[sdiy] Strange uController behaviour (slightly OT)

[Alex Stettler]

Hello!


I'm just programming an older MC68705P3 uController in a self developped
circuit. The MC68705 is an 8-bit EPROM uController.
The uController has an LCD, some keys and two ADCs around it.
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!
So I did some further experiments to decrease the number of possible
error-sources and experienced the following behaviour:

1. If there is opaque tape on the ERPOM-window only plays a role at
power-up. As soon power is applied and the uC started working, it works
correct, with or without a covered EPROM-window.

2. There are four 20cm long I/O lines on a socket, which aren't connected to
anything yet. Just empty wires coming out of the uC PCB. Now, they have a
really strange effect on the whole behaviour: With the leads the uC has
often problems with proper starting, although the software sets them as
outputs right at the beginning of the program. If I deconnect the unused
leads from the uC PCB, the uC works absolut perfectly. If the EPROM-windows
is covered or not doesn't play any role.

So my questions are:
-Why does a uC work better with light instead without?
-Why do unconnected leads affect the behaviour of a uC, even if these lines
are configured as OUTPUTS??
-Could a pulldown-resistor help the uC to get back to normal operation?
-Has anyone else made strange experiences with MC68705 or similar
microcontrollers?

Probably it would make a trendy looking device: A plexiglas casing and some
blue or white coloured LEDs lighting the EPROM-window of the MCU ;-))

But I'd rather prefer normal operation of the MCU.

Many thanks in advance for any help!

Kind regards,
Alex



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Alex Stettler
Student Elektrotechnik / Elektronik vz

Hochschule für Technik und Architektur
Burgdorf

Mobile: +41 79 434 87 03
Fax   : +41 61 901 64 04