On Sat, Nov 15, 2008 at 2:29 PM, Joseph Smith joe@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.