When I use a LM1117T-3.3 as my 3.3V power source, the LED is detected as common Anode. After boot, the RG LED blinks Orange-Red-Orange-Red (or G+R, R, G+R, R). Although operation is just fine, the Red light won’t turn off since the pin is on negative logic and has 2.0volts on it (meaning IMP wants it off, but actually turns it on). FYI Tried various capacitors on input/output of LM1117T-3.3. Wish I had an O-scope, but don’t have it just this moment.
I’m not sure what the root cause of this is, however it would be nice to know how to compensate for this simple type of power supply and have the LED operating the “right” way.
Quick note: Using a hard drive power supply (12V and 5V), 5V -> LM1117T-3.3, I get the strange problem.
Using the USB->5V DF Robot Power Supply -> LM1117T-3.3 the problem goes away.
Must be something odd about the hard drive power supply?
Also of note, I used the 5V from the HDD Power Supply as well as used a HDD Power Supply 12V -> LM7805 -> LM1117T-3.3 and that still has the problem. So it doesn’t seem like giving a cleaner 5V signal from the LM7805 helps.
I have since reworked the circuit to use an LM2576T-005G rather than the LM7805. Still taking 12V in from the same power supply. While the same problem appears, here’s an odd thing that makes me suspect the power supply itself is part of the issue.
This is my setup in general: [wall outlet] -> Cord #1 -> [HDD Power supply] ->12V Wire -> [5V regulator/SMPS] -> [LM1117T-3.3]
If I connect/disconnect “Cord #1”, almost 80% of the time the LED detection is backwards.
If I connect/disconnect “12V Wire”, then the LED detection is backwards only 50% of the time.
Granted there may be other issues, but it seems that letting the HDD Power Supply “boot up” and level out helps. Which is similar to saying I think there’s odd startup power issues with this $10 HDD Power supply.
I’d be thrilled to get a better PS when I run across one. But first… when I can, that 10k resistor test
Yep, this sounds very much like a rise time issue. What’s happening is that because the voltage ramp is slow, the imp is booting and doing the LED type detection before the voltage has got high enough to be able to discern the LED conduction.
The resistor is there to help this, but in certain circumstances (eg high capacitance on that line) you need more current to read the type reliably.
Yeah I read up on the whole rise-time issue yesterday after you mentioned it, which totally explains the lower resistor value solving the issue.
This is not your problem to solve, but I’m just wondering out loud why I have a rise time issue:
The resistor goes to ground, so it’s not really “rising to 0”… or perhaps somehow there’s a bunch of voltage that needs to get “drained out” quickly. But where does that “charge” originate from being that the device was off?
IMP.RED->100 Ohm->LED.RED.PIN->10k Ohm->GND
Anyways, if anyone passing by has ideas, please let me know so I can design better in the future!
I ordered it from Jameco and there’s an STMicroelectronics logo on it.