Why does the LED glow?

RichardF

I just put together my PE kit and all works fine. Interesting though, with the power to the PE turned off (no battery) the power indicating LED (3.3 V regulator output) glows dimly (low voltage) whenever the USB cable is attached to my computer. Doesn't seem to hurt anything but my curiousity is perked up. Looking at the circuit diagram, I can't see how power is getting from the USB cable to the PE 3.3 V bus. Anyone have an idea?
Thanks,
Richard

crgwbr

Not sure of the exact reason, but I think you'll find that if you unplug the propeller chip, it will shut off. Based on this, there is probable just a small leak from tx/rx lines to 3.3v line. Anyway, what ever it is, it's normal.

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I would love to change the world, but they won't give me the source code

People say that if you play Microsoft CD's backwards, you hear satanic things, but that's nothing, because if you play them forwards, they install Windows.

I spent a minute looking at my own code by accident. I was thinking "What the heck is this guy doing?"

Peter Jakacki

It is surprising what little current (microamps) is required to get an LED to glow. It is also not surprising when main power is shut down but other signals etc are still connected that the power led glows. Although the USB power may not be connected to the Propeller you will find that it is powering the USB chip and as has been previously noted the tx from the USB chip idles at +3.3V which is able to source at least several if not tens of milliamps back through the Propeller's substrate diodes onto it's supply. This can also happen to some extent as well if an RS-232 connection is active.

A general note:
On circuits that have brown-out such as the Propeller this may prevent a reset being generated when power is reapplied, not that I have found this to be the case with the Propeller. A simple fix is to place a 1K load resistor across the supply which also helps to discharge supply caps quickly. In some systems if the power is interrupted briefly the system can hang because of this non-reset so this is something to look out for.

*Peter*

P.S. Brownout circuits are meant to prevent this non-reset problem anyway but it is something to be aware of.

Post Edited (Peter Jakacki) : 5/31/2007 11:24:08 PM GMT

RichardF

Thanks for the answers.
Richard