PropBOE Power Supply Problem
RobertDyer
Posts: 23
Recently I have been having a very elusive problem with some smart, serial LEDs (Adafruit NeoPixel Stick w/ 8-WS2812 LEDs). I have the stick connected to one of the servo ports (pin14), and powering the stick with the 5V from that servo connector. I have the power selection jumper connected to the 5V side not the VIN side.
When powering the BOE with one of the Parallax 7.5V wall warts the LED strip just won't behave, regardless of whether or not the USB cable is connected. When I go back to using only the USB connector for power, the LEDs work fine. It appears the I/O pin, or the DC supply pin, get so noisy that the LEDs flicker and act in other inconsistent ways.
I've measured the output of the wall wart, and it is 7.6VDC with only a couple mV of ripple (unloaded). I don't have a scope to check the 5VDC voltage output for ripple, but my DVM only shows about 20mVAC (loaded), though there's no way to tell what the noise waveform may look like. I thought it might be a ground loop problem between the wall wart and my PC, so I reversed the AC connection at the outlet with no change. Next I tried using batteries connected to the barrel connector (no ground loop) - still a problem. ANYTHING connected to the barrel connector seems to give me problems. It appears to me it's an issue with the power supply circuit.
Could I have burned out a component of the DC supply that is making the switcher VERY noisy? I tried connecting a fairly large (2200uF) cap across the 5V supply, but that didn't help at all. Is this a problem that anyone else has reported? If so, can you suggest a fix for me?
By the way, I've had the opposite problem before when using a servo, where the servo appeared to load the USB supply enough that the BOE wouldn't work. I had to use the wall wart to make the servo not mess up the CPU.
Anything you could suggest would be helpful. As it stands right now, I have to be very cautious how I develop with this board.
Thanks.
Robert
When powering the BOE with one of the Parallax 7.5V wall warts the LED strip just won't behave, regardless of whether or not the USB cable is connected. When I go back to using only the USB connector for power, the LEDs work fine. It appears the I/O pin, or the DC supply pin, get so noisy that the LEDs flicker and act in other inconsistent ways.
I've measured the output of the wall wart, and it is 7.6VDC with only a couple mV of ripple (unloaded). I don't have a scope to check the 5VDC voltage output for ripple, but my DVM only shows about 20mVAC (loaded), though there's no way to tell what the noise waveform may look like. I thought it might be a ground loop problem between the wall wart and my PC, so I reversed the AC connection at the outlet with no change. Next I tried using batteries connected to the barrel connector (no ground loop) - still a problem. ANYTHING connected to the barrel connector seems to give me problems. It appears to me it's an issue with the power supply circuit.
Could I have burned out a component of the DC supply that is making the switcher VERY noisy? I tried connecting a fairly large (2200uF) cap across the 5V supply, but that didn't help at all. Is this a problem that anyone else has reported? If so, can you suggest a fix for me?
By the way, I've had the opposite problem before when using a servo, where the servo appeared to load the USB supply enough that the BOE wouldn't work. I had to use the wall wart to make the servo not mess up the CPU.
Anything you could suggest would be helpful. As it stands right now, I have to be very cautious how I develop with this board.
Thanks.
Robert
Comments
The WS2812 LEDs require an input-high voltage of at least 70% of VDD. If you run it off of 5 VDC, it will require at least a 3.5 volt signal, which is a higher voltage than the Propeller microcontroller will generate. If you run it at 4.5 VDC, it will work with a 3.15 volt signal, which would work. USB ports can supply power anywhere between 4.5 and 5.5 VDC. The power supply on your USB port may be running a little low, allowing the WS2812 LEDs to work with the Propeller microcontroller.
Try buffering the Propeller's I/O pin using a 5 volt buffer. It should condition the signal to a voltage range that will work with the WS2812 LEDs, no matter which power supply you are using.
— David Carrier
Parallax Inc.
I was thinking of suggesting just that. If you use a 3.9 k ohm or greater pull-up resistor, it will keep the current through the ESD protection diode within the maximum ratings, and the 0.3 volt drop from the ESD protection diode will allow the pull-up resistor to pull the line to 3.6 volts, enough to register as high on the WS2812. It isn't an ideal solution, as the voltage is right at the threshold, and you will have to modify your driver object, but it should work in a pinch.
— David Carrier
Parallax Inc.