LCD problem when I touch the lcd case

[Sharagim]

Hi,
I have a device which is in inside a metal case, it has a 16x2 character LCD.
Sometimes when I touch the LCD case I see unknown character on my LCD and sometimes it stop responding and Micro watchdog reset the board. is there any idea to solve this problem ?

 

[ARQuattr]

It sounds like an ESD pulse is causing the problems. Is the LCD well-grounded to the board? It may also help if the metal case is grounded. Or alternatively isolate the LCD electrically from the case.

 

[Sharagim]

I think the same.
Let me explain more here.
My case is grounded to outlet ground. I have a "GND ground plane(Digital GND)" on my board too.
But seems the "LCD case" is not connected to GND pins on the lcd, I found two jumper pad which by shorting them I can connect the lcd case to GND pins. and on other side if I connect the "GND ground plane(Digital GND)" to Case ground I can see this problem happening like 1/10 times. I dont know if it is a good idea to connect "Ground" to "Digital Ground" or not , and if I should connect the case to them or not!

 

[ARQuattr]

Most likely tying your digital ground to the earth ground should be fine and this is a very common approach, but there are various factors that affect the decision for your system. If you digital ground is not at a different potential from ground you may want to do this. You could try first tying them together with something like a 1MOhm resistor and measure the voltage across it to make sure it's close to zero, and then using solid wire. Make sure you input power is properly grounded.

 

[FvM]

I believe most character LCD modules have the metal frame connected to display ground, but possibly not all. Allowing ESD events hitting parts of an elecronic circuit without dedicated protection and some kind of error detection and recovery means inviting temporary failure and permanent damage.

In case of a display, there should be a transparent window with a breakdown voltage above expectable ESD voltage levels (e.g. 16 kV) and sufficient creepage distance around it's edges.

If the bus lines between the processor and the LCD controller have some length, ESD hitting the instrument case can still trigger false display characters in some cases. EMI standards allow at worst case temporary function loss of an instrumnet, but it most self-recover without user action. Refreshing the display periodically would fulfill this requirement.

I remember an instrument design where we implemented permanent background readback of the display RAM, calculating a checksum. Only in case of a changed display content the controller was reset and the display refreshed, almost unnoticed by the operator and without flickering in normal operation.

 

[D.A.(Tony)Stewart]

If LCD cover is plastic, high ESD discharge fields can corrupt charge in LCD and it will remember that or get damaged and you won't forget.

Use glass cover or antistat coating on plastic. Use a plastic case. Metal cases are only for EMI experts as sub-ns ESD events dont follow the path you always expect, like lightning and create high E-field or ground spikes.