Keypad causes display hang

[SIS]

I'm using the LK204-25-V (wide voltage version) that is experiencing problems with certain keys on the keypad. Keys with the scancode of 0x4b thru 0x4f cause the display to behave erratically if the keypad is affixed to a non-grounded piece of metal. If the metal backing for the keypad is not grounded, i.e. floating and theoretically acting as a capacitor, then no problems occur.
The erratic behaviour that occurs is one of the follow (each has occured numerous times):
1. Keycode is sent properly, but future keypresses don't work
2. Keycode is invalid, zero or 101 or greater
3. Backlight goes out
4. Display briefly goes blank
5. Display shows garbage, sometimes all pixels go on
There may be a few other symptoms, but in all cases the keypad no longer functions. No intervention via the serial port is able to recover the display, the display must be power-cycled to resume operation.

Now, even stranger, is that if the keypad connector is reversed, all keys function normally including those with the scancodes 0x4b thru 0x4f that caused problems normally. With the keypad in its normal position, keys 0x4b and 0x4c consistently cause problems everytime they are pressed, the others, 0x4d,4e and 0x4f, cause the same problem occasionally.
These keys all appear to be connected to pin 8 of the keypad connector. I have tried other keypads, even from a different manufacturer that all exhibit the same problems with these same scancodes. I've also tried another LK204-25-V, with the same results.

Has anyone seen an similar problems? Any insight would be helpful.

[Tom]

Thank you for posting,

Can you please tell me what firmware version, and what pcb revision you have?

[SIS]

The PCB rev is 1.23, and the CPU has sticker on it that says V1.8, I assume this is the firmware version.

In addition there is another sticker on the board, just above the regulator, that says '1114-04M12'.

[Tom]

Are you using a plastic membrane keypad? Also have you tried shorting the rows and columns to try to get similar results?

[SIS]

Yes, I am using a plastic membrane keypad. Are you suggesting to short a row to a column to simulate the offending keys, or are you suggesting to short two or more columns or rows to each other?

[Tom]

I am suggesting to a row and column together to simulate a keystroke on the keypad.

[SIS]

After many hours trying to come up with a repeatable scenario of failure, the keypad still hangs when certains keys are pressed.
The hang is intermittent and not repeatable without the keypad connected ( i.e. shorting rows to columns does not cause the failure, at least not in an amount of time that is reasonable to test).
And, now the orientation of the keypad connector doesn't make a difference, certains keys cause a hang in either position, although the keys that affect the display are different for each orientation.

I have found that the problematic keys are always connected to pin 8 of the display and therefore RB6/PGC of the onboard PIC. I suspect that this is an EMI issue, because the display often resets to the splash screen when the hang occurs. I've looked at the PIC app notes regarding keypads and have tried putting the suggested 100 Ohm resistors in series on both the rows and columns, but with no success. I have attempted additional shielding and ferrite cores on the power and display connectors, but nothing seems to make a difference. The only way to stop the problem is to have a large gap between the keypad and the grounded metal housing of the device.

Do you have any suggestions on other things to try, I'm to the point where I must solve this problem or buy 200 displays a year from somewhere else.

Thanks

[Tom]

Hi Sis,

Does your keypad have ground strapping?

[SIS]

The back of the membrane keypad is adhered to a grounded surface, but the keypad itself does not include grounded top layer. The display is mounted under the keypad in a cutout in the metal housing, just big enough for the display, which shows through a clear section of the keypad. I ordered an identical keypad with the additional top grounding layer, but have not yet received it.

Have you seen problems like this fixed by using a keypad with a top grounding layer?

[Tom]

SIS,

We have come across this problem in the past, and it has helped to use a keypad with the top grounding layer.

Let me know how you make out, after you tested the new keypad.

If you need anything else let me know.

Best Regards,

[SIS]

I tried the new keypad with the additional grounding layer and it made no difference.

Do you suspect an EMI issue? If so, is there anything else that can be done to the display/keypad to reduce the effects, while I try to find the source and reduce its emissions?

[Miles]

Yes, I do suspect it's an EMI issue. Can you contact me at myero@matrixorbital.ca and provide me with your phone number and I'll contact you immediately and we can discuss some of the steps we are taking to reduce these problems.