Lpc2000

----- Original Message ----- 
From: "David Willmore" <willmore@...>
To: <lpc2100@yahoogroups.com>
Sent: Sunday, January 11, 2004 5:28 PM
Subject: Re: [lpc2100] MAX3232 V+ connection


> > I've just read the fine print on the MAX3232 data sheet and found that
the
> > V+ reservoir capacitor on pin 2 may be taken to *either* Vcc or Gnd.
> >
> > However, there is an error associated with my use of this chip. I'd
assumed
> > that the transmitter inputs have pull-up resistors like the MAX202. They
> > don't, and the unused input (pin 10) should be tied to Vcc or Gnd.
>
> Ahhhhhhhh!!! I had a sinking suspision it might be something like that.
> Enough noise from the charge pumps will leak into the input to make it
> toggle and then the huge output current spike will probably cause it to
> go back and then forward, etc.--at about the max toggle rate that the
> chip can support.  This will put a huge drain on the -V and +V, so that
> the charge pump will effectively short Vcc to Vdd in their pumping
efforts.
>

Your chips might be 'latching up' due to the floating input. Some CMOS chips
are prone to this, they take large amounts of current and get very hot. The
Inmos transputer was prone to this if the lid was touched whilst the device
was operating (I got a badly burnt finger from one), and I've also had it
happen with a Scenix SX28.

> So, I'll tie the unused CMOS input to whatever supply rail is handy.  Do
> I need to do anything with the unused RS232 input?


That's OK, it does have a 5k pull-down resistor. You can see it in the
diagram on page 12 of the data sheet (Typical Operating Circuits), with the
note about C3 I mentioned.

Leon
--
Leon Heller, G1HSM
Email: aqzf13@...
My low-cost Philips LPC210x ARM development system:
http://www.geocities.com/leon_heller/lpc2104.html