Re: Problem with PIC & LCD display



Steve H 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

Is it a low drop-out type ?


on the circuit board is decoupled using 100nf ceramic capacitors.

On both input and output ?


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),

So you have a problem with noise coupling through your power supply.


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.

This may conceivably be due to electrostatic discharge.


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.

You're SURE it's operating correctly aside from the LCD display ? How have
you determined this ?


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.

You should consider adding some 'bulk capacitance - say 100+ uF on the 5V
rail to ground). Also make sure your grounding methodology is 'clean'.

Graham

.

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