2x16 lcd on quickstart prop

bigirv

Ok heres the question if you have a device thats expecting 5v dc and you give it that will the signal also have to come from something running off of 5v. I tried using the bread board on my bs2 h/w board with 2 jumpers 1 for the vdd and 1 for ground. then the other one was coming from the prop quickstart board. I know it was mentioned in the other post that for a prop to talk to a bs2 a resistor had ot be used to keep the prop from getting fried. does the ground have to go back to the controlling device or does the signal have to run off of the same voltage? Sorry if I confused anyone :P.

Dr_Acula

Running 3V3 to a LCD display running on 5V you just connect the wires up. 3.3V will be recognised as a "high" logic level by the LCD display.

If you were sending data the other way then you need protection resistors, eg 2.7k, but for LCD displays the data only ever goes one way so you won't need these resistors.

So - make all the grounds the same. Your microcontroller runs from 3.3V. Run your display from 5V. These displays generally need 6 data lines and you just connect them with no resistors needed.

bigirv

its the breadboard friendly 3 pin model not the parallel. but ill try the ground

bigirv

good news at BigIrv lil evil lab. Connecting the vss to the prop, bs2, and lcd fixed the issue, which probably cause the com issue between the 2 props or the prop and bs2. Off topic why does the devices have to share the same ground or at least for the serial communication between 2 micro controllers, and what is the absolute lowest i can use between the prop/bs2 I know that it was stated 3.3k is there 1 i can use thats lower or higher that may be in the whats a micro controller package?

GeckoKev

The devices have to share the same ground because any time you want current to flow through a wire (or serial connection) you have to give it some sort of return path back to the source. So lets say you have circuit A using a battery, and circuit B using a different battery. If you want a Tx line to be able to send data from A to B, then it's going to need some small amount of current to flow through that Tx line. Therefore you have to give it a return path back, so we typically do this by simply tying the grounds together. Of course if you need them to be electrically isolated from one another (without the grounds directly connected), then there are other methods to accomplish the goal.

By the way, like the "evil lab" comment! :thumb:

Cheers,
-Kevin

MagIO2

Well ... I think that's only the answer for: "Why do you need two different connections?"

For example some serial interfaces use a positive and a negative voltage for communication - no ground!

Ground is simply the name of the reference in any circuit. Each circuit can have it's own reference-level if you -for example- measure it's ground level against protected earth. The circuits locally only make sure that any Vcc needed is constant against it's own Vss.
Each wire also works as an antenna which charges via radio frequencies all around us .... If you now connect two differnent circuits for communication (only one wire), they don't neccesarily have the same reference level due to different charges, so best thing that can happen is that it works ... or only sometimes ... or never ... or even destroy your circuit, as the levels are to different, for example due to static charge.
Same thing happens if you walk over some kind of carpet and then touch something out of metal, it's called statical discharge.

So, connecting ground between different circuits makes all use the same level as a reference.

But when talking about resistors .... for unexperienced users doing some breadboarding I'd alwas suggest to use resistors, because it increases survival-chances in case of wiring and programming-errors.