On Sat, 23 Apr 2011 12:53:45 -0700 (PDT), Jeroni Paul
wrote:

On 23 abr, 19:31, Rich Webb wrote:
Hold it in a reset state longer on startup. These guys are unusual in
that their RST pin is active high, so keep that in mind.

The naive way to implement reset in this family is just a cap to the
positive supply rail, a resistor to ground, and the RST pin in between.
If this is what they used, it sounds like you're just on the edge of
reliability and may be able to make it happy by boosting the RC time
constant a tad by using a larger C or R, depending on which is easier to
get to.

There are also "supervisory" chips that are more reliable and can also
take care to reset on brown-outs. Some of these hold reset active for a
fixed interval but, IIRC, some set the time with an external R or C.

--
Rich Webb * * Norfolk, VA

Thank you for the replies.
I tried to keep the reset input active manually for several seconds
but still behaves the same. What happens while RST is high is the
micro sets some outputs high and turns on all the hardware. When I
remove RST all comes back to st-by. From then on the behaviour is the
same. For reset it uses a KIA7436P:
http://www.alldatasheet.com/datashee.../KIA7436P.html

Humph. So much for the easy fix. ;-)

Any chance that the manufacturer has a firmware fix for this? The
AT89S52 is in-system programmable so it might be possible to load new
firmware in the field. If there is such a thing available they may not
want to release it to an end user (too much chance of bricking the unit)
but you sound like you know what you're doing and *if* there is one
available you might be able to convince them you're okay.

Otherwise, it may be time to get creative. One possibility might be to
run the CPU at a slightly slower speed by increasing the load capacitors
at the crystal. Or, replace the 12 MHz crystal with an 11.0592 MHz
(should be available). Might work, depending on what other timing
sensitive parts that it talks to.

--
Rich Webb Norfolk, VA