competing grounds

[Bill Weiler]

I have a uC board that can be powered by a battery and USB. When I
download code through the UART I connect only the USB power. If I
wanted to download code with the battery powering the board, could I
just connect the grounds? Someone said I could do this, but it seems
to me dissimilar grounds would cause a potentially bad current flow.
Is this person wrong?

Bill

[Dave Curtis]

Speaking simplistically, you can, and usually want to, tie all grounds together so that all parts of the circuit have the same ground reference. (Signals crossing ground reference regimes require special handling.) What you must watch out for is letting the power rails get tied together.

USB power is going to be 5V. What is the battery supply voltage? You can use a diode or two to make sure that the power supplies don't fight each other. It's easy to show in a drawing, but hard to describe in e-mail. Is there any way you can post a quick sketch? I'd probably be thinking of putting in a diode so that you can run off either power source and plug/unplug the USB at will.

If that isn't confusing enough, depending on your battery technology, a battery under 5V might be OK with being tied directly to 5V in a "charging" configuration, but that depends on the battery chemistry and the total impedance in what effectively becomes the charging circuit. Usually, you need a current limiter, at minimum. Fancy batteries like LiPo's need a charge controller.

-dave

> --
> You received this message because you are subscribed to the Google Groups "HomeBrew Robotics Club" group.
> To post to this group, send email to hbrob...@googlegroups.com.
> To unsubscribe from this group, send email to hbrobotics+...@googlegroups.com.
> For more options, visit this group at http://groups.google.com/group/hbrobotics?hl=en.
>

[Bill Weiler]

What I have is a 2 cell Lipo, dropped to 5V in a DC-to-DC converter,
then dropped to 3.3V using a linear regulator which powers the board.
On the PC side, I have a USB-to-UART converter, which I connect to
UART0 on my board. I get garbage characters on the PC terminal when I
power through the battery, so what I am really wondering is: if I
connect the USB-to-UART converter's ground to ground on my battery
powered board, will this fix the garbage? or cause damage? Thanks.

Bill W.