LCD shorts my circuit !

Archiver

I've built a datalogging circuit using the stamp II, using the
Hitachi LCD. I'm running the LCD in 4-bit mode. There are data and
clock lines shared by two external EEPROMs, 3 ADCs, and 2 bits of the
LCD databus.

The rig works fine in normal operation. However sometimes, if the
power is flicked on and off, there is a short circuit somewhere and
the power source overloads. The solution is to unplug the power, let
everything rest a bit then turn it back on again. and no worries.

The short doesn't appear every time the power is switched on and off,
but it does seem to happen if power is switched off during a loop in
the program. It hasn't ever happened while the LCD was disconnected.

I don't understand why or how this could happen. I can't see how
there could be conflicts and all of the outputs are protected by
resistors. The LCD doesn't even have any outputs.

The other pins are:
LCD register select
LCD enable
ADC enable (3)
EEPROM enable (2)
active-low push buttons (5)

If anyone has ever met anything like this before ?! I'd be most
appreciative of any advice.

cheers,
Dave, Leeds UK

Archiver

I would make sure that the voltage regulator you are using can supply the
amount of current that is required at all times. Remember that electrolytic
caps are short circuits until they are charged or while things are changing!.
I would try to decouple the LCD power connections if you think it is the
culprit. Even though this probably has little to do with your problem...
don't forget that the LCD data pins are actually both inputs and outputs
depending on the control instructions. You can read the data FROM the LCD by
sending the correct command. This actually forces the LCD to output data on
the DATA pins. So your statement about the LCD not having any outputs is not
100% correct! Hope this helps!


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Archiver

Thanks stacks!
I didn't know the LCD was capable of outputs! But this doesn't seem to be the
problem.
But.. there's still something real fishy going on. After some further
investigation, there doesn't seem to be any conflict with the LCD pins. The
short circuit occurs precisely when pin 7 is switched from input to output-high.
But pin 7 doesn't have anything to do with the LCD! (It is Chip Select* for an
EEPROM. The LCD runs off pins 0-5) ?????
The short never occurs during the first program run after compile, only when
power is cut off and then reconnected afterwards. The circuit works, but
something in the programming stuffs it up.
It doesn't seem to matter whether the EEPROM chip is connected or not. Yet the
short never occurs with the LCD removed. The whole thing is really peculiar.
cheers,
dave
azeasi@a... wrote:I would make sure that the voltage regulator you are using
can supply the
amount of current that is required at all times. Remember that electrolytic
caps are short circuits until they are charged or while things are changing!.
I would try to decouple the LCD power connections if you think it is the
culprit. Even though this probably has little to do with your problem...
don't forget that the LCD data pins are actually both inputs and outputs
depending on the control instructions. You can read the data FROM the LCD by
sending the correct command. This actually forces the LCD to output data on
the DATA pins. So your statement about the LCD not having any outputs is not
100% correct! Hope this helps!


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Archiver

Thanks stacks!

I didn't know the LCD was capable of outputs! But this doesn't seem
to be the problem.

.. there's still something real fishy going on. After some further
investigation, there doesn't seem to be any conflict with the LCD
pins. The short circuit occurs precisely when pin 7 is switched from
input to output-high. But pin 7 doesn't have anything to do with the
LCD! (It is Chip Select* for an EEPROM. The LCD runs off pins 0-
5) ?????

The short never occurs during the first program run after compile,
only when power is cut off and then reconnected afterwards. The
circuit works, but something in the programming stuffs it up.

It doesn't seem to matter whether the EEPROM chip is connected or
not. Yet the short never occurs with the LCD removed. The whole thing
is really peculiar.

cheers,

dave




--- In basicstamps@y..., azeasi@a... wrote:
> I would make sure that the voltage regulator you are using can
supply the
> amount of current that is required at all times. Remember that
electrolytic
> caps are short circuits until they are charged or while things are
changing!.
> I would try to decouple the LCD power connections if you think it
is the
> culprit. Even though this probably has little to do with your
problem...
> don't forget that the LCD data pins are actually both inputs and
outputs
> depending on the control instructions. You can read the data FROM
the LCD by
> sending the correct command. This actually forces the LCD to output
data on
> the DATA pins. So your statement about the LCD not having any
outputs is not
> 100% correct! Hope this helps!
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

Since the outputs from the STAMP are current limited it doesn't seem that it
should short out your circuit, but....try putting a small ressitor in series
with pin 7 . A 270 ohm resistor sould not affect the ability of the Stamp to
drive the chip select pin but will prevent an excessive amount of current
from being drawn out otf the Stamp I/O
pin. Are you sure that the chip select pin is correct on your EPROM?


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