Power Issue (I think)

I am powering my IMP via USB and using a Sparkfun 5v display. I power the display from the VIN pin on the April board.

The weird thing is that it works perfectly on 90% on the USB ports I use the other 10% I get corrupt data on the display.

Also, I use a USB charger to power the April then the display is corrupt 90% of the time.

Any idea why this would happen?



I started using a Sparkfun display recently as well, and am noticing the same thing. I had been using the Adafruit RGB displays with serial backpack, and I stopped because the backpack was really big, but I might figure out how to go back. The RGB backlit displays seem to be much more easy to read.

I’m not getting data corruption, I’m getting an extra “character” at the end of a string.

Remember that some of the Sparkfun displays are actually 3.3V, even if they say 5V on the display. I’m not sure about the 20x4’s but some of the 16x2’s are like that.

The power from your laptop might be cleaner, but you are probably limited to 100mA from a USB port. The Imp can occasionally draw up to 400mA, so that probably isn’t something you want to do a lot.

What is the available current from your USB power supplies? There are good ones, and not so good ones.

I have tried every USB adapter in the house… Apple, Android, random 3rd party and all off them give me the corrupt display.

I have not measured the voltage yet.

I thought powering my project by USB would be convenient as I could use an adapter into 110v or my laptop.

My thought is that it’s a 5v display and you’re giving it 3.3v signals from the imp. 3.3v is really not high enough to be totally compatible with 5v logic (really, you need 0.7x VDD for that, which is 3.5v).

This means that the higher the voltage the display is powered with, the more the imp’s signalling is in a grey area, binary-wise, and hence the corruption.

The real solution is to buffer the imp’s transmit to 5v levels…

Thanks Hugo, so you think it’s not the power I’m taking from the IMP VIN but the serial signal that could be the issue?

Weird how it works from my computer USB and not the wall charger USB.

I’ll try and measure the voltage on the charger vs the the voltage on my laptop to see if that gives any other clues.


Interesting… My computer USB measured 4.92v and the adapter measured 5.2v


Are you using this serial backpack with the display?


If so, it does require 5V TTL serial, and Hugo’s assumption is correct, that you are in a “gray” signal area. I haven’t done this, but I believe there are a number of logic level convertors around…I’m sure someone else can chime in here with an answer.

I used this LCD…


Yep, that board wants 5V serial. I believe you can just use something like this:

@philspitler yes, the voltage of the supply vs the imp voltage.

The imp has a regulated supply, so the imp’s TX line will always be very close to 3.3v.

If the display is powered by 4.92v, then the logic 1 threshold is 0.74.92=3.444v.
If the display is powered by 5.2v, then the logic 1 threshold is 0.7

With a lower voltage running the display, the imp high output is closer to, but still not quite at, the required level to be unequivocally recognized as a 1.

So it seems like I would need a 3.3v display or the Logic level convert linked above.

Do you agree that these are my only 2 options.

Thanks again.

Yep, unless you can find a 4.5v power supply to run the LCD from :slight_smile:

hmmmm. How much power does the IMP use? I think I spotted a 4.5v PSU in my collection recently.

I need enough power to power the IMP and the screen.

The Imp card can draw up to 400 mA.

Ok, I found a 4.5v power adapter in my box of junk and it works great.

Thanks for helping me troubleshoot.


Now jealous of your junk box. I bet you have -18v regulators in there too :wink: