[Elecraft] K2 - Strange Problem ?

[G3VVT at aol.com]

In a message dated 30/11/04 16:46:51 GMT Standard Time,  
stewart at baker.nildram.co.uk 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