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!!!

Thanks in advance

Gus
	

Hi Gus,

Could you describe what kind of problems you had and on which UART. I'm 
also using 5V powered MAX232 to drive my UART lines without series 
resistors.

marko

Gus 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!!!
>
>Thanks in advance
>
>Gus
>
>
>
>
>
> 
>Yahoo! Groups Links
>
>
>
> 
>
>
>
>  
>
	

Gus,
> 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!!!
>   
I had a MAX232 running at 5V connected to an LPC2106, no series
resistors.
No problems at all running at 115200 Baud.

Regards,

    Rob

I didn't have a 5V voltage in my board, so I used MAX3232, which is
the 
3.3V version of MAX232. There is an ST3232 also, which is equivalent. 
You can use that chip if you think that 5V is your problem (this might 
also save you some 5V circuitry).

Guille

--- In lpc2000@lpc2..., Rob Jansen <rob@m...> wrote:
>
> Gus,
> > 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!!!
> >   
> I had a MAX232 running at 5V connected to an LPC2106, no series 
resistors.
> No problems at all running at 115200 Baud.
> 
> Regards,
> 
>     Rob
>
	

Hi Gus,

  I just currently designed a LPC2138 part with a ICL3222ECPN running
at 5VDC and have no problems with speeds up to 115.2K. I do have a 51
ohm series resistor on all I/O lines including the RXD and TXD of
USART0 to make sure that no EMI problems occur.

regards,
Charles
	--- In lpc2000@lpc2..., "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!!!
> 
> Thanks in advance
> 
> Gus
>
	

Hello, Gus.

Saturday, January 28, 2006, 0:28:55, you wrote:

G>    On my board I have LPC2138 connected to MAX232 running at 5V. I had
G>  problems with UART until I added a 4.7K resistor in series. and the
G>  voltage high on it now is 2.3V

G>  Are the pins real 5V tolerant? The data sheet says UART RX is 5V 
G>  tolerant!!!

G>  Thanks in advance

G>  Gus

I use ADM232LJN (5V chip) with LPC2129 without any problems, but not
yet tried to hook any 5V peripherials to 2138.

By the way, which revision of chip you currently have? (just quote last
line of chip marking if you don't know where to look for it).
	-- 
Best regards,
 Yuri                          mailto:yuri@yuri...
	

I'm not an EMI expert,
> I do have a 51
> ohm series resistor on all I/O lines including the RXD and TXD of
> USART0 to make sure that no EMI problems occur.
>
> regards,
> Charles
>   

I did do some ESD research/design once for a web tablet. Although the IC 
used (forget which one) was said to be ESD safe, firing at the touch 
screen with a Human Body Model ESD tester resulted in error. I then 
added R/C combinations to resolve this problem.

But I'm definitely not an EMI expert and would even think that adding 
resistors may make the problem worse ...
Are there any special considerations to take into account with (apart 
from PCB design and use of separate ground/power planes) ?

Regards,

    Rob

P.s: let me state that I was _not_ the original designer of that web 
tablet :o)

Just to chip in as well, I have never had a problem with 5V 232 chips at 
any baud-rate etc. The fact that you measured 2.3V when you used a 
series resistor indicates that you should perhaps check your 3.3V 
supply. Perhaps you have damaged these inputs because the 5V was present 
when your 3.3V supply was still ramping. The 5V tolerant inputs depend 
upon the 3.3V supply being present to operate effectively. The chips do 
work as they are spec'd, you need to look at your implementation.

Is your VDD supply at a nominal 3.3V and is it clean? Is your 5V supply 
the same? What do they look like (ramping) when power is first applied? 
I think if you scoped the inputs on both sides you can get a clear 
picture of what is really happening anyway.

*Peter*

Gus 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!!!
>   

----- Original Message ----- 
From: "Rob Jansen" <rob@rob@...>
To: <lpc2000@lpc2...>
Sent: Saturday, January 28, 2006 6:11 AM
Subject: [lpc2000] EMI - was Re: lpc2138 5V tolerant is not really tolerant!
	> I'm not an EMI expert,
>> I do have a 51
>> ohm series resistor on all I/O lines including the RXD and TXD of
>> USART0 to make sure that no EMI problems occur.
>>
>> regards,
>> Charles
>>
>
> I did do some ESD research/design once for a web tablet. Although the IC
> used (forget which one) was said to be ESD safe, firing at the touch
> screen with a Human Body Model ESD tester resulted in error. I then
> added R/C combinations to resolve this problem.
>
> But I'm definitely not an EMI expert and would even think that adding
> resistors may make the problem worse ...
> Are there any special considerations to take into account with (apart
> from PCB design and use of separate ground/power planes) ?

Devices like AVX TransGuards can be very useful.

Leon 

---
[This E-mail has been scanned for viruses but it is your responsibility 
to maintain up to date anti virus software on the device that you are
currently using to read this email. ]
	

--- In lpc2000@lpc2..., Rob Jansen <rob@m...> wrote:
>
> I'm not an EMI expert,
> > I do have a 51
> > ohm series resistor on all I/O lines including the RXD and TXD of
> > USART0 to make sure that no EMI problems occur.
> >
> > regards,
> > Charles
> >   
> 
> I did do some ESD research/design once for a web tablet. Although
the IC 
> used (forget which one) was said to be ESD safe,
firing at the touch 
> screen with a Human Body Model ESD tester resulted in error. I then 
> added R/C combinations to resolve this problem.
> 
> But I'm definitely not an EMI expert and would even think that adding 
> resistors may make the problem worse ...
> Are there any special considerations to take into account with (apart 
> from PCB design and use of separate ground/power planes) ?
> 
> Regards,
> 
>     Rob
> 
> P.s: let me state that I was _not_ the original designer of that web 
> tablet :o)
>
Hi Rob,

  I am not either even though I have been designing our products to
eliminate EMI for CE approvals for the past 4 years on our products. 

  I have also used what Leon suggested as well (the AVX transguards),
but I am using some resistor packs from CTS that are 0.5mm pitch which
allows me to put them about 3mm away from the processor pins.

  We also designed a motion control stage for a large Canandian
company and they wanted 47 ohm resistors on all I/O's from a large 12
x 14 inch, multilayer board. They still had problems with the EMI, but
then we added some ferrits to the motor stages and that helped them
drop to 5db below class B.

  We do have clients (one in particular) who are and suggested adding
an resistor to each processor line to eliminate any EMI and to help
with ESD. They suggested a 30 to 51 ohm resistor on all processor
lines along with 100pf to 1nF capacitor on ports that extend off the
board. We have followed this advice 3 times and each of our boards
that went through CE, class B with no problems. There are still other
considerations like board layout, capacitor and other components to
help reduce the noise as well.

  We design allot of motion control and there is plenty of noise that
is generated that we have to take into account. One saving grace is
the CE approved power supply. We ran experiments with one at an
approved lab and found that it eliminates all the conducted emmissions
from the system if one is there. That saved us from having to put
chokes and such on our boards since we could be pumping as much as 12
amps through the board at any one time.

regards,
Charles