Posted by
Stewart Baker on
Dec 01, 2004; 7:00am
URL: http://elecraft.85.s1.nabble.com/K2-Strange-Problem-tp372185p372188.html
On Tue, 30 Nov 2004 15:45:37 EST,
[hidden email] wrote:
> 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
Hi Bob,
Thank you for the detailed explanation of your experiences with PIC resets.
Microchip data sheets/books show a number of ways that a reset can be achieved
on their PIC's. Tying MCLR and VDD together is unsatisfactory. The way you
describe with the R,C and diode gives a very reliable reset everytime and is
independent of the supply rise time. The diode makes sure that the MCLR line
always starts from a known state by discharging the C. As you mention glitches
on the supply also need to be addressed.
Although not directly involved with the "strange problem" I consider the MCU
reset a point of weakness in the K2 design, and am contemplating modifying the
Control board to add a proper reset.
I too would be interested in comment from Eric or Wayne.
73
Stewart G3RXQ
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