Re: Problem with PIC & LCD display
- From: allanhvii@xxxxxxxxx
- Date: Sat, 3 May 2008 15:17:32 -0700 (PDT)
On May 1, 12:51 pm, Steve H <ste...@xxxxxxxxxxxxxxxx> wrote:
I'm having a problem with a fairly simple project incorporating a
16F877A PIC and a 4x20 alphanumeric display module. The circuit
interfaces with other logic ICs, though these can be removed from the
circuit to isolate the problem.
The circuit is fed, under test conditions, from a linear 12v power
supply. The 5v regulator on the circuit board is decoupled using 100nf
ceramic capacitors. There is similar decoupling on the PIC itself, and
at other places e.g. decoupling the 5v supply on the connector to the
LCD display. I have also fitted a 100uH coil in the supply to the 5v
regulator to reduce any effects from spurious noise on the supply line.
The problem I'm having difficulty solving is that the display will
intermittently go blank. It will go blank, seemingly when the display is
written to by the PIC, either (a) due to a spike/noise on the mains
supply (this can be intermittently reproduced by repeatedly turning on
and off a piece of mains powered equipment on the same circuit), or (b)
when the 0v rail (not even necessarily close to the LCD display) is
touched by for example a multimeter probe. This is the most consistent
way of producing the fault. The multimeter can be turned off and the
other probe not connected to anything for this to happen. The PIC
doesn't appear to be affected by any of this as it can be shown to still
be operating as expected.
I've experienced similar issues before in my limited experience of
building small projects with PICs. I had issues with a 16F84A resetting
itself when the 0v rail close to the IC was touched with a screwdriver.
Can anybody suggest what vital consideration for circuit design I'm
missing. I'm at a bit of a loss as to what I'm doing wrong.
Thanks.
--
Steve H
Steve:
I've been having similar problems with my new design.
I swapped out the LCD which was getting scrambled with an old unit of
a different model. The old unit works fine. Just to be sure, I put the
original (newer) model back in again, and it still fails. I'll be
ordering more LCD's!
I think the new (failing) units are using the NT3881 controller chip,
which is HD44780 "compatible", whereas the old (working) unit is
assembled with the real HD44780 IC. You may want to check which IC is
used on your unit. I suspect the circuitry (reset or otherwise) on the
NT3881 is not working properly. To be fair, I can't be sure what IC is
being used on the new units because it is bonded & epoxied onto the
PCB, but the resistors, etc. on both boards match the NT3881 specs,
and the Hantronix model calls out this IC (or equivalent).
Does your board have components near IC1 such as R6=91k; R1,R2,R4,R5 =
2.2k; R3 = 0? You may want to order a few LCD's with different
controllers.
Allan
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