Your problem appears to be source current/impedance related, since the
problem is getting better by adding bulk capacitance to the VCC of the
driver chip. You appear to be seeing noise/supply dips corresponding to
the switching of the charge pump in the driver chip. Adding more
capacitance to the PIC output will probably help. The only danger to
doing this is that the PIC will see a virtual short to ground when the
port pin goes high to turn the driver on since the capacitor will be
discharged. A small series resistor to limit the current will protect
the PIC output but also slow down the risetime of VCC to the driver.
Something on the order of 100-220 Ohms will work fine.

I don't have the datasheet for the Maxim part handy... what is the
supply current specification for the part?

Matt

On Thu, 8 Jan 2004 20:07:35 -0600, James Nick Sears wrote:
> That probably would have been a good idea.  I have added another leaded 1uF
> cap to the setup and now the supply noise is down to about .2V, which I
> think should be on the edge of being workable.  However whenever a character
> is received from the RS232 side (from the PC) the supply dips about .6-.7
> volts for about .2ms and then about .1ms later it dips again about half as
> far and for half as long.  Coinciding with these (I assume - I am not set up
> to measure both simultaneously) there are two dips in the !INVALID output
> from the buffer which have approximately the same time scale but drop about
> 3.5V (of a ~5V Vcc) and are much more squared off than the supply noise
> which gradually dips and rises again (which could just be the effect of the
> capacitance I suppose).
>
> How much capacitance can a PIC pin practically drive without causing
> problems?  I know the datasheet says 50pF but that is rated to keep the
> output rise/fall time within spec, right?  I don't care about a slow rise
> time (I already poll the pin in my code to wait for it to come high and then
> wait some more just to be sure) I just don't want to toast the chip or cause
> any weird side effect.
>
> I have 3.3uF and 10uF electrolytics I could try in addition to the 1uF chip
> cap.  Think this is a bad idea?

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics