Lpc2000

This may be too obvious, but I think it is worth mentioning: 3.3V 
logic outputs from LPC2xxx will be enough to feed most logic 
connections; i.e., if you want to interface TTL or LS logic, a 
logic "1" from the LPC will be at least 2.0V (-4 mA for non open-
drain outputs), so you can interface 74LSxxx or 74HCTxxx without 
worrying.

Concerning the op-amp solution for the A/D, I think you either way 
will end up using resistors to make it <1 gain, and you will need to 
deal with the op-amp's own temperature drift and input offset. Except 
for the impedance problem, I don't see the advantage on using an op-
amp. I'd go with a plain 0.1% resistors divisor. 0.1% resistors are 
relatively cheap (<$0.19). 0.05% resistors are also available, but 
they are more expesive. Depending on the number of samples per second 
needed you could use a capacitor to improve the impedance of the 
sampled value, as suggested some days ago in this forum.

Guille

--- In lpc2000@yahoogroups.com, "Danish Ali" <danish@...> wrote:
>
> Q2 is probably the only one I feel confident to comment on:
> I would use a 74HCT logic gate to buffer the output if the
> target swing is 0 to 5V. If there are a lot of lines to
> buffer you can't go far wrong with a 74HCT373.
> Unless, that is, you intend to drive relays or the like
> when a ULN200x really shines.
> 
> For bidirectional lines (where I can have a pin that says
> which direction I want) I use a 74LVC4245A and it seems to
> work.
> In the past I have tried (and failed) to use an ADG3308
> "automatic direction-sensing" bidirectional level shifter.
> I'm not suggesting that it can't be made to work, but do
> breadboard that chip before getting a PCB made. I would
> be grateful for comments as to what I might have been
> doing wrong with the ADG3308 because it seems so useful!
> 
> I think the data sheet says GPIO pins are only 5V-tolerant
> when the 3.3V supply is present. (Quite what will happen
> when it is not present I'm not sure, but that's what I read).
> Since my +3.3V is derived from the +5V I don't worry about
> this.
> 
> As to unity0724's hope that (when used as GPIO) the I2C
> lines can have pull-ups, the complication is that an I2C
> device must not load the lines when its power is not present.
> The easiest fix for this is to omit the pmos output
> transistor altogether, and this is what Philips do.
> Other approaches might be possible but I can't think how.
> 
> Regards,
> Danish
> --- In lpc2000@yahoogroups.com, "unity0724" <unity0724@> wrote:
> >
> > --- In lpc2000@yahoogroups.com, "dijucthat" <bigdaddy81@> wrote:
> > >
> > > I have a project that was setup for a 5v AVR that I would like 
to 
> > > convert to an LPC2294.
> > >   
> > > 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)
> > 
> > 
> > 
> > Umm... may be..
> > On LPC, to output a '1' with 5V pull up:
> > - Set output bit to 1 (this will drive output to 3.3V)
> > - The output will not go >3.3V as Upper PMOS transister seems to
> >   be sinking current from external 5V pull up when on.
> > - Once the Output is forced to 3.3V by an active pullup 
transistor,
> >   set the port bit to input.
> > - The external pull up resistor will then pull to 5V, slower.
> >   (Do not let port pin be output, it will not get pullup up to 5v)
> > - Hope this will be "quite" similar with your AVR port pin with 
> >   both active (for few clocks) and passive pullup
> > - ...Actually, I've not tried this on LPC yet...
> > To output a '0':
> > - Just set port bit to '0' and configure port pin to output.
> > 
> > (Hope philips will use I/O direction to control I2C ports instead
> > of open drain outputs.  Then we could have push-pull outputs on
> > all I2C pins)
>