Posted by
G3VVT on
Nov 30, 2004; 8:45pm
URL: http://elecraft.85.s1.nabble.com/K2-Strange-Problem-tp372185p372187.html
In a message dated 30/11/04 16:46:51 GMT Standard Time,
[hidden email] writes:
That's what I was doing, however, because anything capacitive slows the rise
of
the 5A rail, and hence the correct MCU initialisation, I was chasing my tail.
This is a problem I have come across before about 5 years back where we had
a newly developed repeater logic controlled by a PIC chip.
I had the prototype running the local 2m repeater and the problem was that
there were what appeared to be clock pulses appearing on the through audio of
the logic. Attempts to decouple the 5V supply rail were disastrous in that it
locked up the PIC chip. Eventually traced to the gremlin of delayed voltage
rise on the chip due to charging of the decoupling capacitors. This is poison
to the PIC chips as they require apparently a very fast rise time on the
supply voltage. In our case the problem was self inflicted and was due to trying
to be clever and using two regulators in tandem. Was cured by reverting to
only one regulator fed direct from the incoming DC supply and only just enough
decoupling to clean up the supply line.
There is an additional support you can give the PIC chip in start up I
discovered when trying to fault find on a partially defective chip.
If instead of connecting the PIC chip MCLR pin direct to 5V (see U6 pin 1,
the MCU on the K2 control board), connect it via a 10K ohm resistor and place a
4.7 to 10uF capacitor on the MCLR pin. This resultant delay allows all the
voltages to stabilise before the chip is enabled. At least it is my belief of
how the modification works. Additionally I added a diode with the anode to
the MCLR pin across the 10K resistor to discharge the capacitor quickly in the
event of a power interruption or the chip would again lock up. This was a
change made to get over a particular problem with a PIC chip, but could be the
answer to some odd glitches experienced recently. We now have this
incorporated into our repeater logic as a standard. An entirely different usage, though
one that could have parallels.
Perhaps Eric or Wayne at Elecraft could comment.
Bob, G3VVT
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