Lpc2000

Thanks for the replies! I think maybe I should have worded Q3 
differently.  What I meant was using an op-amp as an attenuator with 
gain = (less than one). The sensor maxes out at ~4.9v.It seems that 
a resistor attenuator (as suggested by Kurt) should work as well. 
(If the input impedance is sufficient) But I see what you are saying 
about the temp.  One of the sensors is a temp compensated absolute 
press sensor and it would be a shame to lose accuracy in the 
conversion.

Thanks again!
Daniel   


--- In lpc2000@yahoogroups.com, Xtian Xultz <xultz@...> wrote:
>
> Em Qua 15 Mar 2006 15:06, dijucthat escreveu:
> > I have a project that was setup for a 5v AVR that I would like to
> > convert to an LPC2294.
> >
> > Q1: Is it better to use a simple voltage divider on the GPIO 
Input (to
> > get 3.3v) or should I leave it at 5v? (I know it says 
5v "tolerant",
> > but is it safer at 3.3v)
> 
> I made some tests with 5V, including 5V RS232 signals (from 
MAX232) including 
> ISP and I had no problems until here.
> 
> > Q2: On GPIO outputs, can I use a pullup to get 5v, or should I 
use a
> > buffer, ULN2003, etc.? (again, I am concerned with reliability)
> 
> A pull will not work. If you put it, on the pin you will have 3V3, 
even 
> configuring the pin to output (high) or input. I dont know for 
sure, but all 
> the ICs I know have a diode from the pin to the IO supply. So, to 
have 5V as 
> output, you will need a buffer. 
> 
> > Q3: On the sensors designed for 5v operation, is an op-amp with 
gain
> > <1 the best solution to achieve 0-3.3 scale?
> 
> No. If you use a rail-to-rail opamp, with the input at 5V you will 
have maybe 
> 4,9V on the output, in the best case. The same for a signal at 0V. 
If you 
> can, put a resistor divider. That means a lot of problem, the 
principal are 
> current leackage over the resistors, the current leackage from the 
AD input, 
> and the imprecision of the resistors over the temperature. If you 
need and 
> opamp, the best is to use positive and negative supplys, and 
greater than the 
> signal you will apply.
> 
> > Q4: On the ADC inputs, I have read that you should ues pullups 
even if
> > they are not in use.... does this apply to all LPC?
> 
> If you have a pin that will remain as input, ALLWAYS put it on a 
known state, 
> never leave him open. A pullup is a good choice, I am working with 
LPC2106 
> which does not have AD, and I didnt read the 2294 datasheet, but 
maybe you 
> can put him to ground. 
> 
> > Thank you very much for any response!
> >   Daniel.
> 
> I hope I helped (I am a newbie too!)
>