From: Ryan Gammon
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Subject: Grounding advice
Date: Fri, 04 Oct 2002 20:30:36 GMT
NNTP-Posting-Date: Fri, 04 Oct 2002 17:30:36 ADT
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
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
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!