Lpc2000

Hello,

I am designing a board based on LPC2124 and I want to reset the 
device when the core voltage (1.8v) is not good. I have a voltage 
supervisor of 1.8v which makes the reset (0v) but after the reset it 
supplies 1.8v to the reset pin. Is this a legal voltage for the reset 
pin? The manual says that this pin is Schmitt trigger but also says 
that is TTL compatible. So, during reset I will have no problems but 
after reset? Is 1.8v capable to bring LPC2124 out of reset condition?

Thank you very much.

At 07:47 AM 10/24/04 +0000, you wrote:
>I am designing a board based on LPC2124 and I want to reset the
>device when the core voltage (1.8v) is not good. I have a voltage
>supervisor of 1.8v which makes the reset (0v) but after the reset it
>supplies 1.8v to the reset pin. Is this a legal voltage for the reset
>pin? The manual says that this pin is Schmitt trigger but also says
>that is TTL compatible. So, during reset I will have no problems but
>after reset? Is 1.8v capable to bring LPC2124 out of reset condition?

You probably want an open drain reset device rather than a push-pull.  A 
simple pullup will then provide the voltage to take the processor out of 
reset.  As an added bonus you can also parallel reset sources.

Another item to watch for, make sure the reset device holds the reset low 
for the full time period required.  Most do, but a simple comparator is 
likely to be too fast.

BTW the data sheet states the minimum level high input for the reset pin is 
2.0V.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III

Robert Adsett wrote:

>You probably want an open drain reset device rather than a push-pull.  A 
>simple pullup will then provide the voltage to take the processor out of 
>reset.  As an added bonus you can also parallel reset sources.
>

I find that a device that can source the reset pin high after reset is 
more useful. This allows a pull-down resistor that ensures the reset 
line stays low even in abnormally low supply voltage conditions. Most 
reset chips don't do this but I find it practical to dedicate a small 
8-pin PIC12C629 for this job. It's about the same price as a reset chip 
(close enough), comes in very small packages, and includes all the 
brown-out detect and power-on timers etc. Besides being able to program 
the reset time I also include a coms monitoring routine that allows me 
to place the LPC into reset or program mode etc simply by entering a 
unique control string.

Peter Jakacki