Hello,

I am using a TI MAX3221 RS232 driver IC with my 16F877A based datalogger
circuit and am having trouble getting the 3221 supply cleaned up.  It is
powered from a PIC pin and when it is turned on there is a nasty 1V ripple
at about 20kHz on the driver's Vcc pin that appears to coincide with the
switching waveform on C1 and C2 of the charge pump.  I suppose this is from
the chip drawing a quick bit of current to charge the caps.  But I already
tried replacing the bypass cap on the IC with another .1uF (the recommended
value) in case I burned the first one when I installed it (it is an SMD
board) and then with a 1uF and the problem is still there although a little
reduced.

The V+ and V- outputs from the charge pump look fine.  The main problem is
that the !INVALID output from the IC that tells if an external RS232 device
is plugged in has the same ripple as the Vcc pin.  My PIC code checks this
output to see if it should have the USART enabled and stay awake to wait for
Serial Rx interrupt or shut down the USART and go to sleep but due to the
noise it very occasionally powers down the USART while just sitting idle and
consistently does so whenever a character is received.  I created a catch
loop in my code that shuts off some LEDs and halts the program so I know
exactly where the powerdown is occurring and it can only be executed if the
!INVALID pin is sampled low.  So I am about 99% confident that this noise is
to blame.

The MAX3221 is powered from RC5 of the 16F877A via a short (~2cm) .010" pcb
trace and the bypass cap is an 0402 1uF (was 0402 0.1uF).

Any ideas?  I do have a pad for RC4 (currently unused) right next to a pad
for the problematic pin.  I could parallel the two pins with a short jumper
wire to reduce the output impedance by around half and this should help, but
it seems there should be a better solution that should make that
unnecessary.  Opinions?

Thanks,
Nick

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