Lpc2000

--- In lpc2000@yahoogroups.com, "Gus" <gus_is_working@y...> wrote:

> On my board I have LPC2138 connected to MAX232 running at 5V. I had 
> problems with UART until I added a 4.7K resistor in series. and the 
> voltage high on it now is 2.3V
> 
> Are the pins real 5V tolerant? The data sheet says UART RX is 5V 
> tolerant!!!

Admittedly, I've never used the above chips, but are you sure you
don't have the MAX232 drivers hooked up backwards? There is the 0VDC
to +5VDC TTL logic level side, then the +/-5VDC RS-232 side. 

If you have the +/-5VDC side connected to the LPC2138, it would
explain why the LPC2138 doesn't seem 5V tolerant for you. Having -5VDC
applied to an I/O line is probably not what Philips meant by saying
"5V tolerant I/O." 

Also, the +/-5VDC side of the MAX232 is driven from a low current
charge pump power source, which might explain why adding a
4.7K series resistor would drop the voltage down to 2.3V. Otherwise,
maybe the LPC2138's input pin is not configured properly in your
firmware so that the LPC2138 is actually trying to drive the line as
an output low.