Propeller Education Kit power supply mystery

Rsadeika

Well at least it's a mystery to me. I have the propeller kit set up on my EIC-108 breadboard, I have programmed it , and have lit up a few LED's. So it seems that everything is fine.

The mystery, when I plug in the propplug, without a power source present, the power source circuit LED has a dim glow to it. Now, I have tripple checked the wiring, and it looks just like the graphics example in the parts box. According to the schematic, it looks like the power LED is isolated to the power regulator circuit, so I cannot figure out why the LED has a dim glow to it when just the propplug is plugged in. I figure this is probably no big deal, but I would like to know, where is the LED getting its power from.

The breadboard that I am using is almost brand new, I have had a few circuits on the breadboard, which were successfull, no unexpected results. So, before I start ripping stuff apart, does anybody have an explanation as to what the problem could be.

Thanks

Ray

PVJohn

It's getting 5V from USB port. I have the same case, but I don't care about it.

PVJohn

Rsadeika

PVJohn,

If that is true, that the propeller is getting 5V, and the propeller is rated for 3.3V, shouldn't that be of some concern. But, it still does not explain how the voltage makes its way over to the power supply LED.

Thanks

Ray

Paul Baker

It's getting power from the pullup resistors, about 1.7mA worth. It's not enough to power the Propeller except when it's running in RCSLOW and BOE is disabled. The voltage is lower that 3.3 when a load is placed on it. Remember the current is supplied by a resistor, due to Ohm's law, the more current you draw from it the lower the voltage seen is.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

PVJohn

Thanks Paul for your replay. What do we need to change to resolve this?

Rsadeika:

If you don't have any other power source connected to your board, then 5V from USB is the only way to turn on (dim) that LED. I have Prop Demo board rev.C and power LED goes ON only when I turn on the power. Maybe Paul have the solution for our problem.

PVJohn

Paul Baker

It's not considered a bug, just the way the system operates. It has no deleterious effects on any of the components and its not enough to power anything besides an LED dimly or provide backup power to an RTC. As long as it is connected to a powered PC, it will source a little bit of current. Is this just an observation, or am I missing something where this is an undesired behavior?

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

PVJohn

It's just different from Demo board, that's all. I'm not concerned at all.

PVJohn

Paul Baker

I just checked things, and I mispoke a bit. Whats occuring is when the USB is plugged into a powered computer the FTDI chip will drive it's TX pin high. This driving current flows through the clamp diode inside the Propeller's IO pin into Vdd since Vdd is floating and now a lower potential. This current is what is seen dimly driving the LED and is a unavoidable outcome of the setup. This also occurs on the demo board but because there are many more components on it, the load resistance is much lower and any one component's (ie LED) actual share decreases. So the LED is getting so little current it's not enough to illuminate, but it's still there.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Phil Pilgrim (PhiPi)

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Paul,

In situations like this, I like to use an open-drain buffer between the FTDI chip and anything connected to it. I prevents the USB circuit from powering downstream circuitry through its inputs. Another — more important advantage — is that, going the other way, it prevents a high on RX from an externally-powered micro from powering the FTDI chip and possibly driving the USB pins when it's not supposed to. Also, by using a device in the 74LVC family (e.g. the dual-buffer SN74LVC2G07), you can power the buffer from the FTDI chip's 3.3V VccIO supply and still be able to accept 5V signals from a connected micro. Plus, its input structure will not pass current to its Vdd terminal when the device is powered off. Finally, including pullups (but not pick-and-place charges), this arrangement adds only an SC70-6 and two 0603s to the real estate and 13 cents to the cost.

-Phil

Rsadeika

Thanks Guys,

Paul,

The #2 explanation is what I was looking for. I assembled the kit on my own breadboard, so as soon as I saw the dimly lit power LED, I started to think that maybe I had a bad breadboard. Then I looked at the PE Platform Setup doc, and it does mention that it is normal for the dim lit power LED.

But mostly I was curious as how the·'juice' gets over the trough to the regulator circuit. So, I figured it was through the EEProm or the propeller chip itself, but was not sure. Now, you provided me with a good explanation. I think that if I were I teacher that would be an excelent quiz question, just to see if the students were awake.

This was basically an exercise in curiosity, and not a major concern of whether my propeller chip will get fried (although I have fried a few SX52 proto boards). I had to think long and hard as to whether I should post a question like that on this board, but what the heck, the worst that can happen is that nobody responds. I can live with that.

Thanks

Ray

Paul Baker

No problem Ray, glad to help.

Thanks for the solution Phil, I love TI's 1G and 2G 74LVC series. I have a rail each of the 1G57 and 1G58 configurable gates, with only two parts in my box·I can throw any type of gate on a design in a 3mm² area.·Im using a 1G57 as an XNOR with a C0G cap and 1% resistor to generate a short write pulse on a current design.


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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 11/20/2006 6:51:45 PM GMT