Hi everyone,

I recently got an LCD screen HD44780 that I made worked with my Mbed. But lately, the screen does not work at all (nothing happens) whereas the program hasn't changed. It is possible that some electrical connections have been interrupted, but now everything is as when it worked well.

Currently, I try to do a reset of the HD44780 because I guess he has put himself in a state other than the one expected by the Mbed. No reset button is present, so I guess it must be reset manually by software ...

Do you have a solution? Why the screen goes blank? Should I do a reset and how?

Thanks, I am really looking forward to your reply.

Hi Simon, are you sure all the electrical connections are ok, especially pins 15 and 16 on the LCD which power the backlight?

Hi Dave,

Here are my electrical connections :

• Pin 01 : GND
• Pin 02 : Vout
• Pin 03 : GND through a resistor
• Pin 04 : pin I/O 15
• Pin 05 : GND
• Pin 06 : pin I/O 16
• Pin 07 : -
• Pin 08 : -
• Pin 09 : -
• Pin 10 : -
• Pin 11 : pin I/O 17
• Pin 12 : pin I/O 18
• Pin 13 : pin I/O 19
• Pin 14 : pin I/O 20
• Pin 15 : Vout
• Pin 16 : GND

The code I use is :

#include "mbed.h"
#include "TextLCD.h"

TextLCD lcd(p15, p16, p17, p18, p19, p20); // rs, e, d4-d7

int main()
{
    lcd.printf("Test\n");
}

Your connections and code look fine to me, does the backlight come on at all? I.e. does the lcd light up a bit when you power up? If not then if you are sure the connections are ok, i suspect it is blown. If you check out the data sheet here

http://www.crystalfontz.com/products/document/908/CFAH1602BTMIJT_v1.0.pdf

On page 18 it explains the connection to the backlight should be through a resistor as it can easily be damaged, i notice yours is connected directly?

Hi Dave,

Indeed, my backlight is directly connected without resistor. But I don't think the problem is here : this function is independent of the character display. Anyway I have now connected it through a resistor. The LCD light up a bit when I power up.

I really think that it is necessary to manually reset the LCD... :-/

Also check the display contrast pin, if this is the wrong voltage (open circuit or indeed short circuit to the 'wrong' voltage) the display will appear blank. If set by a pot adjust the voltage through the entire range to see if the lcd character matrix/dots become visible.

Standard HD44780 displays dont have a hardware reset pin. They can be reset in software, but that should not be necessary as they are reset when power is switched on.

You may want to test the display by only connecting power (pin2), gnd (pin 1) and contrast voltage (pin 3). That should result in a display that has one row of black squares.

When this doesnt happen it either means you have no power, the contrast is wrong or the display is dead :(

I too have a same issue with MBed ublox C027 board, after firmware reload LCD screen became blank . Dont know what to do, please advise