[sdiy] Strange uController behaviour (slightly OT)

[Emmanuel Amadio]

Hi !
very quickly,

It is possible that theses lines are sampled on start by the uC hardware to
configure some registers.
If the code runs in the uC own ram area it will run perfect with or without
the eeprom once loaded.
Last the uC might be dead :-( Well it's on a socket, right ?

Going to bed now, I'll think about it at work...



-----Message d'origine-----
De : owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]De la part de Alex Stettler
Envoyé : vendredi 16 mai 2003 01:05
À : DIY Synth List
Objet : [sdiy] Strange uController behaviour (slightly OT)


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



**************************************

Alex Stettler
Student Elektrotechnik / Elektronik vz

Hochschule für Technik und Architektur
Burgdorf

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