[sdiy] Strange uController behaviour (slightly OT)

[Neil Johnson]

Hi,

> The strange behaviour of the uController is, that if the EPROM-window is
> covered with opaque tape as this is usually done with EPROM-devices, the uC
> works more or less correct with only the LCD having nonsense characters on
> the screen. As soon as I remove the black tape, the uC works perfect!

Remember that all seminconductors are light-sensitive devices (as well as
heat-sensitive too).  It could be that light is changing the behaviour of
an input gate or somesuch.  Normal operation should be when the quartz
window is covered over---both to protect the EPROM from UV light, and the
rest of the chip from ambient light.

> 2. There are four 20cm long I/O lines on a socket, which aren't connected to

Mmmmm...they could be picking up noise and feeding it into the uController
while it is starting, which would probably be a bad thing.  Do you need
them?  If not, remove them.

> -Why does a uC work better with light instead without?

Errrmmm...it shouldn't.  You have your logic inverted---the light is
masking some other failure of the chip, it is not the cure.

> -Why do unconnected leads affect the behaviour of a uC, even if these lines
> are configured as OUTPUTS??

Because when in reset, and the period between reset and your software
taking control, these pins are configured as inputs.

> -Could a pulldown-resistor help the uC to get back to normal operation?

Very likely, better to try it first and see if it solves the problem.
Indeed, no pin on a micro should be left unconnected, as this can lead to
increased power consumption and/or eratic behaviour (they don't like
inputs floating).

> -Has anyone else made strange experiences with MC68705 or similar
> microcontrollers?

Any microcontroller can behave badly if you don't understand what it is
doing.

For example: mill off the top of a DRAM chip, load it with 0's, wait a
short while, then read out the memory.  You've just made yourself a crude
CCD camera (the DRAM cells being light-sensitive).  Fix up a lens assembly
on top to sharpen the focus.

On a running micro system shuffle your feet on the carpet and then touch
some pins, any pins.  The static electricity charge in your body could
well trip the micro into an undefined state, when it might well do really
odd things.

I've had circuits behave at odd times during certain times of the day.
The cause was light falling on a glass-encapsulated diode (I think it was
a 1N4148).

Thank basic physics, as that is usually the real cause of a problem.

Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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