Using outputs as GND

F

Fowkc

Senior Member
I have limited PCB space, and was wondering if I could save a few tracks and jump wires by doing the following:

I have space PORTC outputs on my 28X. If those outputs are kept low, can they be used as GND points, assuming I keep the current below 20mA? I know you can do it with LEDs, so I don't see why you can't. My plan is to use them for a Dallas I2C RTC and a 512K EEPROM.
 
hippy

hippy

Ex-Staff (retired)
It sounds theoretically possible, but I'm sure it's not recommended, and probably just asking for problems.

Not sure if a low output is even 0V which could have some effects, and at some point during reset and power-up the lines are likely to be inputs and high-impedance. I'm not sure what effect that will have on chips 'powered' that way, or on their I/O pins and consequently things connected to them. If a +5V chip has its 0V sitting at, say, 2.5V then it's being powered at 2.5V, and a 0V signal into it would be -2.5V from the chip's perspective. That's probably not good, and should the PortC line go high, any 0V signal into it would become -5V.
 
Technical

Technical

Technical Support
Staff member
Very bad practice. 0V rails should be consistant across a design.

In theory you could do the opposite (V+ from a PIC output to power a second chip) but don't do it with the ground rail, its asking for trouble.
 
Y

ylp88

Senior Member
If worse comes to worse, put the PICAXE in a socket but pull the pins out of the socket (dual wipe pins often come out with a little force, machined pins can be broken off). This means that those pins no longer require holes or pads which allows room for more tracks. Not necessarily good design practice but probably better than using the pins as a grouding point.

<b><i>ylp88 </b> </i>
 
F

Fowkc

Senior Member
<BLOCKQUOTE><font size=1 face=arial>quote:<hr height=1 noshade>If worse comes to worse, put the PICAXE in a socket but pull the pins out of the socket (dual wipe pins often come out with a little force, machined pins can be broken off). <hr height=1 noshade></BLOCKQUOTE></font><font face='Verdana, Arial, Helvetica' size=2>

Already done that :)

OK, I thought it might not be such a good idea, but wasn't really sure. I'll have to be more creative with other space saving ideas. Thanks guys!
 
J

jodicalhon

New Member
<A href='http://www.siliconchip.com.au/cms/A_103659/article.html' Target=_Blank>External Web Link</a>

Probably is bad practice, but here it's being used as a power-saving technique.
 
You must log in or register to reply here.
Top