From: Ryan Gammon
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Subject: Re: Grounding advice
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Date: Fri, 11 Oct 2002 19:30:47 GMT
NNTP-Posting-Date: Fri, 11 Oct 2002 16:30:47 ADT
I happened to come across the following in AoE (pp853):
"... we suggest as many redundant ground connections as possible...
... Don't worry about about "ground loops" in a digital or RF circuit;
that's a microvolt audio circuit issue"
Ryan Gammon wrote:
> Hello folks,
> I have an avr microcontroller evaluation board connected to a board that
> I've soldered together on a piece of perfboard.
> The perfboard has a xilinx cpld, some sram, a couple of 74hc chips, and
> a 7805 5V regulator.
> I have the cpld configured as a glorified bidirectional shift register,
> controlled by the microcontroller. It has 20 outputs, and is set to use
> a "slow" slew rate. I'm finding that the cpld seems to be occasionally
> receiving extra clock pulses. If I put a voltmeter between one of the
> cpld outputs and gnd, everything seems to work.
> Unfortunately, I don't have an oscilloscope, but I'm guessing this has
> something to do with the way I have ground and power wired up.
> I have two questions:
> 1. Two regulators:
> I have the microcontroller board running off one wall wart supply, and
> my perfboard board running off another. I figure this is ok - the
> tolerance on the regulator's are 5V +/- 0.25 V, and my ic's can take i/o
> voltages up to VCC+0.5V. The transformers in the wall warts should
> isolate my regulators from my house's power (if it didn't, I could get
> large ground loop currents flowing, depending on how the outlets are
> wired, right?)
> Does this logic make any sense?
> 2. Ground loops and "star" grounding
> On my perfboard board, I have everything wired in a star pattern -- all
> the VCC's and GND's run back to two points on the board. My theory here
> is that if several pins on an IC switch, and if the resulting current
> creates ground bounce, other chips on the board should be less affected.
> I am now faced with connecting the ground on my perfboard board to the
> ground on the microcontroller board. There are three ways I can think of
> doing this:
> 1. Connect the grounds of the outputs of the wallwart supplys together
> 2. The microcontroller board has a number of i/o pin headers which
> consist of 8 i/o pins and a VCC and GND pin. There are also two
> dedicated GND header pins on the board. I could connect _one_ of the GND
> pins to the GND on my perfboard.
> 3. I could connect all the header GND's and the wallwart gnd's together,
> abandoning the whole "star pattern" grounding theory.
> And of course there's:
> 4. Power it all off one regulator
> IMO, #1 seems like the best idea to me.
> #2 seems more risky -- the traces running to the header are kinda long
> and skinny, and I'm worried about the inductance.
> I was taught that #3 is bad, but I have a hard time justify why, beyond
> my "it somewhat isolates ic's from the effects of ground
> bounce/inductive spikes" theory.
> #4's a good idea, but I don't want to blow the regulator on the
> microcontroller board if I short something on the perfboard.
> I've tried all of the above with wirewrap wire (no luck), and am
> experimenting with thicker gauges of wire, but without an oscilloscope,
> it's hard to determine whether I'm making things better or worse.
> Thanks for any suggestions or comments on any of the above!