Forums

Reset pin considerations...

Started by trfillos October 24, 2004

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.




An Engineer's Guide to the LPC2100 Series

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