View Full Version : Demo Board Question

02-15-2007, 08:43 AM

Just got a demo board today. Been working on a project on the edu-kit breadboard.
I had the project working fine on the breadboard, and then I moved it over to the demo
board. Basically, the BPP420 LCD screen that was working perfectly on the breadboard
refuses to work on the demo board. I can see that it has 5v power, but won't initialize.
Exact same program and wiring configuration on both boards, so what is different about
the demo board that causes the LCD screen to not work?

Mike Green
02-15-2007, 09:00 AM
Shouldn't happen. How about posting a schematic and the program?

02-15-2007, 09:08 AM
Well for argument's sake I've cleared everything off the board and started from scratch.
The setup is simply as follows:
LCD plugs into 5V power, Pin 2 (but the result is the same for any pin), and VSS.
The program is attached, and works fine on the breadboard but does not work on the demo board.

02-15-2007, 09:18 AM
Sorry, BPP-420.spin is attached.

Post Edited (rapscaLLion) : 2/15/2007 2:27:21 AM GMT

Mike Green
02-15-2007, 10:29 AM
The only thing odd is that "Simple_Serial" is declared both in BPP-420 and in demoboardtest. Since you don't use it in demoboardtest, there shouldn't be a problem. I don't see anything with the program that would cause what you observe. Are you sure the LCD is plugged into +5V power and not Vdd? Have you tried something simple to test out the demo board?

02-15-2007, 11:20 AM
The demo board works fine with simple flashing led and button programs.
I am sure it's plugged into 5v. What I am wondering is if maybe +5v isn't really
a true 5v on this board? I know on the breadboard using a 9v battery, the lcd would
only work when the battery was full, as the battery started to drain the lcd would no
longer respond. I replaced the 9v battery with a 9v 300mA adaptor, and it works fine.

I have tried the demo board with the supplied 6-7.5v adaptor, the 9v and a 12v adaptor.
No change on all three.

02-15-2007, 11:25 AM
I think it has to be the power. If I power the lcd screen via the breadboard and plug the input into the demoboard, it works (albeit with major glitches I assume come from not sharing the same ground). So I can only conclude that the +5v on the demoboard is not really +5v. I don't have anything else plugged into the demo board, what could the problem be?

Mike Green
02-15-2007, 11:29 AM
The +5V supply is really +5V. Be careful that you've plugged the LCD into the single +5V socket rather than the Vdd sockets next to it which are wired to the +3.3V supply used by the processor and other circuitry on the demo board.

02-15-2007, 11:47 AM
Well I'm at a loss. Attached are two photos, one of the board and one of the screen. The image of the board shows my connections and an indicator LED that I put into pin 7 and VSS. The other connections should be pretty obvious, Red in +5V, White in Pin 0, Black in VSS. The image of the screen is showing that it has power. The screen isn't supposed to be filled with black squares, I just turned the contrast all the way up so it would show up in the photo.

Could it be a lack of amperage? The unit power requirements are as follows:
Backlight off: 4.8-5.5 Vdc @ 10mA
Backlight on: 4.8 - 5.5Vdc @ 100mA

Regardless, the unit does not work on the demo board with backlight off or on. The breadboard works fine either way.

Post Edited (rapscaLLion) : 2/15/2007 4:55:36 AM GMT

Mike Green
02-15-2007, 12:12 PM
I assume with the serial line attached to pin 0 that you've changed the program to match (otherwise it would need to be in pin 2 based on what you posted).

I can't explain it. Perhaps you somehow have a defective board. The regulators should be able to handle loads on the order of 500ma. Can you check the voltage at the +5V connection to the LCD? With and without the load of the LCD backlight?

02-15-2007, 12:36 PM
Yes, the program has changed to match. Alas I have no equipment here to check voltage.

Mike Green
02-15-2007, 12:38 PM
I'd put a call in to Parallax Tech Support tomorrow or e-mail them this evening. They can review your posting and get back to you.

02-15-2007, 01:47 PM
Ok, thanks for all the help. I'll also see about finding the voltage tomorrow.

02-15-2007, 02:10 PM
Is there a Test mode dip switch setting on your LCD? If so I'd put it into test mode and see if anything happens.

I'd also remove your LED·which looks suspect because I don't see a resistor anywhere.

02-15-2007, 02:59 PM
Your right about the LED, it was only there for the photo since I forgot to remove it.
It has no resistor which I know is bad, but the presence of the led, resistor or not has
no effect on the lcd problem. The LCD has no test mode that I'm aware of.

Thanks anyways!

Chris Savage
02-15-2007, 11:23 PM
I’ve noticed you have an LED on connected to P7…Does your code also turn this LED on? In any event it would seem that P7 works…Could you try connecting the LCD signal line there and change the code to use that pin and see what happens in the event something happened to P0 on the Demo Board? Thanks.

Chris Savage
Parallax Tech Support

02-16-2007, 10:42 AM
I removed the LED, and tried the LCD on every pin in succession. No change. But still works great on the breadboard, and on an old BS2 board.
I think it must have something to do with the power coming from +5V...
Anyways, I'm going to try and take it to my friend who actually knows electronics (I'm just pretending to know lol), and see if he can figure it out.

02-17-2007, 02:54 AM
You said "major glitches I assume come from not sharing the same ground". I have seen similar problems with different circuits when you don't share a common ground. Unless you are using opto isolation, transformers, or differential paired signals, or wireless (there are probably other exceptions, too), you will need a common reference between interconnected parts and power supplies. I usually call that point "DC common".