In message <gp6k14lkhns618g04kgnh3gdiaiipgsn9p@4ax.com>, Spehro Pefhany
speffSNIP@interlogDOTyou.knowwhat> writes
On Thu, 1 May 2008 17:51:39 +0100, Steve H <steveu@spho.demon.co.uk
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.

Are you sure it's not resetting?


I'm certain the PIC isn't resetting - the LCD module is going blank but
the PIC continues to function. It appears the intelligent LCD module
locks up/resets itself.


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.

What else EXACTLY is this PIC circuit connected to (off board in
particular)? How long is the cable to the LCD display? What kind of
equipment is this? Can you link to a photo? Is it a proper PCB or a
plugboard?

Have no photo to hand and the equipment isn't with me just now. Fully
assembled it comprises the PIC with LCD, 4 control buttons and several
logic inputs/outputs, four of which control 4 to 16-line latches that
switch solenoids via darlington drivers. The unit is part of a gas
sampling multiplexer system.

Currently, a lot of the circuitry is disconnected - there are 3
individual PCBs (etched boards). It's working in a bare-bones sort of
state at the moment, pretty much the PIC and display only

An earlier version of the device, identical but utilising in place of
the 16F877A/LCD a pair of 16F84As and driving a 2 x 7segment LED
display, has worked in the field flawlessly for 3 years.


Best regards,
Spehro Pefhany
Best regards,

On Thu, 1 May 2008 21:08:07 +0100, Steve H <steveu@spho.demon.co.uk
wrote:


Have no photo to hand and the equipment isn't with me just now. Fully
assembled it comprises the PIC with LCD, 4 control buttons and several
logic inputs/outputs, four of which control 4 to 16-line latches that
switch solenoids via darlington drivers. The unit is part of a gas
sampling multiplexer system.

Is it the solenoids that are mucking things up?

Currently, a lot of the circuitry is disconnected - there are 3
individual PCBs (etched boards). It's working in a bare-bones sort of
state at the moment, pretty much the PIC and display only

Lash-ups tend to be more sensitive to EMI.

An earlier version of the device, identical but utilising in place of
the 16F877A/LCD a pair of 16F84As and driving a 2 x 7segment LED
display, has worked in the field flawlessly for 3 years.


Best regards,
Spehro Pefhany
Best regards,
Spehro Pefhany

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.

The commercial linear PSU, I'm guessing, would have a filtered input.

Spehro Pefhany writes

What else EXACTLY is this PIC circuit connected to (off board in
particular)? How long is the cable to the LCD display? What kind of
equipment is this? Can you link to a photo? Is it a proper PCB or a
plugboard?

Have no photo to hand and the equipment isn't with me just now. Fully
assembled it comprises the PIC with LCD, 4 control buttons and several
logic inputs/outputs, four of which control 4 to 16-line latches that
switch solenoids via darlington drivers.

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

Steve H wrote:

The commercial linear PSU, I'm guessing, would have a filtered input.

Why would you guess that ?

Guesswork is often where the problem lies. Just try an AC line filter anyway.

Graham

Steve H wrote:

Spehro Pefhany writes

What else EXACTLY is this PIC circuit connected to (off board in
particular)? How long is the cable to the LCD display? What kind of
equipment is this? Can you link to a photo? Is it a proper PCB or a
plugboard?

Have no photo to hand and the equipment isn't with me just now. Fully
assembled it comprises the PIC with LCD, 4 control buttons and several
logic inputs/outputs, four of which control 4 to 16-line latches that
switch solenoids via darlington drivers.

Is the PIC driving the LCD segments directly or does the display have an
onboard controller ?

Another thought btw, add a suitable transorb / TVS across the 5V rails to see
if it helps any. Or even and/or across the 12V input to the 5V regulator.


Graham

In message <gp6k14lkhns618g04kgnh3gdiaiipgsn9p@4ax.com>, Spehro Pefhany
speffSNIP@interlogDOTyou.knowwhat> writes
On Thu, 1 May 2008 17:51:39 +0100, Steve H <steveu@spho.demon.co.uk
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.

Are you sure it's not resetting?


