the signals in question are stateless inputs.

Jason S
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Thinking out of the box:
You could place a simple motor or a relay near controller, connect a usual thread to it and lay it to the second board, where you place a switch, connected to it's ground or it's VCC. (it could be a real wire -> single-wire signal transfer :D )
If those two boards are in optical visibility you could put IR transmitter-receiver.
...something else...

PF4Public
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