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Very strange behaviour with LCD display

Started by See_Mos, Apr 03, 2021, 04:00 PM

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See_Mos

18F1320  It's an old device but I have a few and they are ideal for what I need at the moment.

I have two PCB.  Both have sockets for the 18F1320, a 6 pin connector for the PicKit and an LCD display.  The only difference is the display.  The fist is 4 lines by 20 and the second 2 lines by 20.  Both are wired the same using RB4....RB7 for the data lines.  The fault arose after the failed 2 line display was replaced with a new one.

Here's the puzzle.  I can program a device in the 4 line board and transfer it to the 2 line board and it works as expected.
 
I cannot program a device in the 2 line board unless I disconnect RB4 and RB5 from the display.  Once programmed the device will work in both boards.

The 2 line display is a new RS 532 7470 that I bought a while ago.

trastikata

It is not the PIC, it is the LCD which has low impedance on those pins, can you measure it?

Giuseppe

It could be the power supply of the programmer not holding with the display connected.
this if you power the board via the programmer

tumbleweed

Does the display produce any voltage on the RB4/RB5 lines when you power it up?
If it does, that could be upsetting the programmer (voltage backfeeding to VDD/MCLR through protection diodes).

See_Mos

Obviously it is the display causing the problem as the board was OK with the old display and the spare is OK with the 4 line display.

The measured resistance from the display pin to pin and pins to supply rails is around 700K.

Tumbleweed has the answer, I removed the PIC from the socket and powered the board from the PicKit.  VDD is 4.65V, RB4 and RB5 are 4.6V.  RB6 and RB7 were only 0.2v because Pickit 2 has internal 3K resistor to VSS.  I added 10K pull down resistors on RB4....RB7 and and the voltage dropped to 0.77.  Programming now works so problem solved.