Reset LCD

Pipo · Guest

There is some way to reset an LCD from software?

Because of some high electrical noise, the LCD sometimes hangs up, but if I short the ENABLE pin to ground for some time (less than a second) the LCD go back to normal.

So I need to do this by software. I try output_low(PIN_B0) (enable pin) for some milliseconds but nothing happens.

If I look with an oscope the enable pin, when the high noise is present and the LCD hangs up, the waveform of the ENABLE pin seems to be inverted, for example:

Normal: ______-_____-_____-______

When hang up:-------_--------_--------_-------

What can I do?

Thanks!

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Your question sounds confuse. The enable pin is controlled by the software, so a unusual waveform at this pin
doesn't contain an information about the LCD controller state. You may be right about a necessary controller reset anyway.
Clearly pulling down the enable pin does not reset the controller.

The full initialization sequence, including the setup of bus width from an possibly unknown state, can always reset a
standard Hitachi compatible LCD controller according to my experience. A power supply cycle of course can, too.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

You could have the PIC switch the power to the LCD on/off.
Look on page 22 (page 26 in Acrobat reader) of the PicDem2-Plus (Rohs)
board. It shows a transistor which switches power on/off to the LCD.
The transistor is controlled by a PIC pin.
http://ww1.microchip.com/downloads/en/DeviceDoc/51275d.pdf
I probably would have included a pull-down resistor on the PIC pin
so that if the pin is floating, the transistor base is guaranteed to be
held low. That may not be necessary, I'm not a transistor designer,
but I try to be cautious.

Guest · Posted: Mon Nov 23, 2009 2:38 pm

Thanks for you answer... I know its confusing... but thats the problem... and i can't reset the controller.

If I, after a hang up, execute the init_lcd() function nothing happens.

If I (with a small wire) connect the enable pin to ground, the LCD continues with the execution.

When a noise pulse is present, the LCD goes into a "pause" state... showing the information on the moment of the noise pulse... if, a second noise pulse is present, then, the LCD continues working ok.

I don't know what can I do.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The real solution is to analyze and cleanup the fundamental noise
problem. Use grounded metal shielding for the board. Carefully filter
the incoming power. Anything else is just doing band-aids on the
problem. There are appnotes on this:

Designing Microcontroller Systems for Electrically Noisy Environments
http://ecee.colorado.edu/~mcclurel/iap125.pdf

Guest · Posted: Mon Nov 23, 2009 3:12 pm

Yes, you are right.

The main problem is that the enclosure is ABS plastic... so I can't do a faraday cage or something like that.

I try shielding the LCD pins on a metal grounded cage, but still the same problem.

The LCD pins are connected direct from the pic with metal pins.

Guest · Posted: Mon Nov 23, 2009 8:46 pm

Problem solved... BY SOFTWARE.

The LCD library has a while loop on the send byte function. If in the moment that the pic is entering that while receive a NOISE PULSE, then, the pic get stuck on that while forever!, so I put a "timed while" to prevent that.

Conclusion, the LCD never hang up again.

I know that this is a band-aid, but solves the problem. The entire circuit is in a VERY noisy environment. I'm sure the PCB is designed correctly, I even put a grounded shield for the LCD pins... The noise then became lower... but that does not solve the problem.