On Tue, 30 Nov 2004 15:45:37 EST, [EMAIL PROTECTED] wrote: > In a message dated 30/11/04 16:46:51 GMT Standard Time, > [EMAIL PROTECTED] 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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
