[coreboot] LPCflasher Project

[Tom Sylla]

On Sat, Nov 15, 2008 at 2:29 PM, Joseph Smith <joe at settoplinux.org> wrote:
> Anyone know if USB ports have power feedback protection. What I mean by
> that is if I use the LPCflasher as a inline flasher with a PLCC32 socket
> plug, flash the chip and then power up the motherboard, I don't want the
> VCC's from the motherboard to short out the flasher and or the USB port
> from the host PC. The 3.3V regulator I am using has a internal diode so I
> am not so worried about that. I am just worried about the power source 5V
> USB. I could always use a Schottky diode that will only drop the voltage by
> .6V, but I don't know if 4.4V will suffice???

I am a little bit confused about your usage model. What it sounds like:

You will have a "target" PC for which you are developing firmware.
Some sort of PCB or protoboard with one PLCC socket and one PLCC
stacking connector will be plugged into the ROM socket of that PC.
A ROM device will be plugged in to the PLCC socket of your PCB.
Another PC will be a development system, and a USB and parallel cable
will connect from your PCB to the development PC.
With the target PC *off*, you will re-flash the ROM from the development PC

If that is an accurate description, I think you have a few more things
to be careful about. The hazard you are asking about deals with the 3V
from the target PC turning on and fighting the ~3-5 of your
protoboard. What about while you are "flashing"? You will be injecting
3V from your prototype into the 3V rail of the target motherboard,
which is not generally a good idea. Also, you will be attempting to
drive the LPC signals against a "dead" motherboard, which may or may
not work.

If the ROM is socketed, why not just program the ROM in your device,
then pull it out and put it in the target? If you really want the
convenience of not having to do that, you'll probably have to be a
little more careful about not fighting the target.