I'm certain the PIC isn't resetting - the LCD module is going blank but
the PIC continues to function. It appears the intelligent LCD module
locks up/resets itself.


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.

What else EXACTLY is this PIC circuit connected to (off board in
particular)? How long is the cable to the LCD display? What kind of
equipment is this? Can you link to a photo? Is it a proper PCB or a
plugboard?

Have no photo to hand and the equipment isn't with me just now. Fully
assembled it comprises the PIC with LCD, 4 control buttons and several
logic inputs/outputs, four of which control 4 to 16-line latches that
switch solenoids via darlington drivers. The unit is part of a gas
sampling multiplexer system.

Currently, a lot of the circuitry is disconnected - there are 3
individual PCBs (etched boards). It's working in a bare-bones sort of
state at the moment, pretty much the PIC and display only

An earlier version of the device, identical but utilising in place of
the 16F877A/LCD a pair of 16F84As and driving a 2 x 7segment LED
display, has worked in the field flawlessly for 3 years.


Best regards,
Spehro Pefhany

--
Steve H

Eeyore writes
Steve H wrote:

The commercial linear PSU, I'm guessing, would have a filtered input.

Why would you guess that ?

Guesswork is often where the problem lies. Just try an AC line filter anyway.

You're absolutely correct. There's quite possibly no filter on it at
all.

Eeyore wrote:
Steve H wrote:
Spehro Pefhany writes

What else EXACTLY is this PIC circuit connected to (off board in
particular)? How long is the cable to the LCD display? What kind of
equipment is this? Can you link to a photo? Is it a proper PCB or a
plugboard?

Have no photo to hand and the equipment isn't with me just now. Fully
assembled it comprises the PIC with LCD, 4 control buttons and several
logic inputs/outputs, four of which control 4 to 16-line latches that
switch solenoids via darlington drivers.

Is the PIC driving the LCD segments directly or does the display have an
onboard controller ?

It's an intelligent display module, requiring 4 data bits and 3 control
lines in addition to the supply. It's updated by the PIC twice a second.

Steve H wrote:

Eeyore wrote:
Steve H wrote:
Spehro Pefhany writes

What else EXACTLY is this PIC circuit connected to (off board in
particular)? How long is the cable to the LCD display? What kind of
equipment is this? Can you link to a photo? Is it a proper PCB or a
plugboard?

Have no photo to hand and the equipment isn't with me just now. Fully
assembled it comprises the PIC with LCD, 4 control buttons and several
logic inputs/outputs, four of which control 4 to 16-line latches that
switch solenoids via darlington drivers.

Is the PIC driving the LCD segments directly or does the display have an
onboard controller ?

It's an intelligent display module, requiring 4 data bits and 3 control
lines in addition to the supply. It's updated by the PIC twice a second.

Sounds to me like you're 'crashing' the LCD module's internal controller then.
Does it have local decoupling on the power lines at the module end and
what have
you done to avoid EMI on the data and control lines ?

Graham

Eeyore writes
Steve H wrote:
Eeyore wrote:
Steve H wrote:
Spehro Pefhany writes

What else EXACTLY is this PIC circuit connected to (off board in
particular)? How long is the cable to the LCD display? What kind of
equipment is this? Can you link to a photo? Is it a proper PCB or a
plugboard?

Have no photo to hand and the equipment isn't with me just now. Fully
assembled it comprises the PIC with LCD, 4 control buttons and several
logic inputs/outputs, four of which control 4 to 16-line latches that
switch solenoids via darlington drivers.

Is the PIC driving the LCD segments directly or does the display have an
onboard controller ?

It's an intelligent display module, requiring 4 data bits and 3 control
lines in addition to the supply. It's updated by the PIC twice a second.

Sounds to me like you're 'crashing' the LCD module's internal controller then.
Does it have local decoupling on the power lines at the module end and
what have
you done to avoid EMI on the data and control lines ?

I added a 100nf cap across the supply at the module end, and also tried
it with a 47uf electrolytic in parallel.

The data and control lines are
approx 30cm long, plain wires with no shielding. I'm led to believe this
is a fairly standard arrangement that should work, but am open to
suggestions.