Trouble with the 2x16 LCD and Prop Chip

RICoder

I just got this (http://www.parallax.com/Store/Accessories/Displays/tabid/159/CategoryID/34/List/0/Level/a/ProductID/52/Default.aspx?SortField=ProductName,ProductName) LCD today, and am trying to use it with a prop chip.

It requires 5v power, so it is on its own power source, and the test works on it (meaning setting both switches to off to show the Parallax info on the screen).

That said, set to 9600 baud and sending it data from pin 10 on the prop doesn't do anything to it...is it the 3.3v vs. 5v difference that is causing this? Can I even drive this screen from the prop chip?

sylvie369

It works fine with the Prop. You need to supply the LCD 5V, and of course 3.3V to the Prop. Give them a common ground.
Connect a Prop pin to the LCD data pin through a 1K resistor.

Code will look something like this

Obj
  LCD   : "Debug_LCD"
Pub Main
  LCD.Init(3,9600,2)
  Waitcnt(30_000_000 + cnt)
  LCD.cls
  Counter := 123
  LCD.gotoxy(0,0)
  LCD.dec(Counter)

etc.

RICoder

Ok, can someone tell me why they need a common ground? Is this critical and why?

I feel like I am getting this whole electronics thing now...and knowing the answer to this may be a breakthrough moment for me [noparse]:)[/noparse]

Chris Savage

Without a common ground, neither side has a reference with which to know what the signal level is. The signals are all in relation to ground, so take that away and they are arbitrary to each other. Think of it like this. You are standing on the ground and someone else is standing on the ground beside you. This is common ground. So now anything above you is the same relative height as the other person. But what if the other person was standing on higher ground? Something 20 feet up to you might only be 1 foot up from them. As a note, you should also have a 1K resistor on that I/O line from the Propeller sicne the LCD is pulling that line high to 5V. I hope this helps. Take care.

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Chris Savage
Parallax Engineering